THE STARS
The accumulation of precise data on some of the nearer stars early in the 20th century enabled Ejnar Hertzsprung
and Henry Norris Russell, working independently, to plot a graph of brightness and color, two basic stellar properties. When they plotted intrinsic stellar brightness on one axis and stellar color (equivalent to surface temperature) on the other axis, Hertzsprung and Russell found that, instead of being scattered over the graph, the stars fell into distinct regions: a heavily populated, diagonal band, known as the main sequence, that varies from bright, hot, blue stars to faint, cool, red ones; a horizontal band containing bright, cool, red stars (the giants); and a sparsely populated, horizontal band containing very luminous stars of all colors (the supergiants). In honor of these scientists, graphs of the type they plotted are called Hertzsprung-Russell
diagrams, or simply H-R diagrams.
The features found on the H-R diagrams are a key to modern astrophysics because they are basic to an understanding of stellar evolution. The star's initial mass determines exactly the position of the star on the main sequence. The star gradually changes, however, thus changing its position on the H-R diagrams. As the hydrogen that fuels the star's fusion reaction becomes depleted, the outer layers of the star expand, and it enters the giant phase. Eventually they become unstable and begin to lose massÑsome smoothly, others catastrophically, depending on their masses. Most stars pulsate smoothly; some may brighten rapidly in older age, blowing material off into space to form a planetary nebula. A few giant, unstable stars explode as supernovas. In any case, evolution proceeds to the stellar graveyard. The most common result of evolution is the white dwarf; large stars end up as neutron stars (pulsars) and, possibly, black holes.
THE
GALAXIES The solar system is located in the outer regions of our Galaxy. From the Earth, the visible part of the Galaxy is seen in the night sky as the Milky Way. This visible part is actually a flattened disk about 100,000 light-years wide and with a central bulge. Around it lies a spherical halo of star clusters (see cluster, star) about 200,000 light-years wide, which is surrounded in turn by a much larger corona of dust and gas. The entire system contains matter in quantities equivalent to more than 1,000 billion solar masses. The Sun takes about 200 million years to orbit the galactic center, which lies about 30,000 light-years from the Sun in the direction of the constellation Sagittarius. Prior to the pioneering work of Harlow Shapley in 1917, the Sun was thought to lie near the galactic center. Shapley also demonstrated the galactic halo of star clusters.
The structure of the Galaxy has been mapped by now, using the distances of extremely luminous stars as well as radio observations of the 21-cm line of the hydrogen spectrum. It has been shown to take the form of a typical spiral galaxy (see extragalactic systems). Three basic galactic types exist: spirals, such as the Milky Way and the Andromeda galaxy; irregulars, such as the Magellanic Clouds; and ellipticals. The last exist at both extremes of galactic size; a dwarf elliptical may contain only a few million stars, whereas a giant elliptical may contain trillions of stars. The fact that extragalactic systems are vast, remote collections of stars was not understood until 1929, when Edwin P. Hubble identified a variable star in the Andromeda galaxy and determined its distance.
The Milky Way, Andromeda galaxy, and Magellanic Clouds are all members of a gravitationally bound cluster of galaxies known as the Local Group, which contains some 20 members in all. Other clusters, such as one in the direction of the constellation Virgo, may contain more than 1,000 galaxies, and much larger superclusters also exist; the Local Group may itself be part of a local supercluster.