PROPERTIES OF
GALAXIES Galaxies, as noted, range in size from dwarfs only a few thousand light-years across to immense giants
almost a million light-years wide. The smaller are the most numerous. In our local region there are about ten times as many dwarf
galaxies as large ones like our own. A typical small galaxy is the Sculptor dwarf, not discovered until 1938 in spite of being one of the galaxies nearest to ours. It consists of a smoothly distributed, symmetrical arrangement of faint, old stars. Even in its center the density of stars is far lower than in our neighborhood, so that for a person on a hypothetical planet in the Sculptor the night sky would be nearly black, with only one or two stars visible. The largest normal galaxies, such as the Andromeda galaxy, consist of a huge rotating disk of gas and stars, surrounded by a thin halo of older stars and star clusters (see interstellar
matter; population, stellar). Even larger are the radio galaxies (see radio astronomy), which have suffered some kind of disturbance, causing them to have high-energy gas that radiates radio waves and sometimes X rays (see X-ray astronomy). Many are understood to be the result of galaxy collisions and mergers. The mass of a galaxy can be determined by measuring the motions of its stars and gas. In a disk-shaped galaxy such as Andromeda the stars revolve around the center of the disk, and the rotation rate of the outer part of the disk allows a measure to be made of the mass inside it. It has been found that most galaxies have large amounts of otherwise undetected matter, the nature of which is a mystery. This so-called "dark matter" may consist of a superhalo of very faint stars or planets, or of some unfamiliar form of elementary particleÑor, perhaps, something else (see mass, missing). Some galaxies do not have rotating disks, and their stars tend to move in highly elongated orbits much as comets do in our solar system. Astronomers can measure their velocities and thereby determine the masses of these galaxies, most of which also seem to have large amounts of dark matter. The total measured mass of the Sculptor dwarf galaxy, for example, is 10 million times the mass of the Sun, whereas the mass of its detectable stars is only 3 million suns. The rest is dark matter. A galaxy's apparent brightness depends on its intrinsic luminosity and on its distance. It is not surprising that the total luminosities of galaxies are directly related to the number of stars they contain, which is measured by their stellar masses. The brightest galaxies are therefore hundreds of billions of times intrinsically brighter than the Sun, while the faint dwarf galaxies are only a few million times brighter. There are even some objects that apparently contain only gas and no stars. These objects do not shine at all, except at radio wavelengths. Astronomers often calculate a galaxy's "mass-to-light ratio," measured in terms of the Sun's mass and luminosity. By definition the ratio is one for the Sun, a typical star, and it is close to one for most galaxies. Very old galaxies such as Sculptor have larger ratios because their stars are generally faint dwarf stars that have proportionally more mass than light. Including the dark matter, the Sculptor's mass-to-light ratio is 6. Some similar dwarfs have ratios as large as 100. Ages of galaxies are not easily measured. Because many galaxies form new stars more or less continuously over their lifetimes, many stars that are seen in them are, astronomically speaking, quite young. To find the true age of a galaxy, its oldest stars must be found and age-datedÑa formidable task. In our own Galaxy the oldest stars are approximately 16 billion years old, which is close to the estimated age of the universe as a whole. The oldest stars in nearby galaxies seem of similar age, indicating that in our immediate neighborhood most galaxies seem to have formed at about the same time. When astronomers look deep into intergalactic spaceey see galaxies that are younger simply because it has taken a long time for the light they emit to reach Earth. This effect does not become important until very large distances are reached, such as those for the faintest galaxies seen with the Hubble Space Telescope. At those distancesÑthe most distant seen thus far (1998) is 12.3 billion light-years awayÑmany galaxies are less than half the present age of the universe, andÑapparently as a result of their youthÑthey show unusual shapes and colors. Many are bluer than normal, probably because of active star formation (which produces very bright, blue stars), and their shapes are often distorted by encounters. Such encounters must have been important in the earlier universe, when the universe was smaller and collisions were more likely than they are now. A puzzle needing to be solved is the apparent discrepancy between the measured ages of galaxies and the calculated expansion age of the universe. The universe is expanding at a measurable rate, and it is possible to calculate when this expansion began. Some unknown constants are involved in the calculation, but there are theoretical reasons for choosing certain values for these constants. Using the best present data, the calculated expansion age is about 12 billion years, smaller than the age of most galaxies.