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By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun.
The oldest accurately dated star chart was the result of ancient Egyptian astronomy in BC. The first star catalogue in Greek astronomy was created by Aristillus in approximately BC, with the help of Timocharis.
In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear.
Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars.
They built the first large observatory research institutes, mainly for the purpose of producing Zij star catalogues. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky Way galaxy as a multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in In , Giordano Bruno suggested that the stars were like the Sun, and may have other planets , possibly even Earth-like, in orbit around them,  an idea that had been suggested earlier by the ancient Greek philosophers , Democritus and Epicurus ,  and by medieval Islamic cosmologists  such as Fakhr al-Din al-Razi.
To explain why these stars exerted no net gravitational pull on the Solar System, Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.
The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in Edmond Halley published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions since the time of the ancient Greek astronomers Ptolemy and Hipparchus.
William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the s, he established a series of gauges in directions and counted the stars observed along each line of sight.
From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core. His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction.
The science of stellar spectroscopy was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines —the dark lines in stellar spectra caused by the atmosphere's absorption of specific frequencies.
In , Secchi began classifying stars into spectral types. Cannon during the s. The first direct measurement of the distance to a star 61 Cygni at Parallax measurements demonstrated the vast separation of the stars in the heavens.
In , Friedrich Bessel observed changes in the proper motion of the star Sirius and inferred a hidden companion.
Edward Pickering discovered the first spectroscopic binary in when he observed the periodic splitting of the spectral lines of the star Mizar in a day period.
Detailed observations of many binary star systems were collected by astronomers such as Friedrich Georg Wilhelm von Struve and S.
Burnham , allowing the masses of stars to be determined from computation of orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in The photograph became a valuable astronomical tool.
Karl Schwarzschild discovered that the color of a star and, hence, its temperature, could be determined by comparing the visual magnitude against the photographic magnitude.
The development of the photoelectric photometer allowed precise measurements of magnitude at multiple wavelength intervals. In Albert A.
Michelson made the first measurements of a stellar diameter using an interferometer on the Hooker telescope at Mount Wilson Observatory. Important theoretical work on the physical structure of stars occurred during the first decades of the twentieth century.
In , the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution.
Cecilia Payne-Gaposchkin first proposed that stars were made primarily of hydrogen and helium in her PhD thesis. This allowed the chemical composition of the stellar atmosphere to be determined.
With the exception of supernovae, individual stars have primarily been observed in the Local Group ,  and especially in the visible part of the Milky Way as demonstrated by the detailed star catalogues available for our galaxy.
However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located at a distance of one billion light years  —ten times further than the most distant star cluster previously observed.
In February , astronomers reported, for the first time, a signal of the reionization epoch, an indirect detection of light from the earliest stars formed - about million years after the Big Bang.
In April, , astronomers reported the detection of the most distant "ordinary" i. In May , astronomers reported the detection of the most distant oxygen ever detected in the Universe - and the most distant galaxy every observed by Atacama Large Millimeter Array or the Very Large Telescope - with the team inferring that the signal was emitted They found that the observed brightness of the galaxy is well-explained by a model where the onset of star formation corresponds to only million years after the Universe began, corresponding to a redshift of about The concept of a constellation was known to exist during the Babylonian period.
Ancient sky watchers imagined that prominent arrangements of stars formed patterns, and they associated these with particular aspects of nature or their myths.
Twelve of these formations lay along the band of the ecliptic and these became the basis of astrology. As well as certain constellations and the Sun itself, individual stars have their own myths.
Their names were assigned by later astronomers. Circa , the names of the constellations were used to name the stars in the corresponding regions of the sky.
The German astronomer Johann Bayer created a series of star maps and applied Greek letters as designations to the stars in each constellation.
Later a numbering system based on the star's right ascension was invented and added to John Flamsteed 's star catalogue in his book "Historia coelestis Britannica" the edition , whereby this numbering system came to be called Flamsteed designation or Flamsteed numbering.
The only internationally recognized authority for naming celestial bodies is the International Astronomical Union IAU. A number of private companies sell names of stars, which the British Library calls an unregulated commercial enterprise.
This now-discontinued ISR practice was informally labeled a scam and a fraud,     and the New York City Department of Consumer Affairs issued a violation against ISR for engaging in a deceptive trade practice.
Although stellar parameters can be expressed in SI units or CGS units , it is often most convenient to express mass , luminosity , and radii in solar units, based on the characteristics of the Sun.
In , the IAU defined a set of nominal solar values defined as SI constants, without uncertainties which can be used for quoting stellar parameters:.
However, one can combine the nominal solar mass parameter with the most recent CODATA estimate of the Newtonian gravitational constant G to derive the solar mass to be approximately 1.
Although the exact values for the luminosity, radius, mass parameter, and mass may vary slightly in the future due to observational uncertainties, the IAU nominal constants will remain the same SI values as they remain useful measures for quoting stellar parameters.
Large lengths, such as the radius of a giant star or the semi-major axis of a binary star system, are often expressed in terms of the astronomical unit — approximately equal to the mean distance between the Earth and the Sun million km or approximately 93 million miles.
In , the IAU defined the astronomical constant to be an exact length in meters: Stars condense from regions of space of higher matter density, yet those regions are less dense than within a vacuum chamber.
These regions — known as molecular clouds — consist mostly of hydrogen, with about 23 to 28 percent helium and a few percent heavier elements.
One example of such a star-forming region is the Orion Nebula. Such feedback effects, from star formation, may ultimately disrupt the cloud and prevent further star formation.
All stars spend the majority of their existence as main sequence stars , fueled primarily by the nuclear fusion of hydrogen into helium within their cores.
However, stars of different masses have markedly different properties at various stages of their development. The ultimate fate of more massive stars differs from that of less massive stars, as do their luminosities and the impact they have on their environment.
Accordingly, astronomers often group stars by their mass: The formation of a star begins with gravitational instability within a molecular cloud, caused by regions of higher density — often triggered by compression of clouds by radiation from massive stars, expanding bubbles in the interstellar medium, the collision of different molecular clouds, or the collision of galaxies as in a starburst galaxy.
As the cloud collapses, individual conglomerations of dense dust and gas form " Bok globules ".
As a globule collapses and the density increases, the gravitational energy converts into heat and the temperature rises.
When the protostellar cloud has approximately reached the stable condition of hydrostatic equilibrium , a protostar forms at the core.
The period of gravitational contraction lasts about 10 to 15 million years. These newly formed stars emit jets of gas along their axis of rotation, which may reduce the angular momentum of the collapsing star and result in small patches of nebulosity known as Herbig—Haro objects.
Early in their development, T Tauri stars follow the Hayashi track —they contract and decrease in luminosity while remaining at roughly the same temperature.
Less massive T Tauri stars follow this track to the main sequence, while more massive stars turn onto the Henyey track. Most stars are observed to be members of binary star systems, and the properties of those binaries are the result of the conditions in which they formed.
The fragmentation of the cloud into multiple stars distributes some of that angular momentum. The primordial binaries transfer some angular momentum by gravitational interactions during close encounters with other stars in young stellar clusters.
These interactions tend to split apart more widely separated soft binaries while causing hard binaries to become more tightly bound.
This produces the separation of binaries into their two observed populations distributions. Such stars are said to be on the main sequence , and are called dwarf stars.
Starting at zero-age main sequence, the proportion of helium in a star's core will steadily increase, the rate of nuclear fusion at the core will slowly increase, as will the star's temperature and luminosity.
Every star generates a stellar wind of particles that causes a continual outflow of gas into space. For most stars, the mass lost is negligible.
The time a star spends on the main sequence depends primarily on the amount of fuel it has and the rate at which it fuses it. The Sun is expected to live 10 billion 10 10 years.
Massive stars consume their fuel very rapidly and are short-lived. Low mass stars consume their fuel very slowly.
Stars less massive than 0. The combination of their slow fuel-consumption and relatively large usable fuel supply allows low mass stars to last about one trillion 10 12 years; the most extreme of 0.
Red dwarfs become hotter and more luminous as they accumulate helium. When they eventually run out of hydrogen, they contract into a white dwarf and decline in temperature.
Besides mass, the elements heavier than helium can play a significant role in the evolution of stars. Astronomers label all elements heavier than helium "metals", and call the chemical concentration of these elements in a star, its metallicity.
A star's metallicity can influence the time the star takes to burn its fuel, and controls the formation of its magnetic fields,  which affects the strength of its stellar wind.
Over time, such clouds become increasingly enriched in heavier elements as older stars die and shed portions of their atmospheres.
As stars of at least 0. Their outer layers expand and cool greatly as they form a red giant. As the hydrogen shell burning produces more helium, the core increases in mass and temperature.
In a red giant of up to 2. Finally, when the temperature increases sufficiently, helium fusion begins explosively in what is called a helium flash , and the star rapidly shrinks in radius, increases its surface temperature, and moves to the horizontal branch of the HR diagram.
For more massive stars, helium core fusion starts before the core becomes degenerate, and the star spends some time in the red clump , slowly burning helium, before the outer convective envelope collapses and the star then moves to the horizontal branch.
After the star has fused the helium of its core, the carbon product fuses producing a hot core with an outer shell of fusing helium.
The star then follows an evolutionary path called the asymptotic giant branch AGB that parallels the other described red giant phase, but with a higher luminosity.
The more massive AGB stars may undergo a brief period of carbon fusion before the core becomes degenerate. During their helium-burning phase, a star of more than 9 solar masses expands to form first a blue and then a red supergiant.
Particularly massive stars may evolve to a Wolf-Rayet star , characterised by spectra dominated by emission lines of elements heavier than hydrogen, which have reached the surface due to strong convection and intense mass loss.
When helium is exhausted at the core of a massive star, the core contracts and the temperature and pressure rises enough to fuse carbon see Carbon-burning process.
This process continues, with the successive stages being fueled by neon see neon-burning process , oxygen see oxygen-burning process , and silicon see silicon-burning process.
Near the end of the star's life, fusion continues along a series of onion-layer shells within a massive star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.
The final stage occurs when a massive star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy.
To a very limited degree such a process proceeds, but it consumes energy. Likewise, since they are more tightly bound than all lighter nuclei, such energy cannot be released by fission.
As a star's core shrinks, the intensity of radiation from that surface increases, creating such radiation pressure on the outer shell of gas that it will push those layers away, forming a planetary nebula.
If what remains after the outer atmosphere has been shed is less than 1. White dwarfs lack the mass for further gravitational compression to take place.
Eventually, white dwarfs fade into black dwarfs over a very long period of time. In massive stars, fusion continues until the iron core has grown so large more than 1.
This core will suddenly collapse as its electrons are driven into its protons, forming neutrons, neutrinos, and gamma rays in a burst of electron capture and inverse beta decay.
The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae become so bright that they may briefly outshine the star's entire home galaxy.
When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none seemingly existed before.
A supernova explosion blows away the star's outer layers, leaving a remnant such as the Crab Nebula. Within a black hole, the matter is in a state that is not currently understood.
The blown-off outer layers of dying stars include heavy elements, which may be recycled during the formation of new stars.
These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.
The post—main-sequence evolution of binary stars may be significantly different from the evolution of single stars of the same mass. If stars in a binary system are sufficiently close, when one of the stars expands to become a red giant it may overflow its Roche lobe , the region around a star where material is gravitationally bound to that star, leading to transfer of material to the other.
When the Roche lobe is violated, a variety of phenomena can result, including contact binaries , common-envelope binaries, cataclysmic variables , and type Ia supernovae.
Stars are not spread uniformly across the universe, but are normally grouped into galaxies along with interstellar gas and dust. A typical galaxy contains hundreds of billions of stars, and there are more than billion 10 11 galaxies in the observable universe.
A multi-star system consists of two or more gravitationally bound stars that orbit each other. The simplest and most common multi-star system is a binary star, but systems of three or more stars are also found.
For reasons of orbital stability, such multi-star systems are often organized into hierarchical sets of binary stars.
These range from loose stellar associations with only a few stars, up to enormous globular clusters with hundreds of thousands of stars.
Such systems orbit their host galaxy. It has been a long-held assumption that the majority of stars occur in gravitationally bound, multiple-star systems.
The nearest star to the Earth, apart from the Sun, is Proxima Centauri , which is Travelling at the orbital speed of the Space Shuttle 8 kilometres per second—almost 30, kilometres per hour , it would take about , years to arrive.
Due to the relatively vast distances between stars outside the galactic nucleus, collisions between stars are thought to be rare.
In denser regions such as the core of globular clusters or the galactic center, collisions can be more common. These abnormal stars have a higher surface temperature than the other main sequence stars with the same luminosity of the cluster to which it belongs.
Almost everything about a star is determined by its initial mass, including such characteristics as luminosity, size, evolution, lifespan, and its eventual fate.
Most stars are between 1 billion and 10 billion years old. Some stars may even be close to The oldest star yet discovered, HD , nicknamed Methuselah star, is an estimated The more massive the star, the shorter its lifespan, primarily because massive stars have greater pressure on their cores, causing them to burn hydrogen more rapidly.
The most massive stars last an average of a few million years, while stars of minimum mass red dwarfs burn their fuel very slowly and can last tens to hundreds of billions of years.
Typically the portion of heavy elements is measured in terms of the iron content of the stellar atmosphere, as iron is a common element and its absorption lines are relatively easy to measure.
The portion of heavier elements may be an indicator of the likelihood that the star has a planetary system. Due to their great distance from the Earth, all stars except the Sun appear to the unaided eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere.
The Sun is also a star, but it is close enough to the Earth to appear as a disk instead, and to provide daylight.
Other than the Sun, the star with the largest apparent size is R Doradus , with an angular diameter of only 0. The disks of most stars are much too small in angular size to be observed with current ground-based optical telescopes, and so interferometer telescopes are required to produce images of these objects.
Another technique for measuring the angular size of stars is through occultation. By precisely measuring the drop in brightness of a star as it is occulted by the Moon or the rise in brightness when it reappears , the star's angular diameter can be computed.
The motion of a star relative to the Sun can provide useful information about the origin and age of a star, as well as the structure and evolution of the surrounding galaxy.
The components of motion of a star consist of the radial velocity toward or away from the Sun, and the traverse angular movement, which is called its proper motion.
The proper motion of a star, its parallax , is determined by precise astrometric measurements in units of milli- arc seconds mas per year.
With knowledge of the star's parallax and its distance, the proper motion velocity can be calculated. Together with the radial velocity, the total velocity can be calculated.
Stars with high rates of proper motion are likely to be relatively close to the Sun, making them good candidates for parallax measurements.
When both rates of movement are known, the space velocity of the star relative to the Sun or the galaxy can be computed.
Among nearby stars, it has been found that younger population I stars have generally lower velocities than older, population II stars.
The latter have elliptical orbits that are inclined to the plane of the galaxy. The magnetic field of a star is generated within regions of the interior where convective circulation occurs.
This movement of conductive plasma functions like a dynamo , wherein the movement of electrical charges induce magnetic fields, as does a mechanical dynamo.
Those magnetic fields have a great range that extend throughout and beyond the star. The strength of the magnetic field varies with the mass and composition of the star, and the amount of magnetic surface activity depends upon the star's rate of rotation.
This surface activity produces starspots , which are regions of strong magnetic fields and lower than normal surface temperatures. Coronal loops are arching magnetic field flux lines that rise from a star's surface into the star's outer atmosphere, its corona.
The coronal loops can be seen due to the plasma they conduct along their length. Stellar flares are bursts of high-energy particles that are emitted due to the same magnetic activity.
Young, rapidly rotating stars tend to have high levels of surface activity because of their magnetic field. The magnetic field can act upon a star's stellar wind, functioning as a brake to gradually slow the rate of rotation with time.
Thus, older stars such as the Sun have a much slower rate of rotation and a lower level of surface activity. The activity levels of slowly rotating stars tend to vary in a cyclical manner and can shut down altogether for periods of time.
This generation of supermassive population III stars is likely to have existed in the very early universe i. The combination of the radius and the mass of a star determines its surface gravity.
Giant stars have a much lower surface gravity than do main sequence stars, while the opposite is the case for degenerate, compact stars such as white dwarfs.
The surface gravity can influence the appearance of a star's spectrum, with higher gravity causing a broadening of the absorption lines.
The rotation rate of stars can be determined through spectroscopic measurement , or more exactly determined by tracking their starspots.
Degenerate stars have contracted into a compact mass, resulting in a rapid rate of rotation. However they have relatively low rates of rotation compared to what would be expected by conservation of angular momentum —the tendency of a rotating body to compensate for a contraction in size by increasing its rate of spin.
A large portion of the star's angular momentum is dissipated as a result of mass loss through the stellar wind. The pulsar at the heart of the Crab nebula , for example, rotates 30 times per second.
The surface temperature of a main sequence star is determined by the rate of energy production of its core and by its radius, and is often estimated from the star's color index.
Note that the effective temperature is only a representative of the surface, as the temperature increases toward the core. The stellar temperature will determine the rate of ionization of various elements, resulting in characteristic absorption lines in the spectrum.
The surface temperature of a star, along with its visual absolute magnitude and absorption features, is used to classify a star see classification below.
Smaller stars such as the Sun have surface temperatures of a few thousand K. The energy produced by stars, a product of nuclear fusion, radiates to space as both electromagnetic radiation and particle radiation.
The particle radiation emitted by a star is manifested as the stellar wind,  which streams from the outer layers as electrically charged protons and alpha and beta particles.
Although almost massless, there also exists a steady stream of neutrinos emanating from the star's core. The production of energy at the core is the reason stars shine so brightly: This energy is converted to other forms of electromagnetic energy of lower frequency, such as visible light, by the time it reaches the star's outer layers.
The color of a star, as determined by the most intense frequency of the visible light, depends on the temperature of the star's outer layers, including its photosphere.
In fact, stellar electromagnetic radiation spans the entire electromagnetic spectrum , from the longest wavelengths of radio waves through infrared , visible light, ultraviolet , to the shortest of X-rays , and gamma rays.
From the standpoint of total energy emitted by a star, not all components of stellar electromagnetic radiation are significant, but all frequencies provide insight into the star's physics.
Using the stellar spectrum , astronomers can also determine the surface temperature, surface gravity , metallicity and rotational velocity of a star.
If the distance of the star is found, such as by measuring the parallax, then the luminosity of the star can be derived.
The mass, radius, surface gravity, and rotation period can then be estimated based on stellar models. Mass can be calculated for stars in binary systems by measuring their orbital velocities and distances.
Gravitational microlensing has been used to measure the mass of a single star. The luminosity of a star is the amount of light and other forms of radiant energy it radiates per unit of time.
It has units of power. The luminosity of a star is determined by its radius and surface temperature. Many stars do not radiate uniformly across their entire surface.
The rapidly rotating star Vega , for example, has a higher energy flux power per unit area at its poles than along its equator.
Patches of the star's surface with a lower temperature and luminosity than average are known as starspots.
Small, dwarf stars such as our Sun generally have essentially featureless disks with only small starspots.
Giant stars have much larger, more obvious starspots,  and they also exhibit strong stellar limb darkening. That is, the brightness decreases towards the edge of the stellar disk.