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, 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 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
Minute and second of arc
A minute of arc, arc minute, or minute arc is a unit of angular measurement equal to 1/60 of one degree. Since one degree is 1/360 of a turn, one minute of arc is 1/21600 of a turn, a second of arc, arcsecond, or arc second is 1/60 of an arcminute, 1/3600 of a degree, 1/1296000 of a turn, and π/648000 of a radian. To express even smaller angles, standard SI prefixes can be employed, the number of square arcminutes in a complete sphere is 4 π2 =466560000 π ≈148510660 square arcminutes. The standard symbol for marking the arcminute is the prime, though a single quote is used where only ASCII characters are permitted. One arcminute is thus written 1′ and it is abbreviated as arcmin or amin or, less commonly, the prime with a circumflex over it. The standard symbol for the arcsecond is the prime, though a double quote is commonly used where only ASCII characters are permitted. One arcsecond is thus written 1″ and it is abbreviated as arcsec or asec. In celestial navigation, seconds of arc are used in calculations.
This notation has been carried over into marine GPS receivers, which normally display latitude and longitude in the format by default. An arcsecond is approximately the angle subtended by a U. S. dime coin at a distance of 4 kilometres, a milliarcsecond is about the size of a dime atop the Eiffel Tower as seen from New York City. A microarcsecond is about the size of a period at the end of a sentence in the Apollo mission manuals left on the Moon as seen from Earth, since antiquity the arcminute and arcsecond have been used in astronomy. The principal exception is Right ascension in equatorial coordinates, which is measured in units of hours, minutes. These small angles may be written in milliarcseconds, or thousandths of an arcsecond, the unit of distance, the parsec, named from the parallax of one arcsecond, was developed for such parallax measurements. It is the distance at which the radius of the Earths orbit would subtend an angle of one arcsecond. The ESA astrometric space probe Gaia is hoped to measure star positions to 20 microarcseconds when it begins producing catalog positions sometime after 2016, there are about 1.3 trillion µas in a turn.
Currently the best catalog positions of stars actually measured are in terms of milliarcseconds, apart from the Sun, the star with the largest angular diameter from Earth is R Doradus, a red supergiant with a diameter of 0.05 arcsecond. The dwarf planet Pluto has proven difficult to resolve because its angular diameter is about 0.1 arcsecond, space telescopes are not affected by the Earths atmosphere but are diffraction limited. For example, the Hubble space telescope can reach a size of stars down to about 0. 1″
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, 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 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
Stellar parallax is parallax on an interstellar scale, the apparent shift of position of any nearby star against the background of distant objects. Stellar parallax is so difficult to detect that its existence was the subject of debate in astronomy for thousands of years. It was first observed by Giuseppe Calandrelli who reported parallax in α-Lyrae in his work Osservazione e riflessione sulla parallasse annua dall’alfa della Lira, in 1838 Friedrich Bessel made the first successful parallax measurement ever, for the star 61 Cygni, using a Fraunhofer heliometer at Königsberg Observatory. Once a stars parallax is known, its distance from Earth can be computed trigonometrically, but the more distant an object is, the smaller its parallax. Even with 21st-century techniques in astrometry, the limits of accurate measurement make distances farther away than about 100 parsecs too approximate to be useful when obtained by this technique. Relatively close on a scale, the applicability of stellar parallax leaves most astronomical distance measurements to be calculated by spectral red-shift or other methods.
Stellar parallax measures are given in the units of arcseconds. The distance unit parsec is defined as the length of the leg of a right triangle adjacent to the angle of one arcsecond at one vertex, because stellar parallaxes and distances all involve such skinny right triangles, a convenient trigonometric approximation can be used to convert parallaxes to distance. The distance is simply the reciprocal of the parallax, d =1 / p, for example, Proxima Centauri, whose parallax is 0.7687, is 1 /0.7687 =1.3009 parsecs distant. Stellar parallax is so small that its apparent absence was used as an argument against heliocentrism during the early modern age. James Bradley first tried to measure stellar parallaxes in 1729, the stellar movement proved too insignificant for his telescope, but he instead discovered the aberration of light, the nutation of Earth’s axis, and catalogued 3222 stars. The parsec is defined as the distance for which the annual parallax is 1 arcsecond, annual parallax is normally measured by observing the position of a star at different times of the year as Earth moves through its orbit.
Measurement of annual parallax was the first reliable way to determine the distances to the closest stars, the first successful measurements of stellar parallax were made by Friedrich Bessel in 1838 for the star 61 Cygni using a heliometer. Being very difficult to measure, only about 60 stellar parallaxes had been obtained by the end of the 19th century, astrographs using astronomical photographic plates sped the process in the early 20th century. Automated plate-measuring machines and more sophisticated technology of the 1960s allowed more efficient compilation of star catalogues. In the 1980s, charge-coupled devices replaced photographic plates and reduced optical uncertainties to one milliarcsecond, stellar parallax remains the standard for calibrating other measurement methods. The angles involved in these calculations are very small and thus difficult to measure, the nearest star to the Sun, Proxima Centauri, has a parallax of 0.7687 ±0.0003 arcsec. This angle is approximately that subtended by an object 2 centimeters in diameter located 5.3 kilometers away
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
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, 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, 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, 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
A giant star is a star with substantially larger radius and luminosity than a main-sequence star of the same surface temperature. They lie above the main sequence on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III, the terms giant and dwarf were coined for stars of quite different luminosity despite similar temperature or spectral type by Ejnar Hertzsprung about 1905. Giant stars have radii up to a few hundred times the Sun, stars still more luminous than giants are referred to as supergiants and hypergiants. A hot, luminous main-sequence star may be referred to as a giant, a star becomes a giant star after all the hydrogen available for fusion at its core has been depleted and, as a result, leaves the main sequence. The behaviour of a star depends largely on its mass. For a star with a mass above about 0.25 solar masses, the portion of the star outside the shell expands and cools, but with only a small increase in luminosity, and the star becomes a subgiant.
The inert helium core continues to grow and increase temperature as it accretes helium from the shell, after just a few million years the core reaches the Schönberg–Chandrasekhar limit, rapidly collapses, and may become degenerate. This causes the layers to expand even further and generates a strong convective zone that brings heavy elements to the surface in a process called the first dredge-up. The core continues to gain mass and increase in temperature, if the stars mass, when on the main sequence, was below approximately 0.4 M☉, it will never reach the central temperatures necessary to fuse helium. It will therefore remain a hydrogen-fusing red giant until it runs out of hydrogen and this is entirely theoretical because no star of such low mass has been in existence long enough to evolve to that stage. In stars above about 0.4 M☉ the core eventually reaches 108 K and helium will begin to fuse to carbon and oxygen in the core by the triple-alpha process. §5.9. When the core is degenerate helium fusion begins explosively, but most of the energy goes into lifting the degeneracy, the energy generated by helium fusion reduces the pressure in the surrounding hydrogen-burning shell, which reduces its energy-generation rate.
The overall luminosity of the star decreases, its outer envelope contracts again, when the core helium is exhausted, a star with up to about 8 M☉ has a carbon–oxygen core that becomes degenerate and starts helium burning in a shell. As with the collapse of the helium core, this starts convection in the outer layers, triggers a second dredge-up. This is the giant branch analogous to the red-giant branch but more luminous. They start core-helium burning before the core becomes degenerate and develop smoothly into red supergiants without an increase in luminosity. Stars in the 8-12 M☉ range have somewhat intermediate properties and have been called super-AGB stars and they largely follow the tracks of lighter stars through RGB, HB, and AGB phases, but are massive enough to initiate core carbon burning and even some neon burning. They form oxygen–magnesium–neon cores, which may collapse in an electron-capture supernova, O class main sequence stars are already highly luminous
Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of the Solar System and our galaxy, the history of astrometry is linked to the history of star catalogues, which gave astronomers reference points for objects in the sky so they could track their movements. This can be dated back to Hipparchus, who around 190 BC used the catalogue of his predecessors Timocharis, in doing so, he developed the brightness scale still in use today. Hipparchus compiled a catalogue with at least 850 stars and their positions, hipparchuss successor, included a catalogue of 1,022 stars in his work the Almagest, giving their location and brightness. Ibn Yunus observed more than 10,000 entries for the Suns position for years using a large astrolabe with a diameter of nearly 1.4 metres. In the 15th century, the Timurid astronomer Ulugh Beg compiled the Zij-i-Sultani, like the earlier catalogs of Hipparchus and Ptolemy, Ulugh Begs catalogue is estimated to have been precise to within approximately 20 minutes of arc.
In the 16th century, Tycho Brahe used improved instruments, including large mural instruments, to measure star positions more accurately than previously, Taqi al-Din measured the right ascension of the stars at the Istanbul observatory of Taqi al-Din using the observational clock he invented. When telescopes became commonplace, setting circles sped measurements James Bradley first tried to measure stellar parallaxes in 1729, the stellar movement proved too insignificant for his telescope, but he instead discovered the aberration of light and the nutation of the Earths axis. His cataloguing of 3222 stars was refined in 1807 by Friedrich Bessel and he made the first measurement of stellar parallax,0.3 arcsec for the binary star 61 Cygni. Being very difficult to measure, only about 60 stellar parallaxes had been obtained by the end of the 19th century, astrographs using astronomical photographic plates sped the process in the early 20th century. Automated plate-measuring machines and more sophisticated technology of the 1960s allowed more efficient compilation of star catalogues.
In the 1980s, charge-coupled devices replaced photographic plates and reduced optical uncertainties to one milliarcsecond and this technology made astrometry less expensive, opening the field to an amateur audience. In 1989, the European Space Agencys Hipparcos satellite took astrometry into orbit, operated from 1989 to 1993, Hipparcos measured large and small angles on the sky with much greater precision than any previous optical telescopes. During its 4-year run, the positions and proper motions of 118,218 stars were determined with a degree of accuracy. A new Tycho catalog drew together a database of 1,058,332 to within 20-30 mas, additional catalogues were compiled for the 23,882 double/multiple stars and 11,597 variable stars analyzed during the Hipparcos mission. Today, the catalogue most often used is USNO-B1.0, during the past 50 years,7,435 Schmidt camera plates were used to complete several sky surveys that make the data in USNO-B1.0 accurate to within 0.2 arcsec. In observational astronomy, astrometric techniques help identify stellar objects by their unique motions and it is instrumental for keeping time, in that UTC is basically the atomic time synchronized to Earths rotation by means of exact observations.
Astrometry is an important step in the distance ladder because it establishes parallax distance estimates for stars in the Milky Way
The solar mass is a standard unit of mass in astronomy, equal to approximately 1.99 ×1030 kilograms. It is used to indicate the masses of stars, as well as clusters, nebulae. It is equal to the mass of the Sun, about two kilograms, M☉ = ×1030 kg The above mass is about 332946 times the mass of Earth. Because Earth follows an orbit around the Sun, its solar mass can be computed from the equation for the orbital period of a small body orbiting a central mass. The value he obtained differs by only 1% from the modern value, the diurnal parallax of the Sun was accurately measured during the transits of Venus in 1761 and 1769, yielding a value of 9″. From the value of the parallax, one can determine the distance to the Sun from the geometry of Earth. The first person to estimate the mass of the Sun was Isaac Newton, in his work Principia, he estimated that the ratio of the mass of Earth to the Sun was about 1/28700. Later he determined that his value was based upon a faulty value for the solar parallax and he corrected his estimated ratio to 1/169282 in the third edition of the Principia.
The current value for the parallax is smaller still, yielding an estimated mass ratio of 1/332946. As a unit of measurement, the solar mass came into use before the AU, the mass of the Sun has been decreasing since the time it formed. This occurs through two processes in nearly equal amounts, first, in the Suns core, hydrogen is converted into helium through nuclear fusion, in particular the p–p chain, and this reaction converts some mass into energy in the form of gamma ray photons. Most of this energy eventually radiates away from the Sun, high-energy protons and electrons in the atmosphere of the Sun are ejected directly into outer space as a solar wind. The original mass of the Sun at the time it reached the main sequence remains uncertain, the early Sun had much higher mass-loss rates than at present, and it may have lost anywhere from 1–7% of its natal mass over the course of its main-sequence lifetime. The Sun gains a small amount of mass through the impact of asteroids. However, as the Sun already contains 99.
86% of the Solar Systems total mass, M☉ G / c2 ≈1.48 km M☉ G / c3 ≈4.93 μs I. -J. A Bright Young Sun Consistent with Helioseismology and Warm Temperatures on Ancient Earth and Mars
Canis Major /ˌkeɪnᵻs ˈmeɪdʒər/ is a constellation in the southern celestial hemisphere. In the second century, it was included in Ptolemys 48 constellations and its name is Latin for greater dog in contrast to Canis Minor, the lesser dog, both figures are commonly represented as following the constellation of Orion the hunter through the sky. The Milky Way passes through Canis Major and several open clusters lie within its borders, Canis Major contains Sirius, the brightest star in the night sky, known as the dog star. It is bright because of its proximity to the Solar System, in contrast, the other bright stars of the constellation are stars of great distance and high luminosity. At magnitude 1.5, Epsilon Canis Majoris is the second-brightest star of the constellation, the red hypergiant VY Canis Majoris is one of the largest stars known, while the neutron star RX J0720. 4-3125 has a radius of a mere 5 km. In ancient Mesopotamia, named KAK. SI, in the compendium of Babylonian astronomy and astrology titled MUL.
APIN, the arrow, was linked with the warrior Ninurta, and the bow with Ishtar, daughter of Enlil. Ninurta was linked to the deity Marduk, who was said to have slain the ocean goddess Tiamat with a great bow, the Ancient Greeks replaced the bow and arrow depiction with that of a dog. It was considered to represent one of Orions hunting dogs, pursuing Lepus the Hare or helping Orion fight Taurus the Bull, the ancient Greeks refer only to one dog, but by Roman times, Canis Minor appears as Orions second dog. Alternative names include Canis Sequens and Canis Alter, Canis Syrius was the name used in the 1521 Alfonsine tables. The Roman myth refers to Canis Major as Custos Europae, the dog guarding Europa but failing to prevent her abduction by Jupiter in the form of a bull, and as Janitor Lethaeus, the watchdog. In medieval Arab astronomy, the constellation became Al Kalb al Akbar, islamic scholar Abū Rayḥān al-Bīrūnī referred to Orion as Al Kalb al Jabbār, the Dog of the Giant. Among the Merazig of Tunisia, shepherds note six constellations that mark the passage of the dry, one of them, called Merzem, includes the stars of Canis Major and Canis Minor and is the herald of two weeks of hot weather.
In Chinese astronomy, the constellation of Canis Major lies in the Vermilion Bird. The Military Market was a pattern of stars containing Nu3, Beta, Xi1 and Xi2. The Wild Cockerel was at the centre of the Military Market, schlegel reported that the stars Omicron and Pi Canis Majoris might have been them, while Beta or Nu2 have been proposed. Sirius was Tiānláng, the Celestial Wolf, denoting invasion and plunder, southeast of the Wolf was the asterism Húshǐ, the celestial Bow and Arrow, which was interpreted as containing Delta, Epsilon and Kappa Canis Majoris and Delta Velorum. Alternatively, the arrow was depicted by Omicron2 and Eta and aiming at Sirius, while the bow comprised Kappa, Sigma, Delta and 164 Canis Majoris, and Pi and Omicron Puppis. Both the Māori people and the people of the Tuamotus recognized the figure of Canis Major as a distinct entity, Te Huinga-o-Rehua, called Te Putahi-nui-o-Rehua and Te Kahui-Takurua, was a Maori constellation that included both Canis Minor and Canis Major, along with some surrounding stars
Stellar evolution is the process by which a star changes over the course of time. The table shows the lifetimes of stars as a function of their masses, all stars are born from collapsing clouds of gas and dust, often called nebulae or molecular clouds. Over the course of millions of years, these protostars settle down into a state of equilibrium, nuclear fusion powers a star for most of its life. Initially the energy is generated by the fusion of hydrogen atoms at the core of the main-sequence star, later, as the preponderance of atoms at the core becomes helium, stars like the Sun begin to fuse hydrogen along a spherical shell surrounding the core. This process causes the star to gradually grow in size, passing through the subgiant stage until it reaches the red giant phase. Once a star like the Sun has exhausted its fuel, its core collapses into a dense white dwarf. Stars with around ten or more times the mass of the Sun can explode in a supernova as their inert iron cores collapse into a dense neutron star or black hole.
Stellar evolution is not studied by observing the life of a star, as most stellar changes occur too slowly to be detected. Instead, astrophysicists come to understand how stars evolve by observing numerous stars at various points in their lifetime, in June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z =6.60. Stellar evolution starts with the collapse of a giant molecular cloud. Typical giant molecular clouds are roughly 100 light-years across and contain up to 6,000,000 solar masses, as it collapses, a giant molecular cloud breaks into smaller and smaller pieces. In each of these fragments, the collapsing gas releases gravitational potential energy as heat, as its temperature and pressure increase, a fragment condenses into a rotating sphere of superhot gas known as a protostar. A protostar continues to grow by accretion of gas and dust from the molecular cloud, further development is determined by its mass. Protostars are encompassed in dust, and are more readily visible at infrared wavelengths.
Observations from the Wide-field Infrared Survey Explorer have been important for unveiling numerous Galactic protostars. Protostars with masses less than roughly 0.08 M☉ never reach high enough for nuclear fusion of hydrogen to begin. These are known as brown dwarfs, the International Astronomical Union defines brown dwarfs as stars massive enough to fuse deuterium at some point in their lives. Objects smaller than 13 MJ are classified as sub-brown dwarfs, both types, deuterium-burning and not, shine dimly and die away slowly, cooling gradually over hundreds of millions of years