Hubble's law known as the Hubble–Lemaître law, is the observation in physical cosmology that: Objects observed in deep space—extragalactic space, 10 megaparsecs or more—are found to have a redshift, interpreted as a relative velocity away from Earth. Hubble's law is considered the first observational basis for the expansion of the universe and today serves as one of the pieces of evidence most cited in support of the Big Bang model; the motion of astronomical objects due to this expansion is known as the Hubble flow. Although attributed to Edwin Hubble, the notion of the universe expanding at a calculable rate was first derived from the general relativity equations in 1922 by Alexander Friedmann. Friedmann published a set of equations, now known as the Friedmann equations, showing that the universe might expand, presenting the expansion speed if this was the case. Georges Lemaître, in a 1927 article, independently derived that the universe might be expanding, observed the proportionality between recessional velocity of and distance to distant bodies, suggested an estimated value of the proportionality constant, which when corrected by Hubble became known as the Hubble constant.
Though the Hubble constant H 0 is constant in the velocity-distance space at any given moment in time, the Hubble parameter H, which the Hubble constant is the current value of, varies with time, so the term'constant' is sometimes thought of as somewhat of a misnomer. Moreover, two years Edwin Hubble confirmed the existence of cosmic expansion, determined a more accurate value for the constant that now bears his name. Hubble inferred the recession velocity of the objects from their redshifts, many of which were earlier measured and related to velocity by Vesto Slipher in 1917; the law is expressed by the equation v = H0D, with H0 the constant of proportionality—Hubble constant—between the "proper distance" D to a galaxy, which can change over time, unlike the comoving distance, its velocity v, i.e. the derivative of proper distance with respect to cosmological time coordinate. The SI unit of H0 is s−1, but it is most quoted in /Mpc, thus giving the speed in km/s of a galaxy 1 megaparsec away.
The Hubble constant is about 70 /Mpc. The reciprocal of H0 is the Hubble time. A decade before Hubble made his observations, a number of physicists and mathematicians had established a consistent theory of an expanding universe by using Einstein's field equations of general relativity. Applying the most general principles to the nature of the universe yielded a dynamic solution that conflicted with the then-prevailing notion of a static universe. In 1912, Vesto Slipher measured the first Doppler shift of a "spiral nebula", soon discovered that all such nebulae were receding from Earth, he did not grasp the cosmological implications of this fact, indeed at the time it was controversial whether or not these nebulae were "island universes" outside our Milky Way. In 1922, Alexander Friedmann derived his Friedmann equations from Einstein's field equations, showing that the universe might expand at a rate calculable by the equations; the parameter used by Friedmann is known today as the scale factor which can be considered as a scale invariant form of the proportionality constant of Hubble's law.
Georges Lemaître independently found a similar solution in 1927. The Friedmann equations are derived by inserting the metric for a homogeneous and isotropic universe into Einstein's field equations for a fluid with a given density and pressure; this idea of an expanding spacetime would lead to the Big Bang and Steady State theories of cosmology. In 1927, two years before Hubble published his own article, the Belgian priest and astronomer Georges Lemaître was the first to publish research deriving what is now known as Hubble's Law. According to the Canadian astronomer Sidney van den Bergh, "The 1927 discovery of the expansion of the universe by Lemaître was published in French in a low-impact journal. In the 1931 high-impact English translation of this article a critical equation was changed by omitting reference to what is now known as the Hubble constant." It is now known. Before the advent of modern cosmology, there was considerable talk about the size and shape of the universe. In 1920, the Shapley-Curtis debate took place between Harlow Shapley and Heber D. Curtis over this issue.
Shapley argued for a small universe the size of the Milky Way galaxy and Curtis argued that the universe was much larger. The issue was resolved in the coming decade with Hubble's improved observations. Edwin Hubble did most of his professional astronomical observing work at Mount Wilson Observatory, home to the world's most powerful telescope at the time, his observations of Cepheid variable stars in “spiral nebulae” enabled him to calculate the distances to these objects. These objects were discovered to be at distances which placed them well outside the Milky Way, they continued to be called “nebulae” and it was only that the term “galaxies” replaced it. The parameters that appear in Hubble's law and distances, are not directly measured. In reality we determine, say, a supernova brightness, which provides information about its distance, the redshift z
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
The apparent magnitude of an astronomical object is a number, a measure of its brightness as seen by an observer on Earth. The magnitude scale is logarithmic. A difference of 1 in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The brighter an object appears, the lower its magnitude value, with the brightest astronomical objects having negative apparent magnitudes: for example Sirius at −1.46. 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 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 simply as V, as in "mV = 15" or "V = 15" to describe a 15th-magnitude object; 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 night sky were said to be of first magnitude, whereas the faintest were of sixth magnitude, the limit of human visual perception. Each grade of magnitude was considered twice the brightness of the following grade, although that ratio was subjective as no photodetectors existed; this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest and is believed to have originated with Hipparchus. In 1856, Norman Robert Pogson formalized the system by defining a first magnitude star as a star, 100 times as bright as a sixth-magnitude star, thereby establishing the logarithmic scale still in use today; this implies that a star of magnitude m is about 2.512 times as bright as a star of magnitude m + 1. This figure, the fifth root of 100, became known as Pogson's Ratio; the zero point of Pogson's scale was defined by assigning Polaris a magnitude of 2. Astronomers discovered that Polaris is variable, so they switched to Vega as the standard reference star, assigning the brightness of Vega as the definition of zero magnitude at any specified wavelength.
Apart from small corrections, the brightness of Vega still serves as the definition of zero magnitude for visible and near infrared wavelengths, where its spectral energy distribution approximates that of a black body for a temperature of 11000 K. However, with the advent of infrared astronomy it was revealed that Vega's radiation includes an Infrared excess due to a circumstellar 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 magnitude 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, as a function of wavelength, can be computed. Small deviations are specified between systems using measurement apparatuses developed independently so that data obtained by different astronomers can be properly compared, but of greater practical importance is the definition of magnitude not at a single wavelength but applying to the response of standard spectral filters used in photometry over various wavelength bands.
With the modern magnitude systems, brightness over a wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30; the brightness of Vega is exceeded by four stars in the night sky at visible wavelengths as well as the bright planets Venus and Jupiter, these must be described by negative magnitudes. For example, the brightest star of the celestial sphere, has an apparent magnitude of −1.4 in the visible. Negative magnitudes for other bright astronomical objects can be found in the table below. Astronomers have developed other photometric zeropoint systems as alternatives to the Vega system; the most used is the AB magnitude system, in which photometric zeropoints are based on a hypothetical reference spectrum having constant flux per unit frequency interval, rather than using a stellar spectrum or blackbody curve as the reference. The AB magnitude zeropoint is defined such that an object's AB and Vega-based magnitudes will be equal in the V filter band.
As the amount of light received by a telescope is reduced by transmission through the Earth's atmosphere, any measurement of apparent magnitude is corrected for what it would have been as seen from above the atmosphere. 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 100. Therefore, the apparent magnitude m, in the spectral band x, would be given by m x = − 5 log 100 , more expressed in terms of common logarithms as m x
Right ascension is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the point above the earth in question. When paired with declination, these astronomical coordinates specify the direction of a point on the celestial sphere in the equatorial coordinate system. An old term, right ascension refers to the ascension, or the point on the celestial equator that rises with any celestial object as seen from Earth's equator, where the celestial equator intersects the horizon at a right angle, it contrasts with oblique ascension, the point on the celestial equator that rises with any celestial object as seen from most latitudes on Earth, where the celestial equator intersects the horizon at an oblique angle. Right ascension is the celestial equivalent of terrestrial longitude. Both right ascension and longitude measure an angle from a primary direction on an equator. Right ascension is measured from the Sun at the March equinox i.e. the First Point of Aries, the place on the celestial sphere where the Sun crosses the celestial equator from south to north at the March equinox and is located in the constellation Pisces.
Right ascension is measured continuously in a full circle from that alignment of Earth and Sun in space, that equinox, the measurement increasing towards the east. As seen from Earth, objects noted to have 12h RA are longest visible at the March equinox. On those dates at midnight, such objects will reach their highest point. How high depends on their declination. Any units of angular measure could have been chosen for right ascension, but it is customarily measured in hours and seconds, with 24h being equivalent to a full circle. Astronomers have chosen this unit to measure right ascension because they measure a star's location by timing its passage through the highest point in the sky as the Earth rotates; the line which passes through the highest point in the sky, called the meridian, is the projection of a longitude line onto the celestial sphere. Since a complete circle contains 24h of right ascension or 360°, 1/24 of a circle is measured as 1h of right ascension, or 15°. A full circle, measured in right-ascension units, contains 24 × 60 × 60 = 86400s, or 24 × 60 = 1440m, or 24h.
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 = 1h 30m 00s is at its meridian a star with RA = 20h 00m 00s will be on the/at its meridian 18.5 sidereal hours later. Sidereal hour angle, used in celestial navigation, is similar to right ascension, but increases westward rather than eastward. Measured in degrees, it is the complement of right ascension with respect to 24h, 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 Earth's axis rotates westward about the poles of the ecliptic, completing one cycle in about 26,000 years; this movement, 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, astronomers specify them with reference to a particular year, known as an epoch.
Coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch. Right ascension for "fixed stars" near the ecliptic and equator increases by about 3.05 seconds per year on average, or 5.1 minutes per century, but for fixed stars further from the ecliptic the rate of change can be anything from negative infinity to positive infinity. 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 used standard epoch is J2000.0, January 1, 2000 at 12:00 TT. The prefix "J" indicates. Prior to J2000.0, astronomers used the successive Besselian epochs B1875.0, B1900.0, B1950.0. 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 use of RA was limited to special cases.
With the invention of the telescope, it became possible for astronomers to observe celestial objects in greater detail, provided that the telescope could be kept pointed at the object for a period of time. The easiest way to do, to use an equatorial mount, which allows the telescope to be aligned with one of its two pivots parallel to the Earth's axis. A motorized clock drive is used with an equatorial mount to cancel out the Earth's rotation; as the equatorial mount became adopted for observation, the equatorial coordinate system, which includes right ascension, was adopted at the same time for simplicity. Equatorial mounts could be pointed at objects with known right ascension and declination by the use of setting circles; the first star catalog to use right ascen
Draco is a constellation in the far northern sky. Its name is Latin for dragon, it was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, remains one of the 88 modern constellations today. The north pole of the ecliptic is in Draco. Draco is circumpolar, can be seen all year from northern latitudes. Thuban was the northern pole star from 3942 BC, when it moved farther north than Theta Boötis, until 1793 BC; the Egyptian Pyramids were designed to have one side facing north, with an entrance passage geometrically aligned so that Thuban would be visible at night. Due to the effects of precession, it will again be the pole star around the year AD 21000, it is a blue-white giant star of 309 light-years from Earth. The traditional name of Alpha Draconis, means "head of the serpent". There are three stars under magnitude 3 in Draco; the brighter of the three, the brightest star in Draco, is Gamma Draconis, traditionally called Etamin or Eltanin. It is an orange giant star of 148 light-years from Earth.
The aberration of starlight was discovered in 1728. Nearby Beta Draconis, traditionally called Rastaban, is a yellow giant star of magnitude 2.8, 362 light-years from Earth. Its name shares a meaning with Thuban, "head of the serpent". Draco is home to binary stars. Eta Draconis is a double star with a yellow-hued primary of magnitude 2.8 and a white-hued secondary of magnitude 8.2 located south of the primary. The two are separated by 4.8 arcseconds. Mu Draconis, traditionally called Alrakis, is a binary star with two white components. Magnitude 5.6 and 5.7, the two components orbit each other every 670 years. The Alrakis system is 88 light-years from Earth. Nu Draconis is a similar binary star with 100 light-years from Earth. Both components are of magnitude 4.9 and can be distinguished in a small amateur telescope or a pair of binoculars. Omicron Draconis is a double star divisible in small telescopes; the primary is an orange giant of 322 light-years from Earth. The secondary is of magnitude 7.8.
Psi Draconis is a binary star divisible in binoculars and small amateur telescopes, 72 light-years from Earth. The primary is a yellow-white star of magnitude 4.6 and the secondary is a yellow star of magnitude 5.8. 16 Draconis and 17 Draconis are part of a triple star 400 light-years from Earth, divisible in medium-sized amateur telescopes. The primary, a blue-white star of magnitude 5.1, is itself a binary with components of magnitude 5.4 and 6.5. The secondary is of magnitude 5.5 and the system is 400 light-years away. 20 Draconis is a binary star with a white-hued primary of magnitude 7.1 and a yellow-hued secondary of magnitude 7.3 located east-northeast of the primary. The two have an orbital period of 420 years; as of 2012, the two components are approaching their maximum separation. 39 Draconis is a triple star 188 light-years from Earth, divisible in small amateur telescopes. The primary is a blue star of magnitude 5.0, the secondary is a yellow star of magnitude 7.4, the tertiary is a star of magnitude 8.0.
40 Draconis and 41 Draconis are a binary star divisible in small telescopes. The two orange dwarf stars are 170 light-years from Earth and are of magnitude 5.7 and 6.1. R Draconis is a red Mira-type variable star with a period of about 8 months, its average minimum magnitude is 12.4, its average maximum magnitude is 7.6. It was discovered to be a variable star by Hans Geelmuyden in 1876; the constellation contains the star named Kepler-10, confirmed to be orbited by Kepler-10b, the smallest rocky Earth-sized planet yet detected outside of the Solar System. One of deep-sky objects in Draco is the Cat's Eye Nebula, a planetary nebula 3,000 light-years away, discovered by English astronomer William Herschel in 1786, it is 9th magnitude and was named for its appearance in the Hubble Space Telescope, though it appears as a fuzzy blue-green disk in an amateur telescope. NGC 6543 has a complex shape due to gravitational interactions between the components of the multiple star at its center, the progenitor of the nebula 1,000 years ago.
It is located 9.6 arcminutes away from the north ecliptic pole to the west-northwest. It is related to IC 4677, a nebula that appears as a bar 1.8 arcminutes to the west of the Cat's Eye nebula. In long-term exposures, IC 4677 appears as a portion of a ring surrounding the planetary nebula. There are several faint galaxies in Draco, one of, the lenticular galaxy NGC 5866 that bears its name to a small group that includes the spiral galaxies NGC 5879 and NGC 5907. Another is the Draco Dwarf Galaxy, one of the least luminous galaxies with an absolute magnitude of −8.6 and a diameter of only about 3,500 light years, discovered by Albert G. Wilson of Lowell Observatory in 1954. Another dwarf galaxy found in this constellation is PGC 39058. Draco features several interacting galaxies and galaxy clusters. One such massive cluster is Abell 2218, located at a distance of 3 billion light-years, it acts as a gravitational lens for more distant background galaxies, allowing astronomers to study those galaxies as well as Abell 2218 itself.
One of the most well-known interacting galaxies is Arp 188 called the "Tadpole Galaxy". Named for its appearance, which features a "
Lyon-Meudon Extragalactic Database
The Lyon-Meudon Extragalactic Database was a database of galaxies, created in 1983 at the Lyon Observatory. Each galaxy had a number assigned to it, now known as its PGC number; the Principal Galaxies Catalogue, published in 1989, was based on the Lyon-Meudon Extragalactic Database and contained cross-identifications for it. LEDA was merged with Hypercat to become HyperLEDA in 2000, itself known as PGC2003. LEDA contained information on more than 60 parameters for about 100,000 galaxies, now contains information on over 3 million celestial objects, of which about 1.5 million are galaxies. The database allows astronomers around the world access to its information. HYPERLEDA HyperLeda: A database for physics of galaxies LEDA data base LEDA Database Reference Page