|Themes > Science > Astronomy > The Stars > Stars: Definition and Physical Description|
A Star, is large celestial body composed of gravitationally contained hot gases emitting electromagnetic radiation, especially light, as a result of nuclear reactions inside the star.
The sun is a star. With the sole exception of the sun, the stars appear to be fixed, maintaining the same pattern in the skies year after year.
In fact the stars are in rapid motion, but their distances are so great that their relative changes in position become apparent only over the centuries.
The number of stars visible to the naked eye from earth has been estimated to total 8000, of which 4000 are visible from the northern hemisphere and 4000 from the southern hemisphere.
But at any one time in either hemisphere, only about 2000 stars are visible. The other 2000 are located in the daytime sky and are obscured by the much brighter light of the sun.
Astronomers have calculated that the stars in the Milky Way,(= the galaxy to which the sun belongs), number in the hundreds of billions. The Milky Way, in turn, is only one of several hundred million such galaxies within the viewing range of the larger modern telescopes. The individual stars visible in the sky are simply those that lie closest to the solar system in the Milky Way.
The star nearest to our solar system is the triple star Proxima Centauri, which is about 40 trillion km (about 25 trillion mi) from earth. In terms of the speed of light, the common standard used by astronomers for expressing distance, this triple-star system is about 4.29 light-years distant; light traveling at about 300,000 km per sec (about 186,000 mi per sec) takes more than four years and three months to travel from this star to earth.
Cooler areas of the photosphere, such as the sunspots on the sun, are likely present on other typical stars; their existence on some large nearby stars has been inferred by a technique called speckle interferometry.
The internal structure of the sun and other stars cannot be directly observed, but studies indicate convection currents and layers of increasing density and temperature until the core is reached where thermonuclear reactions take place.
Stars consist mainly of hydrogen and helium, with varying amounts of heavier elements.
The largest stars known are supergiants with diameters that are more than 400 times that of the sun, whereas the small stars known as white dwarfs have diameters that may be only 0.01 times that of the sun.
Giant stars are usually diffuse, however, and may be only 40 times more massive than the sun, whereas white dwarfs are extremely dense and may have masses about 0.1 times that of the sun despite their small size. Supermassive stars are suspected that could be 1000 times more massive than the sun, and, at the lower range, hot balls of gases may exist that are too small to initiate nuclear reactions. One possible such brown dwarf was first observed in 1987, and others have been detected since then.
Star brightness is described in terms of magnitude. The brightest stars may be as much as 1,000,000 times brighter than the sun; white dwarfs are about 1000 times less bright.