Cepheid variable
A Cepheid variable is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. A strong direct relationship between a Cepheid variable's luminosity and pulsation period established Cepheids as important indicators of cosmic benchmarks for scaling galactic and extragalactic distances; this robust characteristic of classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt after studying thousands of variable stars in the Magellanic Clouds. This discovery allows one to know the true luminosity of a Cepheid by observing its pulsation period; this in turn allows one to determine the distance to the star, by comparing its known luminosity to its observed brightness. The term Cepheid originates from Delta Cephei in the constellation Cepheus, identified by John Goodricke in 1784, the first of its type to be so identified. Cepheid variables are divided into two subclasses which exhibit markedly different masses and evolutionary histories: classical Cepheids and type II Cepheids.
Delta Scuti variables are A class stars on or near the main sequence at the lower end of the instability strip and were referred to as dwarf Cepheids. RR Lyrae variables have short periods and lie on the instability strip where it crosses the horizontal branch. Delta Scuti variables and RR Lyrae variables are not treated with Cepheid variables although their pulsations originate with the same helium ionisation kappa mechanism. Classical Cepheids undergo pulsations with regular periods on the order of days to months. Classical Cepheids are Population I variable stars which are 4–20 times more massive than the Sun, up to 100,000 times more luminous; these Cepheids are yellow bright giants and supergiants of spectral class F6 – K2 and their radii change by millions of kilometers during a pulsation cycle. Classical Cepheids are used to determine distances to galaxies within the Local Group and beyond, are a means by which the Hubble constant can be established. Classical Cepheids have been used to clarify many characteristics of our galaxy, such as the Sun's height above the galactic plane and the Galaxy's local spiral structure.
A group of classical Cepheids with small amplitudes and sinusoidal light curves are separated out as Small Amplitude Cepheids or s-Cepheids, many of them pulsating in the first overtone. Type II Cepheids are population II variable stars which pulsate with periods between 1 and 50 days. Type II Cepheids are metal-poor, low mass objects. Type II Cepheids are divided into several subgroups by period. Stars with periods between 1 and 4 days are of the BL Her subclass, 10–20 days belong to the W Virginis subclass, stars with periods greater than 20 days belong to the RV Tauri subclass. Type II Cepheids are used to establish the distance to the Galactic Center, globular clusters, galaxies. A group of pulsating stars on the instability strip have periods of less than 2 days, similar to RR Lyrae variables but with higher luminosities. Anomalous Cepheid variables have masses higher than type II Cepheids, RR Lyrae variables, our sun, it is unclear whether they are young stars on a "turned-back" horizontal branch, blue stragglers formed through mass transfer in binary systems, or a mix of both.
A small proportion of Cepheid variables have been observed to pulsate in two modes at the same time the fundamental and first overtone the second overtone. A small number pulsate in three modes, or an unusual combination of modes including higher overtones. On September 10, 1784, Edward Pigott detected the variability of Eta Aquilae, the first known representative of the class of classical Cepheid variables. However, the eponymous star for classical Cepheids is Delta Cephei, discovered to be variable by John Goodricke a few months later. A relationship between the period and luminosity for classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt in an investigation of thousands of variable stars in the Magellanic Clouds, she published it in 1912 with further evidence. In 1913, Ejnar Hertzsprung attempted to find distances to 13 Cepheids using the motion through the sky, his research would require revision, however. In 1915, Harlow Shapley used Cepheids to place initial constraints on the size and shape of the Milky Way, of the placement of our Sun within it.
In 1924, Edwin Hubble established the distance to classical Cepheid variables in the Andromeda Galaxy, until known as the Andromeda Nebula, showed that the variables were not members of the Milky Way. Hubble's finding settled the question raised in the "Great Debate" of whether the Milky Way represented the entire Universe or was one of numerous galaxies in the Universe. In 1929, Hubble and Milton L. Humason formulated what is now known as Hubble's Law by combining Cepheid distances to several galaxies with Vesto Slipher's measurements of the speed at which those galaxies recede from us, they discovered. However, the expansion of the Universe was posited several years before by Georges Lemaître. In the mid 20th century, significant problems with the astronomical distance scale were resolved by dividing the Cepheids into different classes with different properties. In the 1940s, Walter Baade recognized two separate populations of Cepheids. Classical Cepheids are younger and more massive population I stars, whereas type II Cepheids are older fainter Population II stars.
Classical Cepheids and type
Constellation
A constellation is a group of stars that forms an imaginary outline or pattern on the celestial sphere representing an animal, mythological person or creature, a god, or an inanimate object. The origins of the earliest constellations go back to prehistory. People used them to relate stories of their beliefs, creation, or mythology. Different cultures and countries adopted their own constellations, some of which lasted into the early 20th century before today's constellations were internationally recognized. Adoption of constellations has changed over time. Many have changed in shape; some became popular. Others were limited to single nations; the 48 traditional Western constellations are Greek. They are given in Aratus' work Phenomena and Ptolemy's Almagest, though their origin predates these works by several centuries. Constellations in the far southern sky were added from the 15th century until the mid-18th century when European explorers began traveling to the Southern Hemisphere. Twelve ancient constellations belong to the zodiac.
The origins of the zodiac remain uncertain. In 1928, the International Astronomical Union formally accepted 88 modern constellations, with contiguous boundaries that together cover the entire celestial sphere. Any given point in a celestial coordinate system lies in one of the modern constellations; some astronomical naming systems include the constellation where a given celestial object is found to convey its approximate location in the sky. The Flamsteed designation of a star, for example, consists of a number and the genitive form of the constellation name. Other star patterns or groups called asterisms are not constellations per se but are used by observers to navigate the night sky. Examples of bright asterisms include the Pleiades and Hyades within the constellation Taurus or Venus' Mirror in the constellation of Orion.. Some asterisms, like the False Cross, are split between two constellations; the word "constellation" comes from the Late Latin term cōnstellātiō, which can be translated as "set of stars".
The Ancient Greek word for constellation is ἄστρον. A more modern astronomical sense of the term "constellation" is as a recognisable pattern of stars whose appearance is associated with mythological characters or creatures, or earthbound animals, or objects, it can specifically denote the recognized 88 named constellations used today. Colloquial usage does not draw a sharp distinction between "constellations" and smaller "asterisms", yet the modern accepted astronomical constellations employ such a distinction. E.g. the Pleiades and the Hyades are both asterisms, each lies within the boundaries of the constellation of Taurus. Another example is the northern asterism known as the Big Dipper or the Plough, composed of the seven brightest stars within the area of the IAU-defined constellation of Ursa Major; the southern False Cross asterism includes portions of the constellations Carina and Vela and the Summer Triangle.. A constellation, viewed from a particular latitude on Earth, that never sets below the horizon is termed circumpolar.
From the North Pole or South Pole, all constellations south or north of the celestial equator are circumpolar. Depending on the definition, equatorial constellations may include those that lie between declinations 45° north and 45° south, or those that pass through the declination range of the ecliptic or zodiac ranging between 23½° north, the celestial equator, 23½° south. Although stars in constellations appear near each other in the sky, they lie at a variety of distances away from the Earth. Since stars have their own independent motions, all constellations will change over time. After tens to hundreds of thousands of years, familiar outlines will become unrecognizable. Astronomers can predict the past or future constellation outlines by measuring individual stars' common proper motions or cpm by accurate astrometry and their radial velocities by astronomical spectroscopy; the earliest evidence for the humankind's identification of constellations comes from Mesopotamian inscribed stones and clay writing tablets that date back to 3000 BC.
It seems that the bulk of the Mesopotamian constellations were created within a short interval from around 1300 to 1000 BC. Mesopotamian constellations appeared in many of the classical Greek constellations; the oldest Babylonian star catalogues of stars and constellations date back to the beginning in the Middle Bronze Age, most notably the Three Stars Each texts and the MUL. APIN, an expanded and revised version based on more accurate observation from around 1000 BC. However, the numerous Sumerian names in these catalogues suggest that they built on older, but otherwise unattested, Sumerian traditions of the Early Bronze Age; the classical Zodiac is a revision of Neo-Babylonian constellations from the 6th century BC. The Greeks adopted the Babylonian constellations in the 4th century BC. Twenty Ptolemaic constellations are from the Ancient Near East. Another ten have the same stars but different names. Biblical scholar, E. W. Bullinger interpreted some of the creatures mentioned in the books of Ezekiel and Revelation as the middle signs of the four quarters of the Zodiac, with the Lion as Leo, the Bull as Taurus, the Man representing Aquarius and the Eagle standing in for Scorpio.
The biblical Book of Job also
Exoplanet
An exoplanet or extrasolar planet is a planet outside the Solar System. The first evidence of an exoplanet was not recognized as such; the first scientific detection of an exoplanet was in 1988. The first confirmed detection occurred in 1992; as of 1 April 2019, there are 4,023 confirmed planets in 3,005 systems, with 656 systems having more than one planet. There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star. In several cases, multiple planets have been observed around a star. About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone. Assuming there are 200 billion stars in the Milky Way, it can be hypothesized that there are 11 billion habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included; the least massive planet known is Draugr, about twice the mass of the Moon.
The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b, about 30 times the mass of Jupiter, although according to some definitions of a planet, it is too massive to be a planet and may be a brown dwarf instead. There are planets that are so near to their star that they take only a few hours to orbit and there are others so far away that they take thousands of years to orbit; some are so far out. All of the planets detected so far are within the Milky Way. Nonetheless, evidence suggests that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist; 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 star's 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 considers a wide range of other factors in determining the suitability of a planet for hosting life. Besides exoplanets, there are rogue planets, which do not orbit any star; these tend to be considered as a separate category if they are gas giants, in which case they are counted as sub-brown dwarfs, like WISE 0855−0714. The rogue planets in the Milky Way 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 single star, the designation is formed by taking the name or, more designation of its parent star and adding a lower case letter; the first planet discovered in a system is given the designation "b" and planets are given subsequent letters. 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. Other naming systems exist. For centuries scientists and science fiction writers suspected that extrasolar planets existed, but there was no way of detecting them or of knowing their frequency or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers; the first evidence of an exoplanet was not recognized as such. The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, 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 main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method.
In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not bound to stars, so they're wandering through space or loosely orbiting between stars. We can estimate. In the sixteenth century the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are accompanied by planets. In the eighteenth century the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."In 1952, more than 40 years before the first hot Jupiter was discovere
Argo Navis
Argo Navis, or Argo, was a large constellation in the southern sky that has since been divided into the three constellations of Carina and Vela. The genitive was "Argus Navis", abbreviated "Arg". Flamsteed and other early modern astronomers called the constellation just Navis, genitive "Navis", abbreviated "Nav", it was identified in Greek mythology with the Argo, the ship used by Jason and the Argonauts that sailed to Colchis in search of the Golden Fleece. The original constellation is presently found near the southern horizon of the Mediterranean sky, becoming visible in springtime and sailed westward, skimming along the "river of the Milky Way." Due to precession of the equinoxes, many of the stars of Argo have been shifted farther south since Classical times, far fewer of its stars are visible today from the latitudes of the Mediterranean. This includes Canopus or α Carinae. All the stars of Argo Navis are visible south of the equator, pass near zenith from southern temperate latitudes. Argo Navis was long-known to Greek observers, who are believed to have derived it from Egypt around 1000 BC.
For example, Plutarch identified Argo with the Egyptian constellation called the "Boat of Osiris." Although some academics theorized a Sumerian origin related to the Epic of Gilgamesh, this hypothesis has been rejected as there is no evidence that the Sumerians or other Mesopotamian culture considered these stars, or any portion of them, to form a vessel. Over time, the constellation became identified with ancient Greek myth of Jason and the Argonauts. In his Almagest, Ptolemy described Argo Navis as occupying the portion of the Milky Way between Canis Major and Centaurus, identified stars comprising such details as the "little shield," the "steering-oar," the "mast-holder," and the "stern-ornament", which continued to be reflected in cartographic representations in celestial atlases into the nineteenth century. Another interesting feature of the constellation is that it appeared to be moving backwards against the backdrop of the night sky. Aratus, the Greek poet/historian living in the third century BC, noted this backward progression writing, "Argo by the Great Dog's tail is drawn.
In modern times, Argo Navis was considered unwieldy for scientific purposes due to its enormous size. In his Coelum Australe Stelliferum, published in 1763, the French astronomer Nicolas Louis de Lacaille explained that there were more than a hundred and sixty stars visible to the naked eye in Navis, so he used the set of lowercase and uppercase Latin letters three times on portions of the constellation referred to as "Argûs in carina", "Argûs in puppi", "Argûs in velis". Lacaille replaced Bayer's designations with new ones that followed stellar magnitudes more but used only a single Greek-letter sequence and described the constellation for those stars as "Argûs". Faint non-lettered stars were listed only as in "Argûs"; the final breakup and abolition of Argo Navis was proposed by Sir John Herschel in 1841 and again in 1844. Despite this, the constellation remained in use in parallel with its constituent parts into the 20th century. In 1922, along with the other constellations, it received a three-letter abbreviation: Arg.
The breakup and relegation to a former constellation occurred in 1930 when the IAU defined the 88 modern constellations, formally instituting Carina and Vela. Lacaille's designations were kept in the three separate constellations, so Carina has α, β and ε, Vela has γ and δ, Puppis has ζ, so on; as a result of this breakup, Argo Navis is the only one of the 48 constellations listed by Ptolemy in his Almagest, no longer recognized as a single constellation. In addition, the constellation Pyxis occupies an area near that which in antiquity was considered part of Argo's mast; some authors state that Pyxis was part of the Greek conception of Argo Navis, but magnetic compasses were unknown in ancient Greek times, it does not appear that its stars were included in the original conception. Lacaille considered it a separate constellation, representing one of the modern scientific instruments he placed among the constellations. Pyxis was listed separately, among his 14 new constellations. Lacaille assigned Bayer designations to Pyxis separate from those of Argo, his illustration shows an isolated instrument not related to the figure of the ship.
In 1844, John Herschel suggested formalizing the mast as a new constellation, Malus, to replace Lacaille's Pyxis, but the idea did not catch on. An effort by Edmond Halley to detach the "cloud of mist" at the prow of Argo Navis to form a new constellation named "Robur Carolinum" in honor of his patron, King Charles II, was unsuccessful; the Māori had several names for what was the constellation Argo, including Te Waka-o-Tamarereti, Te Kohi-a-Autahi, Te Kohi. In Vedic astronomy, Indian observers saw Argo Navis as "the Boat." Asterism List of stars in Argo Navis Starry Night Photography: Argo Navis Image Star Tales – Argo Navis Warburg Institute Iconographic Database – Argo
Frederick de Houtman
Frederick de Houtman, or Frederik de Houtman, was a Dutch explorer who sailed along the Western coast of Australia en route to Batavia, known today as Jakarta in Indonesia. He made observations of the southern stars and contributed to the creation of 12 new southern constellations. Frederick de Houtman was born in Gouda, Seventeen Provinces, he assisted fellow Dutch navigator Pieter Dirkszoon Keyser with astronomical observations during the first Dutch expedition to the East Indies in 1595–1597. He sailed in 1598–1599 on a second expedition led by his brother Cornelis de Houtman. Cornelis was killed on that expedition, Frederick was imprisoned by the Sultan of Aceh in northern Sumatra, but used his two years of captivity to study the local Malay language and to make astronomical observations; these observations supplemented. The constellations formed from their observations were first published in 1597 or 1598 on a globe by Petrus Plancius, globes incorporated adjustments based on de Houtman's observations.
Today, credit for these constellations is assigned jointly to Keyser, de Houtman, Plancius, though some of the underlying stars were known beforehand. The constellations are widely associated with Johann Bayer, who included them in his celestial atlas Uranometria in 1603. In 1603, after his return to Holland, de Houtman published his stellar observations in an appendix to his dictionary and grammar of the Malayan and Malagasy languages. In 1619 de Houtman sailed with Jacob d'Edel in the VOC ship Amsterdam, they sighted the Australian coast near present-day Perth. After sailing northwards along the coast he encountered and only narrowly avoided a group of shoals, subsequently called the Houtman Abrolhos. De Houtman made landfall in the region known as Eendrachtsland, which the explorer Dirk Hartog had encountered earlier. In his journal, de Houtman identified these coasts with Marco Polo's land of Beach, or Locach, as shown on maps of the time. De Houtman died in Alkmaar, Dutch Republic. Petrus Plancius First Dutch Expedition to Indonesia Second Dutch Expedition to Indonesia Dutch East India Company in Indonesia European exploration of Australia Janszoon voyage of 1605-6 Voyage of the Pera and Arnhem to Australia in 1623 New Holland Australian places with Dutch names History of the Northern Territory History of Western Australia History of South Australia History of Tasmania "The exploration and mapping of the Australian coastline in the seventeenth and eighteenth centuries".
VOC Historical Society Inc. Retrieved 2017-03-09. Ian Ridpath's Star Tales Knobel, E. B. "On Frederick de Houtman's catalogue of southern stars, the origin of the southern constellations",Monthly Notices of the Royal Astronomical Society, Vol. 77, p. 414-432
Variable star
A variable star is a star whose brightness as seen from Earth fluctuates. This variation may be caused by a change in emitted light or by something blocking the light, so variable stars are classified as either: Intrinsic variables, whose luminosity changes. Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth. Many most, stars have at least some variation in luminosity: the energy output of our Sun, for example, varies by about 0.1% over an 11-year solar cycle. An ancient Egyptian calendar of lucky and unlucky days composed some 3,200 years ago may be the oldest preserved historical document of the discovery of a variable star, the eclipsing binary Algol. Of the modern astronomers, the first variable star was identified in 1638 when Johannes Holwarda noticed that Omicron Ceti pulsated in a cycle taking 11 months; this discovery, combined with supernovae observed in 1572 and 1604, proved that the starry sky was not eternally invariable as Aristotle and other ancient philosophers had taught.
In this way, the discovery of variable stars contributed to the astronomical revolution of the sixteenth and early seventeenth centuries. The second variable star to be described was the eclipsing variable Algol, by Geminiano Montanari in 1669. Chi Cygni was identified in 1686 by G. Kirch R Hydrae in 1704 by G. D. Maraldi. By 1786 ten variable stars were known. John Goodricke himself discovered Beta Lyrae. Since 1850 the number of known variable stars has increased especially after 1890 when it became possible to identify variable stars by means of photography; the latest edition of the General Catalogue of Variable Stars lists more than 46,000 variable stars in the Milky Way, as well as 10,000 in other galaxies, over 10,000'suspected' variables. The most common kinds of variability involve changes in brightness, but other types of variability occur, in particular changes in the spectrum. By combining light curve data with observed spectral changes, astronomers are able to explain why a particular star is variable.
Variable stars are analysed using photometry, spectrophotometry and spectroscopy. Measurements of their changes in brightness can be plotted to produce light curves. For regular variables, the period of variation and its amplitude can be well established. Peak brightnesses in the light curve are known as maxima. Amateur astronomers can do useful scientific study of variable stars by visually comparing the star with other stars within the same telescopic field of view of which the magnitudes are known and constant. By estimating the variable's magnitude and noting the time of observation a visual lightcurve can be constructed; the American Association of Variable Star Observers collects such observations from participants around the world and shares the data with the scientific community. From the light curve the following data are derived: are the brightness variations periodical, irregular, or unique? What is the period of the brightness fluctuations? What is the shape of the light curve? From the spectrum the following data are derived: what kind of star is it: what is its temperature, its luminosity class? is it a single star, or a binary? does the spectrum change with time?
Changes in brightness may depend on the part of the spectrum, observed if the wavelengths of spectral lines are shifted this points to movements strong magnetic fields on the star betray themselves in the spectrum abnormal emission or absorption lines may be indication of a hot stellar atmosphere, or gas clouds surrounding the star. In few cases it is possible to make pictures of a stellar disk; these may show darker spots on its surface. Combining light curves with spectral data gives a clue as to the changes that occur in a variable star. For example, evidence for a pulsating star is found in its shifting spectrum because its surface periodically moves toward and away from us, with the same frequency as its changing brightness. About two-thirds of all variable stars appear to be pulsating. In the 1930s astronomer Arthur Stanley Eddington showed that the mathematical equations that describe the interior of a star may lead to instabilities that cause a star to pulsate; the most common type of instability is related to oscillations in the degree of ionization in outer, convective layers of the star.
Suppose the star is in the swelling phase. Its outer layers expand; because of the decreasing temperature the degree of ionization decreases. This makes the gas more transparent, thus makes it easier for the star to radiate its energy; this in turn will make the star start to contract. As the gas is thereby compressed, it is heated and the degree of ionization again increases. Thi
Orbital pole
An orbital pole is either point at the ends of an imaginary line segment that runs through the center of an orbit and is perpendicular to the orbital plane. Projected onto the celestial sphere, orbital poles are similar in concept to celestial poles, but are based on the body's orbit instead of its equator; the north orbital pole of a revolving body is defined by the right-hand rule. If the fingers of the right hand are curved along the direction of orbital motion, with the thumb extended and oriented to be parallel to the orbital axis the direction the thumb points is defined to be the orbital north; the poles of Earth's orbit are referred to as the ecliptic poles. The ecliptic is the plane; the ecliptic poles are the two points where the ecliptic axis, the imaginary line perpendicular to the ecliptic, intersects the celestial sphere. The two ecliptic poles are mapped below. Due to axial precession, either celestial pole completes a circuit around the nearer ecliptic pole every 25,800 years; as of 1 January 2000, the positions of the ecliptic poles expressed in equatorial coordinates, as a consequence of Earth's axial tilt, are the following: North: right ascension 18h 0m 0.0s, declination +66° 33′ 38.55″ South: right ascension 6h 0m 0.0s, declination −66° 33′ 38.55″It is impossible anywhere for either ecliptic pole to be at the zenith in the night sky.
By definition, the ecliptic poles are located 90° from the Sun's position. Therefore and wherever either ecliptic pole is directly overhead, the Sun must be on the horizon; the ecliptic poles can contact the zenith only in the Antarctic circles. Celestial pole Polar alignment Pole star
