Star

In astronomy, a star (ancient Greek ἀστήρ, ἄστρον astēr, astron and Latin aster, astrum, stella, sidus for 'star, star'; ahd. sterno; astronomical symbol: ✱) is a massive, self-luminous celestial body made of very hot gas and plasma, such as the sun. In addition, a planet in our solar system that is illuminated by the Sun is also commonly called a star, such as the evening star, although it is not a star like the Sun.

That almost all self-luminous celestial bodies visible to the naked eye are Sun-like objects, appearing point-like only because of their great distance, is one of the most important discoveries of modern astronomy. About three-quarters of stars are part of a binary or multiple star system, and many have a planetary system. Stars formed together more often form star clusters. Under favorable conditions, several thousand stars can be distinguished by unaided vision. They all belong to the same galaxy as the Sun, the Milky Way, which consists of over a hundred billion stars. Together with its neighbouring galaxies, this galaxy belongs to the Local Group, one of thousands and thousands of galaxy clusters.

Stars are formed from gas clouds - in certain regions (H-II region) from gaseous molecular clouds - by local strong compression in several phases. They are held together by the gravity of their own mass and are therefore approximately spherical. While a star's interior is several million degrees hot (in the case of the Sun's core, just under 16,000,000 Kelvin), the surface temperature of most of them is roughly between 2,000 K and 20,000 K (in the case of the Sun's photosphere, just under 6,000 K); white dwarfs, as exposed stellar cores, can reach temperatures of up to 100,000 K at their surface. Not only does intense radiation such as light emanate from the glowing stellar surface, but a stream of charged plasma particles (stellar wind) also travels far into space, forming an astrosphere.

Stars can differ considerably in mass and volume, as well as in luminosity and color; in the course of a star's evolution, these properties change. An orienting classification of stars is already possible with the two characteristics absolute brightness and spectral type alone. The properties of stars are also important in determining whether a planet orbiting them could support life or not (see habitable zone).

Etymology

Old High German sterno, Middle High German stern[e], Swedish stjärna stand next to differently formed Old High German sterro and Middle High German sterre, English star. Non-Germanic are related, for example, Greek astḗr, Latin stella. The words go back to Indo-European stē̌r- "star".

Overview

Most stars consist of 99% hydrogen and helium in the form of hot plasma. Their radiant energy is generated in the star's interior by stellar nuclear fusion and reaches the surface by intense radiation and convection. About 90% of stars - the main sequence stars - are like the Sun in a stable equilibrium between gravitational, radiation and gas pressure, in which they remain for many millions to billions of years.

Afterwards they inflate to giant stars and finally shrink to white dwarfs, as which they slowly cool down. These very compact final stages of stellar evolution, as well as the even denser neutron stars, are also counted as stars, although they only emit radiation due to their residual heat.

The nearest and most studied star is the Sun, the center of the solar system. Even in the Middle Ages it was unknown that the sun was a "normal star", but already ancient natural philosophers suspected that it must be hotter than a glowing stone. The Sun is the only star on which structures can be clearly seen from Earth: Sunspots, solar flares and solar flares.

Only a few relatively nearby supergiants, such as Betelgeuse or Mira, are visible in state-of-the-art telescopes as disks that can reveal gross non-uniformities. All other stars are too distant for this; with the available optical instruments they appear as diffraction discs of point-like light sources.

In the past, the term fixed stars was used to distinguish them from tail stars (comets) and variable stars (planets). However, the positions of stars in the sky are not fixed, but their stellar words slowly shift in relation to each other. The measurable proper motion varies in magnitude and, for a comparatively nearby star such as Barnard's Arrow Star, can be about ten seconds of arc per year (10.3″/a). In ten thousand years, therefore, some of today's constellations will have changed significantly.

Depending on darkness and atmospheric conditions, about 2000 to 6000 stars can be seen with the naked eye in the entire sky, but less than 1000 near cities. The sight of these seemingly structureless points of light easily deceives one into thinking that stars span immense ranges of values, not only in terms of their distance, but also in terms of the ranges of variation in temperatures, luminosity, mass density, volume, and lifetime. For example, the outermost layers of red giant stars would be described as a vacuum according to the criteria of terrestrial technology, while neutron stars can be denser than atomic nuclei; with a mass density of 4-1015 kg/m³, a spoon with 12 cm³ of it would weigh about as much as the entire water in Lake Constance (48 km³). The extremely different appearances of stars correspond to considerable differences in their internal structure; turbulent exchange processes often take place between the depth-dependent zones. This article offers a rough overview and refers to further articles.

Celestial bodies in size comparison1 : Mercury < Mars < Venus < Earth2 : Earth < Neptune < Uranus < Saturn < Jupiter3 : Jupiter < Wolf 359 < Sun < Sirius4 : Sirius < Pollux < Arcturus < Aldebaran5 : Aldebaran < Rigel < Antares < Betelgeuse6 : Betelgeuse < Garnet Star < VV Cephei A < VY Canis Majoris