Star

:This article is about celestial bodies. {{otheruses}}

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A star is a massive body of plasma in outer space which is currently producing or has produced energy through nuclear fusion. Unlike a planet, from which most light is reflected, a star emits light because of its intense heat. Scientifically, stars are defined as self-gravitating spheres of plasma in hydrostatic equilibrium, which generate their own energy through the process of nuclear fusion. Small (dwarf) stars such as the Sun generally have essentially featureless disks with only small starspots. Larger (giant) stars have much bigger, much more obvious starspots, and also exhibit strong stellar limb-darkening (the brightness decreases towards the edge of the stellar disk). Stellar astronomy is the study of stars.

Related Topics:
Plasma - Nuclear fusion - Planet - Heat - Self-gravitating - Hydrostatic equilibrium - Energy - Sun - Starspots - Stellar limb-darkening - Stellar astronomy

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All stars except the Sun appear to the human eye as shining points in the nighttime sky that twinkle because of the effect of the Earth's atmosphere. Interferometer telescopes are required in order to produce images of these objects. The Sun is also a star, but it is close enough to Earth to appear as a disk instead, and to provide daylight.

Related Topics:
Sun - Twinkle because of the effect of the Earth's atmosphere - Interferometer - Daylight

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Stars are not spread uniformly across the universe, but are typically grouped into galaxies. A typical galaxy contains hundreds of billions of stars.The majority of stars are gravitationally bound to other stars, forming binary stars. Larger groups called star clusters also exist.

Related Topics:
Universe - Galaxies - Billion - Binary star - Star cluster

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Astronomers estimate that there are at least 70 sextillion (7×1022) stars in the known universe http://news.bbc.co.uk/2/hi/science/nature/3085885.stm. That is 70 000 000 000 000 000 000 000, or 230 billion times as much as the 300 billion in our own Milky Way.

Related Topics:
Sextillion - Known universe - Billion - Milky Way

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Many stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old, which is the observed age of the universe. (See Big Bang theory and stellar evolution.) They range in size from the tiny neutron stars (which are actually dead stars) no bigger than a city, to supergiants like the North Star (Polaris) and Betelgeuse, in the Orion constellation, which have a diameter about 1,000 times larger than the Sun —about 1.6 terametres. However, these have a much lower density than the Sun.

Related Topics:
Big Bang theory - Stellar evolution - Neutron star - Supergiant - North Star - Betelgeuse - Orion - Constellation - Terametre - Sun

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One of the most massive stars known is η Carinae, with 100–150 times as much mass as the Sun. Recent work by Donald Figer, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, suggests that 150 solar masses is the upper limit of stars in the current era of the universe. He used the Hubble Space Telescope to observe about a thousand stars in the Arches cluster, a massive young star cluster near the core of the Milky Way, and found no stars over that limit despite a statistical expectation that there should be several. The reason for this limit is not precisely known, but the Eddington limit is part of the answer. The very 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 star is long extinct, however, and currently only theoretical.

Related Topics:
η Carinae - Donald Figer - Space Telescope Science Institute - Baltimore, Maryland - Hubble Space Telescope - Arches cluster - Star cluster - Core of the Milky Way - Eddington limit - Lithium

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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. Smaller bodies are brown dwarfs, which occupy a poorly-defined grey area between stars and gas giants. The minimum mass a star can have is estimated to be in the vicinity of 75 Jupiters.

Related Topics:
Jupiter - AB Doradus C - Brown dwarf - Gas giant

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The energy produced by stars radiates into space as electromagnetic radiation, as a stream of neutrinos from the star's core, and as a stream of particles from the star's outer layers (its stellar wind). The peak frequency of the light depends on the temperature of the outer layers of the star. Besides the emitted visible light, the ultraviolet and infrared components are typically significant. The apparent brightness of a star is measured by its apparent magnitude.

Related Topics:
Radiates - Electromagnetic radiation - Neutrino - Stellar wind - Brightness - Measured - Apparent magnitude

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The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometres, or 4.2 light years away (light from Proxima Centauri takes 4.2 years to reach Earth). Travelling at the orbit speed of the Space Shuttle (5 miles per second -- almost 30,000 kilometers per hour), it would take about 150,000 years to get there. Distances like this are typical inside galactic discs, where the Sun and Earth are located. Stars can be much closer to each other in the centres of galaxies and globular clusters, or much further apart in galactic halos.

Related Topics:
Proxima Centauri - Light year - Space Shuttle - Globular cluster - Galactic halo

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