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 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.