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Hundreds of stars are visible in this image taken by the Hubble Space Telescope of the Sagittarius Star Cloud in the Milky Way Galaxy.
A star is any massive gaseous celestial body in outer space. Stars appear as shining points in the nighttime sky that twinkle because of the effect of the Earth's atmosphere and their distance from us. The Sun is an exception: it is the only star sufficiently close to Earth to appear as a disc and to provide daylight.
Common language does not always reflect this astronomical usage; the term "star" ordinarily does not include the Sun, and sometimes includes the visible planets and even meteorA meteor is the visible path of a meteoroid that enters the Earth's (or another body's) atmosphere, commonly called a shooting star''. The visibility is due to the heat produced by the atmospheric entry. A very bright meteor, brighter than the apparent mas ("shooting stars" or "falling stars").
The nearest star to the earth, apart from the Sun, is Proxima CentauriThe red dwarf Proxima Centauri (also Alpha Centauri C , part of the Alpha Centauri star system, is the nearest star to Earth, other than the Sun. As the name suggests, it is located in the constellation of Centaurus. Based on the parallax of 772. 42 milli, which is 39.9 trillionThe numeral trillion refers to one of two number values, depending on the context of where and how it is being used. It is the largest numerical value in everyday non-scientific use in the English language. Usage Short scale usage In Brazil, Russia, Turke kilometers = 39.9 Pm ( petaThis article describes the SI prefix peta . For other meanings, see Peta (disambiguation In physics and mathematics, peta (symbol: P is a prefix in the SI system of units denoting 1015, or 1,000,000,000,000,000. For example: 1 petametre 1015 metres 1 petametre) = 4.2 ly ( light yearA light year abbreviated ly is the distance light travels in one year: roughly 9. 46 × 1012 kilometres (9. 46 petametres, or about 5. 88 × 1012 miles). More specifically, a light year is defined as the distance that a photon would travel, in free space an) = 1.29 pc ( parsecThis article is about the unit of length. In computer programming, Parsec is an XML syntax analyzer, like Lark, or a parsing library for the Haskell programming language. There's an open source computer game named Parsec . The parsec (abbreviated pc is a) away. Thus light from Proxima Centauri takes 4.2 yearA year is the time between two recurrences of an event related to the orbit of the Earth around the Sun. By extension, this can be applied to any planet: for example, "Martian year". Seasonal year A seasonal year is the time between successive recurrencess to reach Earth. If you took the French TGV, one of the fastest trains, on a trip to Proxima Centauri using its highest recorded speed (515.3 kilometers per hour), it would take you about 8.86 million years.
However, between this distance and a few times this distance, there are quite a few other stars, see list of nearest stars.
Astronomers estimate that there are at least 70 sextillion stars in the known universe (7 × 1022). That is 70 000 000 000 000 000 000 000, that is 230 billion times as much as the 300 billion in our own Milky Way.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 estimated 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 billion kilometers. However, these have a much lower density than the Sun. One of the most massive stars is Eta Carinae, with 100-150 times as much mass as the Sun.
Scientifically, stars are defined as self-gravitating spheres of plasma in hydrostatic equilibrium, which generate their own energy through the process of nuclear fusion.
The energy produced by stars radiates into space as electromagnetic radiation (mostly visible light), and as a stream of neutrinos. The apparent brightness of a star is measured by its apparent magnitude.
Stellar astronomy is the study of stars and the phenomena exhibited by the various forms/developmental stages of stars.Many stars are gravitationally bound to other stars, forming binary stars. Larger groups called star clusters also exist. Stars are not spread uniformly across the universe, but are typically grouped into galaxies. A typical galaxy contains hundreds of billions of stars.
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