1.
Constellation
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A constellation is formally defined as a region of the celestial sphere, with boundaries laid down by the International Astronomical Union. The constellation areas mostly had their origins in Western-traditional patterns of stars from which the constellations take their names, in 1922, the International Astronomical Union officially recognized the 88 modern constellations, which cover the entire sky. They began as the 48 classical Greek constellations laid down by Ptolemy in the Almagest, Constellations in the far southern sky are late 16th- and mid 18th-century constructions. 12 of the 88 constellations compose the zodiac signs, though the positions of the constellations only loosely match the dates assigned to them in astrology. The term constellation can refer to the stars within the boundaries of that constellation. Notable groupings of stars that do not form a constellation are called asterisms, when astronomers say something is “in” a given constellation they mean it is within those official boundaries. Any given point in a coordinate system can unambiguously be assigned to a single constellation. Many astronomical naming systems give the constellation in which an object is found along with a designation in order to convey a rough idea in which part of the sky it is located. For example, the Flamsteed designation for bright stars consists of a number, the word constellation seems to come from the Late Latin term cōnstellātiō, which can be translated as set of stars, and came into use in English during the 14th century. It also denotes 88 named groups of stars in the shape of stellar-patterns, the Ancient Greek word for constellation was ἄστρον. Colloquial usage does not draw a distinction between constellation in the sense of an asterism and constellation in the sense of an area surrounding an asterism. The modern system of constellations used in astronomy employs the latter concept, the term circumpolar constellation is used for any constellation that, from a particular latitude on Earth, never sets below the horizon. From the North Pole or South Pole, all constellations south or north of the equator are circumpolar constellations. In the equatorial or temperate latitudes, the term equatorial constellation has sometimes been used for constellations that lie to the opposite the circumpolar constellations. They generally include all constellations that intersect the celestial equator or part of the zodiac, usually the only thing the stars in a constellation have in common is that they appear near each other in the sky when viewed from the Earth. In galactic space, the stars of a constellation usually lie at a variety of distances, since stars also travel on their own orbits through the Milky Way, the star patterns of the constellations change slowly over time. After tens to hundreds of thousands of years, their familiar outlines will become unrecognisable, the terms chosen for the constellation themselves, together with the appearance of a constellation, may reveal where and when its constellation makers lived. The earliest direct evidence for the constellations comes from inscribed stones and it seems that the bulk of the Mesopotamian constellations were created within a relatively short interval from around 1300 to 1000 BC
2.
Equuleus
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Its name is Latin for little horse, a foal. It was one of the 48 constellations listed by the 2nd century astronomer Ptolemy and it is the second smallest of the modern constellations, spanning only 72 square degrees. It is also very faint, having no stars brighter than the fourth magnitude, the brightest star in Equuleus is Alpha Equulei, traditionally called Kitalpha, a yellow star magnitude 3.9,186 light-years from Earth. Its traditional name means the section of the horse, there are few variable stars in Equuleus. Only around 25 are known, most of which are faint, gamma Equulei is an alpha CVn star, ranging between magnitudes 4.58 and 4.77 over a period of around 12½ minutes. It is a white star 115 light-years from Earth, and has a companion of magnitude 6.1,6 Equulei. R Equulei is a Mira variable that ranges between magnitudes 8.0 and 15.7 over nearly 261 days, Equuleus contains some double stars of interest. γ Equ consists of a star with a magnitude around 4.7. Epsilon Equulei is a triple star also designated 1 Equulei. The system,197 light-years away, has a primary of magnitude 5.4 that is itself a binary star, the secondary is of magnitude 7.4 and is visible in small telescopes. The components of the primary are becoming closer together and will not be divisible in amateur telescopes beginning in 2015, δ Equ is a binary star with an orbital period of 5.7 years, which at one time was the shortest known orbital period for an optical binary. The two components of the system are never more than 0.35 arcseconds apart, due to its small size and its distance from the plane of the Milky Way, Equuleus contains no notable deep sky objects. Some very faint galaxies between magnitudes 13 and 15 include NGC7015, NGC7040, NGC7045 and NGC7046, in Greek mythology, one myth associates Equuleus with the foal Celeris, who was the offspring or brother of the winged horse Pegasus. Celeris was given to Castor by Mercury, other myths say that Equuleus is the horse struck from Poseidons trident, during the contest between him and Athena when deciding which would be the superior. Because this section of stars rises before Pegasus, it is often called Equus Primus, Equuleus is also linked to the story of Philyra and Saturn. Created by Hipparchus and included by Ptolemy, it abuts Pegasus, in Chinese astronomy, the stars that correspond to Equuleus are located within the Black Tortoise of the North. Burnhams Celestial Handbook, An observers guide to the universe beyond the solar system, stars and Planets Guide, Collins, London. The Deep Photographic Guide to the Constellations, Equuleus Star Tales – Equuleus Equueleus Constellation at Constellation Guide Warburg Institute Iconographic Database
3.
Right ascension
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Right ascension is the angular distance measured eastward along the celestial equator from the vernal equinox to the hour circle of the point in question. When combined with declination, these astronomical coordinates specify the direction of a point on the sphere in the equatorial coordinate system. Right ascension is the equivalent of terrestrial longitude. Both right ascension and longitude measure an angle from a direction on an equator. Right ascension is measured continuously in a circle from that equinox towards the east. Any units of measure could have been chosen for right ascension, but it is customarily measured in hours, minutes. Astronomers have chosen this unit to measure right ascension because they measure a stars location by timing its passage through the highest point in the sky as the Earth rotates. The highest point in the sky, called meridian, is the projection of a line onto the celestial sphere. A full circle, measured in units, contains 24 × 60 × 60 = 86 400s, or 24 × 60 = 1 440m. Because right ascensions are measured in hours, they can be used to time the positions of objects in the sky. For example, if a star with RA = 01h 30m 00s is on the meridian, sidereal hour angle, used in celestial navigation, is similar to right ascension, but increases westward rather than eastward. Usually measured in degrees, it is the complement of right ascension with respect to 24h and it is important not to confuse sidereal hour angle with the astronomical concept of hour angle, which measures angular distance of an object westward from the local meridian. The Earths axis rotates slowly westward about the poles of the ecliptic and this effect, known as precession, causes the coordinates of stationary celestial objects to change continuously, if rather slowly. Therefore, equatorial coordinates are inherently relative to the year of their observation, coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch. The right ascension of Polaris is increasing quickly, the North Ecliptic Pole in Draco and the South Ecliptic Pole in Dorado are always at right ascension 18h and 6h respectively. The currently used standard epoch is J2000.0, which is January 1,2000 at 12,00 TT, the prefix J indicates that it is a Julian epoch. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, the concept of right ascension has been known at least as far back as Hipparchus who measured stars in equatorial coordinates in the 2nd century BC. But Hipparchus and his successors made their star catalogs in ecliptic coordinates, the easiest way to do that is to use an equatorial mount, which allows the telescope to be aligned with one of its two pivots parallel to the Earths axis
4.
Declination
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In astronomy, declination is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declinations angle is measured north or south of the celestial equator, the root of the word declination means a bending away or a bending down. It comes from the root as the words incline and recline. Declination in astronomy is comparable to geographic latitude, projected onto the celestial sphere, points north of the celestial equator have positive declinations, while those south have negative declinations. Any units of measure can be used for declination, but it is customarily measured in the degrees, minutes. Declinations with magnitudes greater than 90° do not occur, because the poles are the northernmost and southernmost points of the celestial sphere, the Earths axis rotates slowly westward about the poles of the ecliptic, completing one circuit in about 26,000 years. This effect, known as precession, causes the coordinates of stationary celestial objects to change continuously, therefore, equatorial coordinates are inherently relative to the year of their observation, and astronomers specify them with reference to a particular year, known as an epoch. Coordinates from different epochs must be rotated to match each other. The currently used standard epoch is J2000.0, which is January 1,2000 at 12,00 TT, the prefix J indicates that it is a Julian epoch. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, the declinations of Solar System objects change very rapidly compared to those of stars, due to orbital motion and close proximity. This similarly occurs in the Southern Hemisphere for objects with less than −90° − φ. An extreme example is the star which has a declination near to +90°. Circumpolar stars never dip below the horizon, conversely, there are other stars that never rise above the horizon, as seen from any given point on the Earths surface. Generally, if a star whose declination is δ is circumpolar for some observer, then a star whose declination is −δ never rises above the horizon, as seen by the same observer. Likewise, if a star is circumpolar for an observer at latitude φ, neglecting atmospheric refraction, declination is always 0° at east and west points of the horizon. At the north point, it is 90° − |φ|, and at the south point, from the poles, declination is uniform around the entire horizon, approximately 0°. Non-circumpolar stars are visible only during certain days or seasons of the year, the Suns declination varies with the seasons. As seen from arctic or antarctic latitudes, the Sun is circumpolar near the summer solstice, leading to the phenomenon of it being above the horizon at midnight
5.
Apparent magnitude
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The apparent magnitude of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth. The brighter an object appears, the lower its magnitude value, the Sun, at apparent magnitude of −27, is the brightest object in the sky. It is adjusted to the value it would have in the absence of the atmosphere, furthermore, the magnitude scale is logarithmic, a difference of one in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry, apparent magnitudes are used to quantify the brightness of sources at ultraviolet, visible, and infrared wavelengths. An apparent magnitude is measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V filter band would be denoted either as mV or often simply as V, the scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes. The brightest stars in the sky were said to be of first magnitude, whereas the faintest were of sixth magnitude. Each grade of magnitude was considered twice the brightness of the following grade and this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest, and is generally believed to have originated with Hipparchus. This implies that a star of magnitude m is 2.512 times as bright as a star of magnitude m +1 and this figure, the fifth root of 100, became known as Pogsons Ratio. The zero point of Pogsons scale was defined by assigning Polaris a magnitude of exactly 2. However, with the advent of infrared astronomy it was revealed that Vegas radiation includes an Infrared excess presumably due to a disk consisting of dust at warm temperatures. At shorter wavelengths, there is negligible emission from dust at these temperatures, however, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the scale was extrapolated to all wavelengths on the basis of the black body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance for the zero magnitude point, with the modern magnitude systems, brightness over a very wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30, astronomers have developed other photometric zeropoint systems as alternatives to the Vega system. The AB magnitude zeropoint is defined such that an objects AB, the dimmer an object appears, the higher the numerical value given to its apparent magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of exactly 100. Since an increase of 5 magnitudes corresponds to a decrease in brightness by a factor of exactly 100, each magnitude increase implies a decrease in brightness by the factor 5√100 ≈2.512. Inverting the above formula, a magnitude difference m1 − m2 = Δm implies a brightness factor of F2 F1 =100 Δ m 5 =100.4 Δ m ≈2.512 Δ m
6.
Cosmic distance ladder
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The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an object is possible only for those objects that are close enough to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at distances and methods that work at larger distances. Several methods rely on a candle, which is an astronomical object that has a known luminosity. The ladder analogy arises because no single technique can measure distances at all ranges encountered in astronomy, instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, and so on. Each rung of the ladder provides information that can be used to determine the distances at the next higher rung, at the base of the ladder are fundamental distance measurements, in which distances are determined directly, with no physical assumptions about the nature of the object in question. The precise measurement of stellar positions is part of the discipline of astrometry, direct distance measurements are based upon the astronomical unit, which is the distance between the Earth and the Sun. Historically, observations of transits of Venus were crucial in determining the AU, in the first half of the 20th century, observations of asteroids were also important. Keplers laws provide precise ratios of the sizes of the orbits of objects orbiting the Sun, radar is used to measure the distance between the orbits of the Earth and of a second body. From that measurement and the ratio of the two sizes, the size of Earths orbit is calculated. The Earths orbit is known with a precision of a few meters, the most important fundamental distance measurements come from trigonometric parallax. As the Earth orbits the Sun, the position of stars will appear to shift slightly against the more distant background. These shifts are angles in a triangle, with 2 AU making the base leg of the triangle. The amount of shift is small, measuring 1 arcsecond for an object at the 1 parsec distance of the nearest stars. Astronomers usually express distances in units of parsecs, light-years are used in popular media, because parallax becomes smaller for a greater stellar distance, useful distances can be measured only for stars whose parallax is larger than a few times the precision of the measurement. Parallax measurements typically have an accuracy measured in milliarcseconds, the Hubble telescope WFC3 now has the potential to provide a precision of 20 to 40 microarcseconds, enabling reliable distance measurements up to 5,000 parsecs for small numbers of stars. By the early 2020s, the GAIA space mission will provide similarly accurate distances to all bright stars. Stars have a velocity relative to the Sun that causes proper motion, for a group of stars with the same spectral class and a similar magnitude range, a mean parallax can be derived from statistical analysis of the proper motions relative to their radial velocities
7.
Parsec
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The parsec is a unit of length used to measure large distances to objects outside the Solar System. One parsec is the distance at which one astronomical unit subtends an angle of one arcsecond, a parsec is equal to about 3.26 light-years in length. The nearest star, Proxima Centauri, is about 1.3 parsecs from the Sun, most of the stars visible to the unaided eye in the nighttime sky are within 500 parsecs of the Sun. The parsec unit was likely first suggested in 1913 by the British astronomer Herbert Hall Turner, named from an abbreviation of the parallax of one arcsecond, it was defined so as to make calculations of astronomical distances quick and easy for astronomers from only their raw observational data. Partly for this reason, it is still the unit preferred in astronomy and astrophysics, though the light-year remains prominent in science texts. This corresponds to the definition of the parsec found in many contemporary astronomical references. Derivation, create a triangle with one leg being from the Earth to the Sun. As that point in space away, the angle between the Sun and Earth decreases. A parsec is the length of that leg when the angle between the Sun and Earth is one arc-second. One of the oldest methods used by astronomers to calculate the distance to a star is to record the difference in angle between two measurements of the position of the star in the sky. The first measurement is taken from the Earth on one side of the Sun, and the second is approximately half a year later. The distance between the two positions of the Earth when the two measurements were taken is twice the distance between the Earth and the Sun. The difference in angle between the two measurements is twice the angle, which is formed by lines from the Sun. Then the distance to the star could be calculated using trigonometry. 5-parsec distance of 61 Cygni, the parallax of a star is defined as half of the angular distance that a star appears to move relative to the celestial sphere as Earth orbits the Sun. Equivalently, it is the angle, from that stars perspective. The star, the Sun and the Earth form the corners of a right triangle in space, the right angle is the corner at the Sun. Therefore, given a measurement of the angle, along with the rules of trigonometry. A parsec is defined as the length of the adjacent to the vertex occupied by a star whose parallax angle is one arcsecond
8.
Stellar classification
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In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with absorption lines, each line indicates an ion of a certain chemical element, with the line strength indicating the abundance of that ion. The relative abundance of the different ions varies with the temperature of the photosphere, the spectral class of a star is a short code summarizing the ionization state, giving an objective measure of the photospheres temperature and density. Most stars are classified under the Morgan–Keenan system using the letters O, B, A, F, G, K, and M. Each letter class is subdivided using a numeric digit with 0 being hottest and 9 being coolest. The sequence has been expanded with classes for other stars and star-like objects that do not fit in the system, such as class D for white dwarfs. In the MK system, a luminosity class is added to the class using Roman numerals. This is based on the width of absorption lines in the stars spectrum. The full spectral class for the Sun is then G2V, indicating a main-sequence star with a temperature around 5,800 K, the conventional color description takes into account only the peak of the stellar spectrum. This means that the assignment of colors of the spectrum can be misleading. There are no green, indigo, or violet stars, likewise, the brown dwarfs do not literally appear brown. The modern classification system is known as the Morgan–Keenan classification, each star is assigned a spectral class from the older Harvard spectral classification and a luminosity class using Roman numerals as explained below, forming the stars spectral type. The spectral classes O through M, as well as more specialized classes discussed later, are subdivided by Arabic numerals. For example, A0 denotes the hottest stars in the A class, fractional numbers are allowed, for example, the star Mu Normae is classified as O9.7. The Sun is classified as G2, the conventional color descriptions are traditional in astronomy, and represent colors relative to the mean color of an A-class star, which is considered to be white. The apparent color descriptions are what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars. However, most stars in the sky, except the brightest ones, red supergiants are cooler and redder than dwarfs of the same spectral type, and stars with particular spectral features such as carbon stars may be far redder than any black body. O-, B-, and A-type stars are called early type
9.
Star catalogue
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A star catalogue or star catalog, is an astronomical catalogue that lists stars. In astronomy, many stars are referred to simply by catalogue numbers, there are a great many different star catalogues which have been produced for different purposes over the years, and this article covers only some of the more frequently quoted ones. Star catalogues were compiled by many different ancient peoples, including the Babylonians, Greeks, Chinese, Persians, most modern catalogues are available in electronic format and can be freely downloaded from space agencies data center. Completeness and accuracy is described by the weakest apparent magnitude V, from their existing records, it is known that the ancient Egyptians recorded the names of only a few identifiable constellations and a list of thirty-six decans that were used as a star clock. They are better known by their Assyrian-era name Three Stars Each and these star catalogues, written on clay tablets, listed thirty-six stars, twelve for Anu along the celestial equator, twelve for Ea south of that, and twelve for Enlil to the north. In Ancient Greece, the astronomer and mathematician Eudoxus laid down a set of the classical constellations around 370 BC. His catalogue Phaenomena, rewritten by Aratus of Soli between 275 and 250 BC as a poem, became one of the most consulted astronomical texts in antiquity. It contains descriptions of the positions of the stars, the shapes of the constellations, approximately in the 3rd century BC, the Greek astronomers Timocharis of Alexandria and Aristillus created another star catalogue. Hipparchus completed his star catalogue in 129 BC, which he compared to Timocharis and this led him to determine the first value of the precession of the equinoxes. In the 2nd century, Ptolemy of Roman Egypt published a star catalogue as part of his Almagest, ptolemys catalogue was based almost entirely on an earlier one by Hipparchus. It remained the star catalogue in the Western and Arab worlds for over eight centuries. The earliest known inscriptions for Chinese star names were written on oracle bones, sources dating from the Zhou Dynasty which provide star names include the Zuo Zhuan, the Shi Jing, and the Canon of Yao in the Book of Documents. The Lüshi Chunqiu written by the Qin statesman Lü Buwei provides most of the names for the twenty-eight mansions, an earlier lacquerware chest found in the Tomb of Marquis Yi of Zeng contains a complete list of the names of the twenty-eight mansions. Star catalogues are traditionally attributed to Shi Shen and Gan De, the Shi Shen astronomy is attributed to Shi Shen, and the Astronomic star observation to Gan De. It was not until the Han Dynasty that astronomers started to observe and record names for all the stars that were apparent in the night sky, not just those around the ecliptic. A star catalogue is featured in one of the chapters of the late 2nd-century-BC history work Records of the Grand Historian by Sima Qian and contains the schools of Shi Shen and Gan Des work. For his Spiritual Constitution of the Universe of 120 AD, the astronomer Zhang Heng compiled a star catalogue comprising 124 constellations, Chinese constellation names were later adopted by the Koreans and Japanese. A large number of star catalogues were published by Muslim astronomers in the medieval Islamic world and these were mainly Zij treatises, including Arzachels Tables of Toledo, the Maragheh observatorys Zij-i Ilkhani and Ulugh Begs Zij-i-Sultani
10.
Hipparcos
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Hipparcos was a scientific satellite of the European Space Agency, launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the measurement of the positions of celestial objects on the sky. This permitted the determination of proper motions and parallaxes of stars, allowing a determination of their distance. When combined with radial velocity measurements from spectroscopy, this pinpointed all six quantities needed to determine the motion of stars, the resulting Hipparcos Catalogue, a high-precision catalogue of more than 118,200 stars, was published in 1997. The lower-precision Tycho Catalogue of more than a million stars was published at the same time, Hipparcos follow-up mission, Gaia, was launched in 2013. Problems were dominated by the effects of the Earths atmosphere, but were compounded by complex optical terms, thermal and gravitational instrument flexures, a formal proposal to make these exacting observations from space was first put forward in 1967. Although originally proposed to the French space agency CNES, it was considered too complex and its acceptance within the European Space Agencys scientific programme, in 1980, was the result of a lengthy process of study and lobbying. The spacecraft carried a single all-reflective, eccentric Schmidt telescope, with an aperture of 29 cm, a special beam-combining mirror superimposed two fields of view,58 degrees apart, into the common focal plane. This complex mirror consisted of two mirrors tilted in opposite directions, each occupying half of the entrance pupil. The telescope used a system of grids, at the surface, composed of 2688 alternate opaque and transparent bands. The apparent angle between two stars in the fields of view, modulo the grid period, was obtained from the phase difference of the two star pulse trains. An additional photomultiplier system viewed a beam splitter in the path and was used as a star mapper. Its purpose was to monitor and determine the attitude, and in the process. These measurements were made in two broad bands approximately corresponding to B and V in the UBV photometric system. The positions of these stars were to be determined to a precision of 0.03 arc-sec. The spacecraft spun around its Z-axis at the rate of 11.25 revolutions/day at an angle of 43° to the Sun, the Z-axis rotated about the sun-satellite line at 6.4 revolutions/year. The spacecraft consisted of two platforms and six panels, all made of aluminum honeycomb. The solar array consisted of three sections, generating around 300 W in total
11.
Smithsonian Astrophysical Observatory catalogue
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The Smithsonian Astrophysical Observatory Star Catalog is an astrometric star catalogue. It was published by the Smithsonian Astrophysical Observatory in 1966 and contains 258,997 stars, the catalogue was compiled from various previous astrometric catalogues, and contains only stars to about ninth magnitude for which accurate proper motions were known. Names in the SAO catalogue start with the letters SAO, followed by a number, the numbers are assigned following 18 ten-degree bands of declination, with stars sorted by right ascension within each band. SAO158687 is the star that was occulted by Uranus in March 1977, leading to the discovery of rings around Uranus
12.
SIMBAD
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SIMBAD is an astronomical database of objects beyond the Solar System. It is maintained by the Centre de données astronomiques de Strasbourg, the first on-line interactive version, known as Version 2, was made available in 1981. Version 3, developed in the C language and running on UNIX stations at the Strasbourg Observatory, was released in 1990, fall of 2006 saw the release of Version 4 of the database, now stored in PostgreSQL, and the supporting software, now written entirely in Java. As of 10 February 2017, SIMBAD contains information for 9,099,070 objects under 24,529,080 different names, the minor planet 4692 SIMBAD was named in its honour. Planetary Data System – NASAs database of information on SSSB, maintained by JPL, nASA/IPAC Extragalactic Database – a database of information on objects outside the Milky Way, also maintained by JPL. NASA Exoplanet Archive – an online astronomical exoplanet catalog and data service Bibcode SIMBAD, Strasbourg SIMBAD, Harvard
13.
Star
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A star is a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun, many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the stars in the Universe, including all stars outside our galaxy, indeed, most are invisible from Earth even through the most powerful telescopes. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the stars lifetime, near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity, and many properties of a star by observing its motion through space, its luminosity. The total mass of a star is the factor that determines its evolution. Other characteristics of a star, including diameter and temperature, change over its life, while the environment affects its rotation. A plot of the temperature of stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram. Plotting a particular star on that allows the age and evolutionary state of that star to be determined. A stars life begins with the collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, the remainder of the stars interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The stars internal pressure prevents it from collapsing further under its own gravity, a star with mass greater than 0.4 times the Suns will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements at the core or in shells around the core, as the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars. Meanwhile, the core becomes a remnant, a white dwarf. Binary and multi-star systems consist of two or more stars that are bound and generally move around each other in stable orbits. When two such stars have a close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, historically, stars have been important to civilizations throughout the world
14.
Subgiant
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A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as true giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution of a star, the term subgiant was first used in 1930 for class G and early K stars with absolute magnitudes between +2.5 and +4. Luminosity-class-IV stars are the subgiants, located between main-sequence stars and red giants, O class stars and stars cooler than K1 are rarely given subgiant luminosity classes. The subgiant branch is a stage in the evolution of low to intermediate mass stars, stars with a subgiant spectral type are not always on the evolutionary subgiant branch, and vice versa. For example, the stars FK Com and 31 Com both lie in the Hertzsprung Gap and are likely evolutionary subgiants, but both are often assigned giant luminosity classes, the spectral classification can be influenced by metallicity, rotation, unusual chemical peculiarities, etc. The initial stages of the subgiant branch in a star like the sun are prolonged with little indication of the internal changes. One approach to identifying evolutionary subgiants include chemical abundances such as Lithium which is diluted in subgiants, as the fraction of hydrogen remaining in the core of a main sequence star decreases, the core temperature increases and so the rate of fusion increases. This causes stars to evolve slowly to high luminosities as they age, once a main sequence star ceases to fuse hydrogen in its core, the core begins to collapse under its own weight. This causes it to increase in temperature and hydrogen fuses in a shell outside the core, low- and intermediate-mass stars expand and cool until at about 5,000 K they begin to increase in luminosity in a stage known as the red-giant branch. The transition from the sequence to the red giant branch is known as the subgiant branch. The shape and duration of the subgiant branch varies for stars of different masses, stars less massive than about 0.4 M☉ are convective throughout most of the star. These stars continue to fuse hydrogen in their cores until essentially the entire star has converted to helium. Stars of this mass have main-sequence lifetimes many times longer than the current age of the Universe, stars less massive than the Sun have non-convective cores with a strong temperature gradient from the centre outwards. When they exhaust hydrogen at the centre of the star, a shell of hydrogen outside the central core continues to fuse without interruption. The star is considered to be a subgiant at this point although there is little change visible from the exterior, the helium core mass is below the Schönberg–Chandrasekhar limit and it remains in thermal equilibrium with the fusing hydrogen shell. Its mass continues to increase and the star very slowly expands as the hydrogen shell migrates outwards, any increase in energy output from the shell goes into expanding the envelop of the star and the luminosity stays approximately constant. The subgiant branch for stars is short, horizontal, and heavily populated. After several billion years, the core becomes too massive to support its own weight
15.
Sun
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The Sun is the star at the center of the Solar System. It is a perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 109 times that of Earth, and its mass is about 330,000 times that of Earth, accounting for about 99. 86% of the total mass of the Solar System. About three quarters of the Suns mass consists of hydrogen, the rest is mostly helium, with smaller quantities of heavier elements, including oxygen, carbon, neon. The Sun is a G-type main-sequence star based on its spectral class and it formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into a disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core and it is thought that almost all stars form by this process. The Sun is roughly middle-aged, it has not changed dramatically for more than four billion years and it is calculated that the Sun will become sufficiently large enough to engulf the current orbits of Mercury, Venus, and probably Earth. The enormous effect of the Sun on Earth has been recognized since prehistoric times, the synodic rotation of Earth and its orbit around the Sun are the basis of the solar calendar, which is the predominant calendar in use today. The English proper name Sun developed from Old English sunne and may be related to south, all Germanic terms for the Sun stem from Proto-Germanic *sunnōn. The English weekday name Sunday stems from Old English and is ultimately a result of a Germanic interpretation of Latin dies solis, the Latin name for the Sun, Sol, is not common in general English language use, the adjectival form is the related word solar. The term sol is used by planetary astronomers to refer to the duration of a solar day on another planet. A mean Earth solar day is approximately 24 hours, whereas a mean Martian sol is 24 hours,39 minutes, and 35.244 seconds. From at least the 4th Dynasty of Ancient Egypt, the Sun was worshipped as the god Ra, portrayed as a falcon-headed divinity surmounted by the solar disk, and surrounded by a serpent. In the New Empire period, the Sun became identified with the dung beetle, in the form of the Sun disc Aten, the Sun had a brief resurgence during the Amarna Period when it again became the preeminent, if not only, divinity for the Pharaoh Akhenaton. The Sun is viewed as a goddess in Germanic paganism, Sól/Sunna, in ancient Roman culture, Sunday was the day of the Sun god. It was adopted as the Sabbath day by Christians who did not have a Jewish background, the symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions
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Naked eye
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Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered naked, in astronomy, the naked eye may be used to observe events that can be viewed without equipment, such as an astronomical conjunction, the passage of a comet, or a meteor shower. Sky lore and various tests demonstrate a wealth of phenomena that can be seen with the unaided eye. The basic accuracies of the eye are, Quick autofocus from distances of 25 cm to 50 cm to infinity. Angular resolution, about 1 arcminute, approximately 0. 02° or 0.0003 radians, field of view, simultaneous visual perception in an area of about 160° × 175°. Faint stars up to +8 magnitude under a dark sky. Photometry to ±10% or 1% of intensity – in a range between night and day of 1,10,000,000,000, symmetries of 10–20, see the measurements of Tycho Brahe and the Egyptians. Even a few hundred kilometers away from an area where the sky can appear to be very dark. For most people, these are likely to be the best observing conditions within their reach, under such typical dark sky conditions, the naked eye can see stars with an apparent magnitude up to +6m. Under perfect dark sky conditions where all light pollution is absent, the angular resolution of the naked eye is about 1′, however, some people have sharper vision than that. There is anecdotal evidence that people had seen the Galilean moons of Jupiter before telescopes were invented, Uranus, when discovered in 1781, was the first planet discovered using technology rather than being spotted by the naked eye. In practice, the extinction and dust reduces this number somewhat. In the center of a city, where the limiting magnitude due to extreme amounts of light pollution can be +4m or less. Colors can be seen but this is limited by the fact that the eye uses rods instead of cones to view fainter stars, the visibility of diffuse objects such as star clusters and galaxies is much more strongly affected by light pollution than is that of planets and stars. Under typical dark conditions only a few objects are visible. The Triangulum Galaxy is a difficult averted vision object and only visible at all if it is higher than 50° in the sky, the globular clusters M3 in Canes Venatici and M92 in Hercules are also visible with the naked eye under such conditions. Under really dark sky conditions, however, M33 is easy to see, many other Messier objects are also visible under such conditions. The most distant objects that have seen by the naked eye are nearby bright galaxies such as Centaurus A, Bodes Galaxy, Sculptor Galaxy
17.
Binoculars
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Binoculars or field glasses are two telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes when viewing distant objects. Most are sized to be held using both hands, although sizes vary widely from opera glasses to large pedestal mounted military models. Almost from the invention of the telescope in the 17th century the advantages of mounting two of them side by side for binocular vision seems to have been explored, most early binoculars used Galilean optics, that is, they used a convex objective and a concave eyepiece lens. The Galilean design has the advantage of presenting an image but has a narrow field of view and is not capable of very high magnification. This type of construction is used in very cheap models. They also have large exit pupils making centering less critical and the field of view works well in those applications. These are typically mounted on a frame or custom-fit onto eyeglasses. An improved image and higher magnification is achieved in binoculars employing Keplerian optics, since the Keplerian configuration produces an inverted image, different methods were used to turn the image right way up. In aprismatic binoculars with Keplerian optics each tube has one or two additional lenses between the objective and the ocular and these lenses are used to erect the image. The binoculars with erecting lenses have a disadvantage, their length is too long. Such binoculars were popular in the 1800s, but became obsolete shortly after the Karl Zeiss company introduced improved prism binoculars in the 1890s, Optical prisms added to the design are another way to turn the image right way up, usually in a Porro prism or roof-prisms design. Porro prism binoculars are named after Italian optician Ignazio Porro who patented this image erecting system in 1854, Binoculars of this type use a Porro prism in a double prism Z-shaped configuration to erect the image. This feature results in binoculars that are wide, with lenses that are well separated. Thus, the size of binoculars is reduced, Binoculars using roof prisms may have appeared as early as the 1870s in a design by Achille Victor Emile Daubresse. Most roof prism binoculars use either the Abbe-Koenig prism or the Schmidt-Pechan prism designs to erect the image and they have objective lenses that are approximately in line with the eyepieces. Roof-prisms designs create an instrument that is narrower and more compact than Porro prisms, there is also a difference in image brightness. Roof-prisms designs also require tighter tolerances for alignment of their optical elements and this adds to their expense since the design requires them to use fixed elements that need to be set at a high degree of collimation at the factory. Porro prisms binoculars occasionally need their prism sets to be re-aligned to bring them into collimation, the fixed alignment in roof-prism designs means the binoculars normally will not need re-collimation
18.
Exoplanet
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An exoplanet or extrasolar planet is a planet that orbits a star other than the Sun. The first scientific detection of an exoplanet was in 1988, HARPS has discovered about a hundred exoplanets while the Kepler space telescope has found more than two thousand. Kepler has also detected a few thousand candidate planets, of which about 11% may be false positives, on average, there is at least one planet per star, with a percentage having multiple planets. About 1 in 5 Sun-like stars have an Earth-sized planet in the habitable zone, the least massive planet known is Draugr, which is about twice the mass of the Moon. There are planets that are so near to their star that they take only a few hours to orbit, some are so far out that it is difficult to tell whether they are gravitationally bound to the star. Almost all of the planets detected so far are within the Milky Way, the nearest exoplanet is Proxima Centauri b, located 4.2 light-years from Earth and orbiting Proxima Centauri, the closest star to the Sun. The discovery of exoplanets has intensified interest in the search for extraterrestrial life, there is special interest in planets that orbit in a stars habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface. The study of planetary habitability also considers a range of other factors in determining the suitability of a planet for hosting life. The rogue planets in the Milky Way possibly number in the billions, the convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union. For exoplanets orbiting a star, the designation is normally formed by taking the name or, more commonly, designation of its parent star. The first planet discovered in a system is given the designation b, if several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size. A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets, a limited number of exoplanets have IAU-sanctioned proper names. Various detection claims made in the century were rejected by astronomers. The first scientific detection of an exoplanet began in 1988, However, the first confirmed detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmation of an exoplanet orbiting a star was made in 1995. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method, in the eighteenth century the same possibility was mentioned by Isaac Newton in the General Scholium that concludes his Principia. Claims of exoplanet detections have been made since the nineteenth century, some of the earliest involve the binary star 70 Ophiuchi. In 1855 William Stephen Jacob at the East India Companys Madras Observatory reported that orbital anomalies made it highly probable that there was a body in this system
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Orbital resonance
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Orbital resonances greatly enhance the mutual gravitational influence of the bodies, i. e. their ability to alter or constrain each others orbits. In most cases, this results in an interaction, in which the bodies exchange momentum. Under some circumstances, a resonant system can be stable and self-correcting, examples are the 1,2,4 resonance of Jupiters moons Ganymede, Europa and Io, and the 2,3 resonance between Pluto and Neptune. Unstable resonances with Saturns inner moons give rise to gaps in the rings of Saturn, thus the 2,3 ratio above means Pluto completes two orbits in the time it takes Neptune to complete three. In the case of resonance relationships between three or more bodies, either type of ratio may be used and the type of ratio will be specified. Since the discovery of Newtons law of gravitation in the 17th century. The stable orbits that arise in a two-body approximation ignore the influence of other bodies and it was Laplace who found the first answers explaining the remarkable dance of the Galilean moons. It is fair to say that this field of study has remained very active since then. Before Newton, there was consideration of ratios and proportions in orbital motions, in what was called the music of the spheres. In general, a resonance may involve one or any combination of the orbit parameters. Act on any scale from short term, commensurable with the orbit periods, to secular. Lead to either long-term stabilization of the orbits or be the cause of their destabilization, a mean-motion orbital resonance occurs when two bodies have periods of revolution that are a simple integer ratio of each other. Depending on the details, this can either stabilize or destabilize the orbit, stabilization may occur when the two bodies move in such a synchronised fashion that they never closely approach. For instance, The orbits of Pluto and the plutinos are stable, despite crossing that of the much larger Neptune, the resonance ensures that, when they approach perihelion and Neptunes orbit, Neptune is consistently distant. Other Neptune-crossing bodies that were not in resonance were ejected from that region by strong perturbations due to Neptune. There are also smaller but significant groups of resonant trans-Neptunian objects occupying the 1,1,3,5,4,7,1,2 and 2,5 resonances, among others, with respect to Neptune. In the asteroid belt beyond 3.5 AU from the Sun, orbital resonances can also destabilize one of the orbits. For small bodies, destabilization is actually far more likely, for instance, In the asteroid belt within 3.5 AU from the Sun, the major mean-motion resonances with Jupiter are locations of gaps in the asteroid distribution, the Kirkwood gaps
20.
Gravity
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Gravity, or gravitation, is a natural phenomenon by which all things with mass are brought toward one another, including planets, stars and galaxies. Since energy and mass are equivalent, all forms of energy, including light, on Earth, gravity gives weight to physical objects and causes the ocean tides. Gravity has a range, although its effects become increasingly weaker on farther objects. The most extreme example of this curvature of spacetime is a hole, from which nothing can escape once past its event horizon. More gravity results in time dilation, where time lapses more slowly at a lower gravitational potential. Gravity is the weakest of the four fundamental interactions of nature, the gravitational attraction is approximately 1038 times weaker than the strong force,1036 times weaker than the electromagnetic force and 1029 times weaker than the weak force. As a consequence, gravity has an influence on the behavior of subatomic particles. On the other hand, gravity is the dominant interaction at the macroscopic scale, for this reason, in part, pursuit of a theory of everything, the merging of the general theory of relativity and quantum mechanics into quantum gravity, has become an area of research. While the modern European thinkers are credited with development of gravitational theory, some of the earliest descriptions came from early mathematician-astronomers, such as Aryabhata, who had identified the force of gravity to explain why objects do not fall out when the Earth rotates. Later, the works of Brahmagupta referred to the presence of force, described it as an attractive force. Modern work on gravitational theory began with the work of Galileo Galilei in the late 16th and this was a major departure from Aristotles belief that heavier objects have a higher gravitational acceleration. Galileo postulated air resistance as the reason that objects with less mass may fall slower in an atmosphere, galileos work set the stage for the formulation of Newtons theory of gravity. In 1687, English mathematician Sir Isaac Newton published Principia, which hypothesizes the inverse-square law of universal gravitation. Newtons theory enjoyed its greatest success when it was used to predict the existence of Neptune based on motions of Uranus that could not be accounted for by the actions of the other planets. Calculations by both John Couch Adams and Urbain Le Verrier predicted the position of the planet. A discrepancy in Mercurys orbit pointed out flaws in Newtons theory, the issue was resolved in 1915 by Albert Einsteins new theory of general relativity, which accounted for the small discrepancy in Mercurys orbit. The simplest way to test the equivalence principle is to drop two objects of different masses or compositions in a vacuum and see whether they hit the ground at the same time. Such experiments demonstrate that all objects fall at the rate when other forces are negligible
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Mass
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In physics, mass is a property of a physical body. It is the measure of a resistance to acceleration when a net force is applied. It also determines the strength of its gravitational attraction to other bodies. The basic SI unit of mass is the kilogram, Mass is not the same as weight, even though mass is often determined by measuring the objects weight using a spring scale, rather than comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity and this is because weight is a force, while mass is the property that determines the strength of this force. In Newtonian physics, mass can be generalized as the amount of matter in an object, however, at very high speeds, special relativity postulates that energy is an additional source of mass. Thus, any body having mass has an equivalent amount of energy. In addition, matter is a defined term in science. There are several distinct phenomena which can be used to measure mass, active gravitational mass measures the gravitational force exerted by an object. Passive gravitational mass measures the force exerted on an object in a known gravitational field. The mass of an object determines its acceleration in the presence of an applied force, according to Newtons second law of motion, if a body of fixed mass m is subjected to a single force F, its acceleration a is given by F/m. A bodys mass also determines the degree to which it generates or is affected by a gravitational field and this is sometimes referred to as gravitational mass. The standard International System of Units unit of mass is the kilogram, the kilogram is 1000 grams, first defined in 1795 as one cubic decimeter of water at the melting point of ice. Then in 1889, the kilogram was redefined as the mass of the prototype kilogram. As of January 2013, there are proposals for redefining the kilogram yet again. In this context, the mass has units of eV/c2, the electronvolt and its multiples, such as the MeV, are commonly used in particle physics. The atomic mass unit is 1/12 of the mass of a carbon-12 atom, the atomic mass unit is convenient for expressing the masses of atoms and molecules. Outside the SI system, other units of mass include, the slug is an Imperial unit of mass, the pound is a unit of both mass and force, used mainly in the United States
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Semi-major and semi-minor axes
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In geometry, the major axis of an ellipse is its longest diameter, a line segment that runs through the center and both foci, with ends at the widest points of the perimeter. The semi-major axis is one half of the axis, and thus runs from the centre, through a focus. Essentially, it is the radius of an orbit at the two most distant points. For the special case of a circle, the axis is the radius. One can think of the axis as an ellipses long radius. The semi-major axis of a hyperbola is, depending on the convention, thus it is the distance from the center to either vertex of the hyperbola. A parabola can be obtained as the limit of a sequence of ellipses where one focus is fixed as the other is allowed to move arbitrarily far away in one direction. Thus a and b tend to infinity, a faster than b, the semi-minor axis is a line segment associated with most conic sections that is at right angles with the semi-major axis and has one end at the center of the conic section. It is one of the axes of symmetry for the curve, in an ellipse, the one, in a hyperbola. The semi-major axis is the value of the maximum and minimum distances r max and r min of the ellipse from a focus — that is. In astronomy these extreme points are called apsis, the semi-minor axis of an ellipse is the geometric mean of these distances, b = r max r min. The eccentricity of an ellipse is defined as e =1 − b 2 a 2 so r min = a, r max = a. Now consider the equation in polar coordinates, with one focus at the origin, the mean value of r = ℓ / and r = ℓ /, for θ = π and θ =0 is a = ℓ1 − e 2. In an ellipse, the axis is the geometric mean of the distance from the center to either focus. The semi-minor axis of an ellipse runs from the center of the ellipse to the edge of the ellipse, the semi-minor axis is half of the minor axis. The minor axis is the longest line segment perpendicular to the axis that connects two points on the ellipses edge. The semi-minor axis b is related to the axis a through the eccentricity e. A parabola can be obtained as the limit of a sequence of ellipses where one focus is fixed as the other is allowed to move arbitrarily far away in one direction
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Astronomical unit
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The astronomical unit is a unit of length, roughly the distance from Earth to the Sun. However, that varies as Earth orbits the Sun, from a maximum to a minimum. Originally conceived as the average of Earths aphelion and perihelion, it is now defined as exactly 149597870700 metres, the astronomical unit is used primarily as a convenient yardstick for measuring distances within the Solar System or around other stars. However, it is also a component in the definition of another unit of astronomical length. A variety of symbols and abbreviations have been in use for the astronomical unit. In a 1976 resolution, the International Astronomical Union used the symbol A for the astronomical unit, in 2006, the International Bureau of Weights and Measures recommended ua as the symbol for the unit. In 2012, the IAU, noting that various symbols are presently in use for the astronomical unit, in the 2014 revision of the SI Brochure, the BIPM used the unit symbol au. In ISO 80000-3, the symbol of the unit is ua. Earths orbit around the Sun is an ellipse, the semi-major axis of this ellipse is defined to be half of the straight line segment that joins the aphelion and perihelion. The centre of the sun lies on this line segment. In addition, it mapped out exactly the largest straight-line distance that Earth traverses over the course of a year, knowing Earths shift and a stars shift enabled the stars distance to be calculated. But all measurements are subject to some degree of error or uncertainty, improvements in precision have always been a key to improving astronomical understanding. Improving measurements were continually checked and cross-checked by means of our understanding of the laws of celestial mechanics, the expected positions and distances of objects at an established time are calculated from these laws, and assembled into a collection of data called an ephemeris. NASAs Jet Propulsion Laboratory provides one of several ephemeris computation services, in 1976, in order to establish a yet more precise measure for the astronomical unit, the IAU formally adopted a new definition. Equivalently, by definition, one AU is the radius of an unperturbed circular Newtonian orbit about the sun of a particle having infinitesimal mass. As with all measurements, these rely on measuring the time taken for photons to be reflected from an object. However, for precision the calculations require adjustment for such as the motions of the probe. In addition, the measurement of the time itself must be translated to a scale that accounts for relativistic time dilation
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Day
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In common usage, it is either an interval equal to 24 hours or daytime, the consecutive period of time during which the Sun is above the horizon. The period of time during which the Earth completes one rotation with respect to the Sun is called a solar day, several definitions of this universal human concept are used according to context, need and convenience. In 1960, the second was redefined in terms of the motion of the Earth. The unit of measurement day, redefined in 1960 as 86400 SI seconds and symbolized d, is not an SI unit, but is accepted for use with SI. The word day may also refer to a day of the week or to a date, as in answer to the question. The life patterns of humans and many species are related to Earths solar day. In recent decades the average length of a day on Earth has been about 86400.002 seconds. A day, understood as the span of time it takes for the Earth to make one rotation with respect to the celestial background or a distant star, is called a stellar day. This period of rotation is about 4 minutes less than 24 hours, mainly due to tidal effects, the Earths rotational period is not constant, resulting in further minor variations for both solar days and stellar days. Other planets and moons have stellar and solar days of different lengths to Earths, besides the day of 24 hours, the word day is used for several different spans of time based on the rotation of the Earth around its axis. An important one is the day, defined as the time it takes for the Sun to return to its culmination point. Because the Earth orbits the Sun elliptically as the Earth spins on an inclined axis, on average over the year this day is equivalent to 24 hours. A day, in the sense of daytime that is distinguished from night-time, is defined as the period during which sunlight directly reaches the ground. The length of daytime averages slightly more than half of the 24-hour day, two effects make daytime on average longer than nights. The Sun is not a point, but has an apparent size of about 32 minutes of arc, additionally, the atmosphere refracts sunlight in such a way that some of it reaches the ground even when the Sun is below the horizon by about 34 minutes of arc. So the first light reaches the ground when the centre of the Sun is still below the horizon by about 50 minutes of arc, the difference in time depends on the angle at which the Sun rises and sets, but can amount to around seven minutes. Ancient custom has a new day start at either the rising or setting of the Sun on the local horizon, the exact moment of, and the interval between, two sunrises or sunsets depends on the geographical position, and the time of year. A more constant day can be defined by the Sun passing through the local meridian, the exact moment is dependent on the geographical longitude, and to a lesser extent on the time of the year
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Orbital eccentricity
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The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is an orbit, values between 0 and 1 form an elliptical orbit,1 is a parabolic escape orbit. The term derives its name from the parameters of conic sections and it is normally used for the isolated two-body problem, but extensions exist for objects following a rosette orbit through the galaxy. In a two-body problem with inverse-square-law force, every orbit is a Kepler orbit, the eccentricity of this Kepler orbit is a non-negative number that defines its shape. The limit case between an ellipse and a hyperbola, when e equals 1, is parabola, radial trajectories are classified as elliptic, parabolic, or hyperbolic based on the energy of the orbit, not the eccentricity. Radial orbits have zero angular momentum and hence eccentricity equal to one, keeping the energy constant and reducing the angular momentum, elliptic, parabolic, and hyperbolic orbits each tend to the corresponding type of radial trajectory while e tends to 1. For a repulsive force only the trajectory, including the radial version, is applicable. For elliptical orbits, a simple proof shows that arcsin yields the projection angle of a circle to an ellipse of eccentricity e. For example, to view the eccentricity of the planet Mercury, next, tilt any circular object by that angle and the apparent ellipse projected to your eye will be of that same eccentricity. From Medieval Latin eccentricus, derived from Greek ἔκκεντρος ekkentros out of the center, from ἐκ- ek-, eccentric first appeared in English in 1551, with the definition a circle in which the earth, sun. Five years later, in 1556, a form of the word was added. The eccentricity of an orbit can be calculated from the state vectors as the magnitude of the eccentricity vector, e = | e | where. For elliptical orbits it can also be calculated from the periapsis and apoapsis since rp = a and ra = a, where a is the semimajor axis. E = r a − r p r a + r p =1 −2 r a r p +1 where, rp is the radius at periapsis. For Earths annual orbit path, ra/rp ratio = longest_radius / shortest_radius ≈1.034 relative to center point of path, the eccentricity of the Earths orbit is currently about 0.0167, the Earths orbit is nearly circular. Venus and Neptune have even lower eccentricity, over hundreds of thousands of years, the eccentricity of the Earths orbit varies from nearly 0.0034 to almost 0.058 as a result of gravitational attractions among the planets. The table lists the values for all planets and dwarf planets, Mercury has the greatest orbital eccentricity of any planet in the Solar System. Such eccentricity is sufficient for Mercury to receive twice as much solar irradiation at perihelion compared to aphelion, before its demotion from planet status in 2006, Pluto was considered to be the planet with the most eccentric orbit
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Orbital inclination
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Orbital inclination measures the tilt of an objects orbit around a celestial body. It is expressed as the angle between a plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Earth directly above the equator, the plane of the orbit is the same as the Earths equatorial plane. The general case is that the orbit is tilted, it spends half an orbit over the northern hemisphere. If the orbit swung between 20° north latitude and 20° south latitude, then its orbital inclination would be 20°, the inclination is one of the six orbital elements describing the shape and orientation of a celestial orbit. It is the angle between the plane and the plane of reference, normally stated in degrees. For a satellite orbiting a planet, the plane of reference is usually the plane containing the planets equator, for planets in the Solar System, the plane of reference is usually the ecliptic, the plane in which the Earth orbits the Sun. This reference plane is most practical for Earth-based observers, therefore, Earths inclination is, by definition, zero. Inclination could instead be measured with respect to another plane, such as the Suns equator or the invariable plane, the inclination of orbits of natural or artificial satellites is measured relative to the equatorial plane of the body they orbit, if they orbit sufficiently closely. The equatorial plane is the perpendicular to the axis of rotation of the central body. An inclination of 30° could also be described using an angle of 150°, the convention is that the normal orbit is prograde, an orbit in the same direction as the planet rotates. Inclinations greater than 90° describe retrograde orbits, thus, An inclination of 0° means the orbiting body has a prograde orbit in the planets equatorial plane. An inclination greater than 0° and less than 90° also describe prograde orbits, an inclination of 63. 4° is often called a critical inclination, when describing artificial satellites orbiting the Earth, because they have zero apogee drift. An inclination of exactly 90° is an orbit, in which the spacecraft passes over the north and south poles of the planet. An inclination greater than 90° and less than 180° is a retrograde orbit, an inclination of exactly 180° is a retrograde equatorial orbit. For gas giants, the orbits of moons tend to be aligned with the giant planets equator, the inclination of exoplanets or members of multiple stars is the angle of the plane of the orbit relative to the plane perpendicular to the line-of-sight from Earth to the object. An inclination of 0° is an orbit, meaning the plane of its orbit is parallel to the sky. An inclination of 90° is an orbit, meaning the plane of its orbit is perpendicular to the sky
27.
Jupiter mass
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Jupiter mass is the unit of mass equal to the total mass of the planet Jupiter. Jupiter mass is used to describe masses of the gas giants, such as the outer planets and it is also used in describing brown dwarfs. The most massive exoplanets are typically described in terms of Jupiter masses as this provides a convenient scale for comparison, a Jupiter-mass planet at an orbital distance of 1 AU from a Sun-like star causes an amplitude shift of 28 m/s, which is detectable with current technology. The most readily detectable planets through radial velocity measurements have high mass and this produces a selection effect for planets of Jupiter mass. Likewise, Jupiter mass or higher planets are likely to be detected through other means. A planet with a Jupiter mass might not have the same dimensions, in the Solar System, the masses of the outer planets can be listed in Jupiter mass. The other gas giants are far less massive than Jupiter
28.
Jupiter radius
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Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a giant planet with a mass one-thousandth that of the Sun, Jupiter and Saturn are gas giants, the other two giant planets, Uranus and Neptune are ice giants. Jupiter has been known to astronomers since antiquity, the Romans named it after their god Jupiter. Jupiter is primarily composed of hydrogen with a quarter of its mass being helium and it may also have a rocky core of heavier elements, but like the other giant planets, Jupiter lacks a well-defined solid surface. Because of its rotation, the planets shape is that of an oblate spheroid. The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence, a prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope. Surrounding Jupiter is a faint planetary ring system and a powerful magnetosphere, Jupiter has at least 67 moons, including the four large Galilean moons discovered by Galileo Galilei in 1610. Ganymede, the largest of these, has a greater than that of the planet Mercury. Jupiter has been explored on several occasions by robotic spacecraft, most notably during the early Pioneer and Voyager flyby missions and later by the Galileo orbiter. In late February 2007, Jupiter was visited by the New Horizons probe, the latest probe to visit the planet is Juno, which entered into orbit around Jupiter on July 4,2016. Future targets for exploration in the Jupiter system include the probable ice-covered liquid ocean of its moon Europa, Earth and its neighbor planets may have formed from fragments of planets after collisions with Jupiter destroyed those super-Earths near the Sun. Astronomers have discovered nearly 500 planetary systems with multiple planets, Jupiter moving out of the inner Solar System would have allowed the formation of inner planets, including Earth. Jupiter is composed primarily of gaseous and liquid matter and it is the largest of the four giant planets in the Solar System and hence its largest planet. It has a diameter of 142,984 km at its equator, the average density of Jupiter,1.326 g/cm3, is the second highest of the giant planets, but lower than those of the four terrestrial planets. Jupiters upper atmosphere is about 88–92% hydrogen and 8–12% helium by percent volume of gas molecules, a helium atom has about four times as much mass as a hydrogen atom, so the composition changes when described as the proportion of mass contributed by different atoms. Thus, Jupiters atmosphere is approximately 75% hydrogen and 24% helium by mass, the atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds. There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, the outermost layer of the atmosphere contains crystals of frozen ammonia. The interior contains denser materials - by mass it is roughly 71% hydrogen, 24% helium, through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found
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Astronomy (magazine)
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Astronomy is a monthly American magazine about astronomy. Targeting amateur astronomers for its readers, it contains columns on sky viewing, reader-submitted astrophotographs, Astronomy is a magazine about the science and hobby of astronomy. Based near Milwaukee in Waukesha, Wisconsin, it is produced by Kalmbach Publishing, astronomy’s readers include those interested in astronomy, and those who want to know about sky events, observing techniques, astrophotography, and amateur astronomy in general. Astronomy was founded in 1973 by Stephen A. Walther, a graduate of the University of Wisconsin–Stevens Point, the first issue, August 1973, consisted of 48 pages with five feature articles and information about what to see in the sky that month. Issues contained astrophotos and illustrations created by astronomical artists, Walther had worked part time as a planetarium lecturer at the University of Wisconsin–Milwaukee and developed an interest in photographing constellations at an early age. Although even in childhood he was interested to obsession in Astronomy, however he graduated in Journalism from the University of Wisconsin Stevens Point, and as a senior class project he created a business plan for a magazine for amateur astronomers. With the help of his brother David, he was able to bring the magazine to fruition, astroMedia Corp. the company Walther had founded to publish Astronomy, brought in Richard Berry as editor. Berry also created the offshoot Odyssey, aimed at young readers, in 1985, Milwaukee hobby publisher Kalmbach bought Astronomy. In 1992, Richard Berry left the magazine and Robert Burnham took over as chief editor, Kalmbach discontinued Deep Sky and Telescope Making magazines and sold Odyssey. In 1996 Bonnie Gordon, now a professor at Central Arizona College, David J. Eicher, the creator of Deep Sky, became chief editor in 2002. The Astronomy staff also produces other publications, there also was, for a time in the mid-2000s, a Brazilian edition – published by Duetto Editora – called Astronomy Brasil. However, due mainly to low numbers, Duetto ceased its publication in September 2007. Astronomy publishes articles about the hobby and science of astronomy, generally, the front half of the magazine reports on professional science, while the back half of the magazine presents items of interest to hobbyists. Science articles cover topics as cosmology, space exploration, exobiology, research conducted by professional-class observatories. Each issue of Astronomy contains a star map showing the evening sky for the current month and the positions of planets. They include science writer Bob Berman, who writes a column called “Bob Berman’s Strange Universe”, Stephen James O’Meara writes “Stephen James O’Meara’s Secret Sky, ” which covers observing tips and stories relating to deep-sky objects, planets, and comets. Glenn Chaple writes Glenn Chaple’s Observing Basics, a beginner’s column, Phil Harrington writes Phil Harrington’s Binocular Universe, about observing with binoculars. Telescope Insider interviews people who are a part of the telescope-manufacturing industry, in each issue of Astronomy Magazine, readers will find star and planet charts, telescope observing tips and techniques, and advice on taking photography of the night sky
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Celestial coordinate system
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In astronomy, a celestial coordinate system is a system for specifying positions of celestial objects, satellites, planets, stars, galaxies, and so on. Coordinate systems can specify a position in 3-dimensional space, or merely the direction of the object on the celestial sphere, the coordinate systems are implemented in either spherical coordinates or rectangular coordinates. Spherical coordinates, projected on the sphere, are analogous to the geographic coordinate system used on the surface of the Earth. These differ in their choice of fundamental plane, which divides the sphere into two equal hemispheres along a great circle. Rectangular coordinates, in units, are simply the cartesian equivalent of the spherical coordinates, with the same fundamental plane. Each coordinate system is named after its choice of fundamental plane, the following table lists the common coordinate systems in use by the astronomical community. The fundamental plane divides the sphere into two equal hemispheres and defines the baseline for the latitudinal coordinates, similar to the equator in the geographic coordinate system. The poles are located at ±90° from the fundamental plane, the primary direction is the starting point of the longitudinal coordinates. The origin is the distance point, the center of the celestial sphere. The horizontal, or altitude-azimuth, system is based on the position of the observer on Earth, the positioning of a celestial object by the horizontal system varies with time, but is a useful coordinate system for locating and tracking objects for observers on Earth. It is based on the position of relative to an observers ideal horizon. The equatorial coordinate system is centered at Earths center, but fixed relative to the celestial poles, the coordinates are based on the location of stars relative to Earths equator if it were projected out to an infinite distance. The equatorial describes the sky as seen from the solar system, the equatorial system is the normal coordinate system for most professional and many amateur astronomers having an equatorial mount that follows the movement of the sky during the night. Celestial objects are found by adjusting the telescopes or other instruments scales so that they match the equatorial coordinates of the object to observe. There are also subdivisions into mean of date coordinates, which average out or ignore nutation, and true of date, the fundamental plane is the plane of the Earths orbit, called the ecliptic plane. The geocentric ecliptic system was the coordinate system for ancient astronomy and is still useful for computing the apparent motions of the Sun, Moon. The heliocentric ecliptic system describes the orbital movement around the Sun. The system is used for computing the positions of planets and other solar system bodies
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Bayer designation
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A Bayer designation is a stellar designation in which a specific star is identified by a Greek letter, followed by the genitive form of its parent constellations Latin name. The original list of Bayer designations contained 1,564 stars, most of the brighter stars were assigned their first systematic names by the German astronomer Johann Bayer in 1603, in his star atlas Uranometria. Bayer assigned a lower-case Greek letter, such as alpha, beta, gamma, for example, Aldebaran is designated α Tauri, which means Alpha of the constellation Taurus. A single constellation may contain fifty or more stars, but the Greek alphabet has only twenty-four letters, when these ran out, Bayer began using Latin letters, upper case A, followed by lower case b through z, for a total of another 24 letters. Bayer never went beyond z, but later added more designations using both upper and lower case Latin letters, the upper case letters following the lower case ones in general. Examples include s Carinae, d Centauri, G Scorpii, and N Velorum, the last upper-case letter used in this way was Q. Bayer catalogued only a few stars too far south to be seen from Germany, in most constellations, Bayer assigned Greek and Latin letters to stars within a constellation in rough order of apparent brightness, from brightest to dimmest. Since the brightest star in a majority of constellations is designated Alpha, in Bayers day, however, stellar brightness could not be measured precisely. Within each magnitude class, Bayer made no attempt to arrange stars by relative brightness, as a result, the brightest star in each class did not always get listed first in Bayers order. Occasionally the order looks quite arbitrary, of the 88 modern constellations, there are at least 30 in which Alpha is not the brightest star, and four of those lack an alpha star altogether. Orion provides an example of Bayers method. Bayer first designated Betelgeuse and Rigel, the two 1st-magnitude stars, as Alpha and Beta from north to south, with Betelgeuse coming ahead of Rigel, Bayer then repeated the procedure for the stars of the 2nd magnitude, labeling them from gamma through zeta in top-down order. The First to Rise in the East order is used in a number of instances, Castor and Pollux of Gemini may be an example of this, Pollux is brighter than Castor, but the latter rises earlier and was assigned alpha. In this case, Bayer may also have influenced by the traditional order of the mythological names Castor and Pollux. Although the brightest star in Draco is Eltanin, Thuban was assigned alpha by Bayer because, due to precession, sometimes there is no apparent order, as exemplified by the stars in Sagittarius, where Bayers designations appear almost random to the modern eye. Alpha and Beta Sagittarii are perhaps the most anomalously designated stars in the sky, the order of the letters assigned in Sagittarius does correspond to the magnitudes as illustrated on Bayers chart, but the latter do not agree with modern determinations of the magnitudes. Bayer designations added by later astronomers generally were ordered by magnitude, in Libra, for example, the new designations sigma, tau, and upsilon were chosen to avoid conflict with Bayers earlier designations, even though several stars with earlier letters are not as bright. In Cygnus, for example, Bayers fixed stars run through g, Bayer did not intend such labels as catalog designations, but some have survived to refer to astronomical objects, P Cygni for example is still used as a designation for Nova Cyg 1600
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Gamma Equulei
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Gamma Equulei is a double star in the northern constellation of Equuleus. It is located at a distance of around 118 light-years from Earth and has an apparent visual magnitude of 4.7, the primary component is a chemically peculiar star with a stellar classification of A9 Sr Eu and an apparent magnitude of 4.80. Gamma Equulei undergoes periodic pulsations in luminosity, which places it among the category of roAp stars, the surface magnetic field undergoes long term variation with a period of 91.1 ±3.6 years, ranging from +577 G to –1,101 G. It has a magnitude 9.05 companion at a separation of 1.26 arcseconds
