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A topology for a computer network in which one computer occupies the role of a central node and all the remaining computers are linked solely to that central node. Communication between any two of the computers must pass through the central node.

A sphere of matter held together entirely by its own gravitational field, and generating energy by means of nuclear fusion reactions in its deep interior. The important distinguishing feature of a star is the presence of a natural nuclear reactor in its core, where the pressure of the overlying mass of material is sufficient to cause nuclear reactions, the principal one of which is the conversion of hydrogen to helium. About

5% of the mass becomes electromagnetic radiation. The minimum mass needed to make a star is c. 7% the mass of the Sun; the maximum about 100 times as great as the Sun.
A star is a massive, compact body of plasma in outer space that is held together by its own gravity and, unlike a planet, is sufficiently massive to sustain nuclear fusion in a very dense, hot core region. This fusion of atomic nuclei generates the energy that is continuously radiated from the outer layers of the star during much of its life span.

Individual stars differ in their total mass, composition, and age. Initially, stars are composed primarily of hydrogen, with some helium and heavier trace elements that determine their metallicity. Part of the matter is then recycled into the interstellar environment and used to form a new generation of more metal-rich stars.

Binary and multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution. For example, a nova occurs when a white dwarf accretes matter from a companion star. The Gregorian calendar, used nearly everywhere in the world, is a solar calendar based on the position of the Earth relative to the nearest star, the Sun.

Early astronomers such as Tycho Brahe identified new stars in the heavens, suggesting that the heavens were not immutable. In 1584 Giordano Bruno suggested that the stars were actually other suns, and may have Earth-like planets in orbit around them. By the following century the idea of the stars as distant suns was reaching a consensus among astronomers, and it would be the theologian Richard Bentley who would prompt Isaac Newton to suggest that the stars were equally distributed in every direction, resulting in no net gravitational pull.

The Italian astronomer Geminiano Montanari recorded seeing variability in the star Algol 1667. Edmond Halley would then publish the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed position from the time of the ancient Greek astronomers Ptolemy and Hipparchus. But it would not be until 1838 that the first direct measurement of the distance to the star 61 Cygni was made by Friedrich Bessel using the parallax technique. Parallax measurements demonstrated the vast separation of the stars in the heavens.

Star designations

The concept of the constellation arose as far back in history as Babylonian period. Many of the more prominent individual stars were also given names, particularly with Arabic or Latin designations.

As well as certain constellations and the Sun itself, stars as a whole have their own myths. They were thought to be the souls of the dead or gods. An example of this is the star Algol, which was thought to represents the eye of the Gorgon Medusa.

To the Ancient Greeks, some "stars", later identified as planets, represented various important deities, from which the names of the planets Mercury, Venus, Mars, Jupiter and Saturn were taken. (Uranus and Neptune were also Greek and Roman gods, but neither planet was known in Antiquity because of their low brightness. The German astronomer Johann Bayer created a series of star maps and applied greek letters as designations to the
stars in each constellation. Numerous additional systems have since been created as star catalogues appeared.

The only body which has been recognized by the scientific community as having the authority to name stars or other celestial bodies is the International Astronomical Union (IAU). A number of private companies (for instance, the "International Star Registry") purport to sell names to stars; however, these names are neither recognized by the scientific community nor used by them, and many in the astronomy community view these organizations as frauds preying on people ignorant of how stars are in fact named.

Units of measurement

Most stellar parameters are expressed in SI units by convention, but CGS units are also used (e.g., expressing luminosity in ergs per second). Mass, luminosity, and radii are usually given in solar units, based on the characteristics of the Sun:

solar mass:	M_bigodot = 1.9891 times 10^{30} kg
solar luminosity:	L_bigodot = 3.827 times 10^{26} watts
solar radius:	R_bigodot = 6.960 times 10^{8}m

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 (AU) — One example of such a star-forming nebula is the Orion Nebula. As massive stars are formed from these clouds, they powerfully illuminate the clouds from which they formed, creating an H II region.

Protostar formation

The formation of a star begins with a gravitational instability inside a molecular cloud, often triggered by shockwaves from supernovae or the collision of two galaxies (as in a starburst galaxy). Once a region reaches a sufficient density of matter to satisfy the criteria for Jeans Instability it begins to collapse under its own gravitational force.

As the cloud collapses, individual Bok globules form with up to 50 solar masses of material. When the protostellar cloud has approximately reached hydrostatic equilibrium, a protostar forms at the core. These pre-main sequence stars are often surrounded by a protoplanetary disk. The protostar then follows a
Hayashi track on the Hertzsprung-Russell diagram. The contraction will proceed until the Hayashi boundary is reached, and thereafter contraction will continue on a Kelvin-Helmholtz timescale with the temperature remaining stable. For more massive protostars, at the end of the Hayashi track they will slowly collapse in near hydrostatic equilibrium, following the Henyey track. The period of gravitational contraction lasts for about 10-15 million years.

Early stars of less than 2 solar masses are called T Tauri stars, while those with greater mass are Herbig Ae/Be stars. These newly-born stars emit jets of gas along their axis of rotation, producing small patches of nebulosity known as Herbig-Haro objects.

Main sequence

Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high-temperature and high-pressure reactions near the core. As a consequence, in order to maintain the required rate of nuclear fusion at the core, the star will slowly increase in temperature and luminosity. The Sun, for example, is estimated to have increased in luminosity by about 40% since it reached the main sequence 4.6 billion years ago.

Every star generates a stellar wind of particles that causes a continual outflow of gas into space. Consequently the total main sequence lifetime of a star can be estimated from its mass relative to the Sun's as follows:

tau_{ms} sim 10^{10} cdot left frac{M}{M_bigodot} right ^{-2.5} years

where M is the mass of the star and tau_{ms} is the star's estimated main sequence lifetime in years.

Large stars burn their fuel very rapidly and are short-lived. At the end of their lives, they simply become dimmer and dimmer, fading into black dwarfs. However, since the lifespan of such stars is greater than the current age of the universe (13.7 billion years), no black dwarfs exist yet.

Besides mass, the portion of elements heavier than helium can play a significant role in the evolution of stars. control the formation of magnetic fields, and modifies the strength of the stellar wind. Older, population II stars have substantially less metallicity than the younger, population I stars due to the composition of the molecular clouds from which they formed. This process continues, with the successive stages being fueled by oxygen, neon, silicon, and sulfur. The core that remains will be a tiny ball of electron degenerate matter not massive enough for further compression to take place, supported only by degeneracy pressure, called a white dwarf. These too will fade into brown, and then black dwarfs over a very long stretch of time. Electron degenerate matter is not plasma, even though stars are generally referred to as being spheres of plasma.

In larger stars, defined as having more than 1.4 solar masses after explosion, fusion continues until an iron core accumulates that is too large to be supported by electron degeneracy pressure. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none existed before.

Eventually, most of the matter in a star is blown away by the supernovae explosion (forming nebulae such as the Crab Nebula) and what remains will be a neutron star (sometimes a pulsar or X-ray burster) or, in the case of the largest stars (more than 3 solar masses after explosion), a black hole. In neutron stars and black holes, the star is not in a plasma state of matter, but either neutron degenerate matter or a state of matter not currently understood within the black hole.

The blown-off outer layers of dying stars include heavy elements which may be recycled during new star formation. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.

Appearance and distribution

Due to their great distance from the Earth, all stars except the Sun appear to the human eye as shining points in the night sky that twinkle because of the effect of the Earth's atmosphere. The disks of stars are much too small in angular size to be observed with current ground-based optical telescopes, and so Interferometer telescopes are required in order to produce images of these objects. Other than the Sun, the star with the largest apparent size is R Doradus, with an angular diameter of only 0.057 arcseconds.

It has been a long-held assumption that the majority of stars occur in gravitationally-bound, multiple-star systems, forming binary stars. 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 100 billion (10¹¹) galaxies in the observable universe.

Astronomers estimate that there are at least 70 sextillion (7×10²²) stars in the known universe. That is 230 billion times as many as the 300 billion in our own Milky Way.

The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion (10¹²) kilometres, or 4.2 light-years away. 10⁵ years. Distances like this are typical inside galactic discs, where the solar system is located. Stars can be much closer to each other in the centres of galaxies and in globular clusters, or much farther apart in galactic halos. These abnormal stars appear on a different part of the evolutionary track of the HR-diagram, effectively forming a merged star that has a higher surface temperature than the other main sequence stars in the cluster with the same luminosity.


Small, dwarf stars such as the Sun generally have essentially featureless disks with only small starspots. That is, the brightness decreases towards the edge of the stellar disk. Red dwarf flare stars such as UV Ceti may also possess prominent starspot features.

Age and size

Almost everything about a star is determined by its initial mass, including its destiny and fate, as well as its essential characteristics, such as lifespan, luminosity, and size. Stars range in size from neutron stars no bigger than a city to supergiants like Betelgeuse in the Orion constellation, which has a diameter about 1,000 times larger than the Sun—about 1.6 billion kilometres. However, Betelgeuse has a much lower density than the Sun.

Many stars are between 1 billion and 10 billion years old. the observed age of the universe. (See Big Bang theory and stellar evolution.) 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 about one million years, while stars of minimum mass (red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years.

Most of our understanding of stars comes from theoretical models and simulations based on spectral observations and measurements of the diameters of stars. The first measurement of the diameter of a star other than the Sun was made in 1921 by Albert Abraham Michelson on the Hooker telescope.

One of the most massive stars known is Eta Carinae, with 100–150 times as much mass as the Sun; A recent study of the Arches cluster suggests that 150 solar masses is the upper limit for stars in the current era of the universe. The reason for this limit is not precisely known, but is partially due to Eddington luminosity.

The first stars to form after the Big Bang may have been larger, up to 300 solar masses or more, due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive, population III stars is long extinct, however, and currently only theoretical.

With a mass only 93 times that of Jupiter, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core. For stars with similar metallicity to the Sun, the theoretical minimum mass the star can have, and still undergo fusion at the core, is estimated to be about 75 times the mass of Jupiter. When the metallicity is very low, however, a recent study of the faintest stars found that the minimum star size seems to be about 8.3% of the solar mass, or about 87 times the mass of Jupiter. Smaller bodies are called brown dwarfs, which occupy a poorly-defined grey area between stars and gas giants.

Radiation

The energy produced by stars, as a by-product of nuclear fusion, radiates into space as both electromagnetic radiation and particle radiation. The particle radiation emitted by a star is manifested as the stellar wind (which exists as a steady stream of electrically charged particles, such as free protons, alpha particles, and beta particles, emanating from the star?s outer layers) and as a steady stream of neutrinos emanating from the star?s core.

The production of energy at the core is the reason why stars shine so brightly: every time two or more atomic nuclei of one element fuse together to form an atomic nucleus of a new heavier element deep inside the core of a star, photons of electromagnetic energy are released from the nuclear fusion reaction, which are then converted to visible light in the star?s outer layers.

The peak frequency and color of the visible light depends on the temperature of the star?s outer layers, including its photosphere. Besides visible light, stars also emit forms of electromagnetic radiation that are invisible to the human eye. In fact, stellar electromagnetic radiation spans across the entire electromagnetic spectrum, from the longest wavelengths of radio waves and infrared to the shortest wavelengths of ultraviolet, X-rays, and gamma rays. All components of stellar electromagnetic radiation, both visible and invisible, are typically significant.

Using the stellar spectrum, astronomers can also determine the surface temperature, surface gravity, metallicity and rotation velocity of a star. The technique of gravitational microlensing will also yield the mass of a star.) With these parameters, astronomers can also estimate the age of the star.

Luminosity

In astronomy, luminosity is the amount of light, and other forms of radiant energy, a star radiates per unit of time. The luminosity of a star can be approximated by treating the emitted energy as a black body radiation. So:

L = 4 pi R^2 sigma T_{e}^4

where L is the luminosity, ? is the Stefan-Boltzmann constant, R is the stellar radius and T is the effective temperature. This same formula can be used to compute the approximate radius of a main sequence star relative to the sun:

frac{R}{R_bigodot} approx left ( frac{T_bigodot}{T} right )^{2} cdot sqrt{frac{L}{L_bigodot}}

Magnitude

The apparent brightness of a star is measured by its apparent magnitude, which is the brightness of a star with respect to the star?s luminosity, distance from Earth, and the altering of the star?s light as it passes through Earth?s atmosphere.

Number of stars brighter than magnitude

Apparent magnitude	Number  of Stars[[cite web	url = www.nso.edu/PR/answerbook/magnitude.html	title = Magnitude	publisher = National Solar Observatory—Sacramento Peak	language = English	accessdate = 2006-08-23]]
0	4
1	15
2	48
3	171
4	513
5	1,602
6	4,800
7	14,000

Intrinsic or absolute magnitude is what the apparent magnitude a star would be if the distance between the Earth and the star were 10 parsecs (32.6 light-years), and it is directly related to a star?s luminosity, measured from the standard distance of 10 parsecs.

Both the apparent and absolute magnitude scales are logarithmic units: one whole number difference in magnitude is equal to a brightness variation of about 2.5 times (the 5th root of 100 or 2.512 to be precise). This means that a first magnitude (+1.00) star is about 2.5 times brighter than a second magnitude (+2.00) star, and approximately 100 times brighter than a sixth magnitude (+6.00) star, which is the faintest star visible to the naked eye.

On both apparent and absolute magnitude scales, the smaller the magnitude number, the brighter the star; Sirius, the brightest star in the night sky, is approximately 23 times more luminous than our Sun, while Canopus, the second brightest star in the night sky, with an absolute magnitude of -5.53, is approximately 14,000 times more luminous than our Sun. There, the faintest red dwarf star was found, with a magnitude of 26, and a white dwarf of the 28th magnitude. These range from 0 (hypergiants) through III (giants) to V (main sequence dwarfs) and VII (white dwarfs). These fall along a narrow band when graphed according to their absolute magnitude and spectral type. Our Sun is a main sequence G2V (yellow dwarf), being of intermediate temperature and ordinary size.

Additional nomenclature, in the form of lower-case letters, can follow the spectral type to indicate peculiar features of the spectrum. This category includes Cepheid and cepheid-like stars, and long-period variables such as Mira.

Eruptive variables are stars that experience sudden increases in luminosity because of flares or mass ejection events. This group includes protostars, Wolf-Rayet stars, and Flare stars, as well as giant and supergiant stars.

Cataclysmic or explosive variables undergo a dramatic change in their properties. A binary star system that includes a nearby white dwarf
can produce certain types of these spectacular stellar explosions, including the nova and a Type 1a supernova. The explosion is created when the white dwarf accretes hydrogen from the companion star, building up mass until the hydrogen undergoes fusion. Some novae are also recurrent, having periodic outbursts of moderate amplitude.

Stars can also vary in luminosity because of extrinsic factors, such as eclipsing binaries, as well as rotating stars that produce extreme starspots. A notable example of an eclipsing binary is Algol, which regularly varies in magnitude from 2.3 to 3.5 over a period of 2.87 days. Smaller stars such as the Sun are just the opposite, with the convective zone located in the outer layers. The convective zones will also vary over time as the star ages and the constitution of the interior is modified.

The portion of a main sequence star that is visible to an observer is called the photosphere. It is within the photosphere that star spots, or regions of lower than average temperature, appear.

Above the level of the photosphere is the stellar atmosphere. In a main sequence star such as the Sun, the lowest level of the atmosphere is the thin chromosphere region, where spicules appear and stellar flares begin. This lost mass is converted into energy, according to the mass-energy relationship E=mc².

In the Sun, with a 10⁷ °K core, hydrogen fuses to form helium in the proton-proton chain reaction:

4 ¹H ? 2 ²H + 2 e⁺ + 2 ?_e (4.0 M eV + 1.0 MeV)

2¹H + 2²H ? + 2?_e (26.7 MeV)

In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon, the carbon-nitrogen-oxygen cycle.

In stars with cores at 10⁸ K and masses between 0.5 and 10 solar masses, helium can be transformed into carbon in the triple-alpha process:

⁴He + ⁴He + 92 keV ? + 7.2 MeV

In massive stars, heavier elements can also be burned in a contracting core through the Neon burning process and Oxygen burning process. As an O-class main sequence star, it would be 8 times the solar radius and 62,000 times the Sun's luminosity.

Fuel material	Temperature (million Kelvin)	Density (kg/cm3)	Burn duration τ
H	37	0.0045	8.1 million years
He	188	0.97	1.2 million years
C	870	170	976 years
Ne	1,570	3,100	0.6 years
O	1,980	5,550	1.25 years
S/Si	3,340	33,400	11.5 days

References