Galaxies: Collections of Stars
Stars exist only in galaxies. Although galaxies appear numerous in many photographs, they only occupy about a hundred-millionth of the vast volume of space of the known universe. The view of the sky from a typical point in space is empty of individual stars, and usually only distant galaxies can be seen. But our own sky is full of stars, which indicates that we live in a privileged place in the Universe. Far from being in a random part of empty space, our environment is atypical of the Universe.
Signposts to the expanding Universe
Galaxies rarely exist in isolation. They are mostly found in groups, called clusters, which may number from two or three to 1000 or more.
In all clusters, the member galaxies are distributed with no more pattern than a tendency to congregate towards the center. What holds them together? The answer is: Gravity. Each galaxy is a captive of the gravitational pull of all its neighbors.
Large clusters of galaxies measure many tens of millions of light-years across. Yet even these may not be the largest organized structures in the Universe. The sketchy data which reaches us from the most distant parts of the Universe suggests that clusters of galaxies themselves group into giant super-clusters, whose scale is greater by yet another factor of 100 or more.
The name of the American astronomer Edwin P. Hubble is inescapably associated with the early study of galaxies. in 1920's Hubble measured the velocities of galaxies in a number of clusters and found that the faint clusters are receding from us at great speed. Taking the brightest galaxy in a cluster, the fainter it is, the faster it is moving away. Because the brightest galaxy in a cluster is always around the same true brightness, its faintness indicates how far away it is. So the more distant a cluster of galaxies, it seems, the greater its speed of recession from us. This relationship is called: Hubble's Law.
Hubble's Law is the only tool astronomers can use to estimate distances in the farthest reaches of the Universe. A measurement of the recession velocity (the so-called redshift) implies the distance to an object. Nearby galaxies act as standards, since their distances can be worked out in other ways, but this is a difficult business, and the calibration may be considerably in error. The currently accepted value for the expansion rate (Known as the Hubble Constant) is 15 km/sec for every million light years distance.
(A recent 2011 estimate of the Hubble constant, which used a new infrared camera on the Hubble Space Telescope (HST) to measure the distance and redshift for a collection of astronomical objects, gives a value of H0 = 73.8 ± 2.4 (km/s)/Mpc. An alternate approach using data from galactic clusters gave a value of H0 = 67.0 ± 3.2 (km/s)/Mpc.)
The force behind
Why does material collect into clusters of galaxies, stars and planets? Once again, gravity is the important force. The expanding Universe would not have been entirely uniform. Patches denser than the rest of must have existed from the beginning. Having a greater concentration of mass, the gravitational pull of these denser patches attracted gas from less dense regions. Each became the seed of a growing concentration. The collapsing clouds themselves contained smaller denser patches which became the nuclei of smaller collapses, and so on. Today we view this process at a stage when much of the gas in the Universe has collapsed to form stars and planets, lying within galaxies, clusters, and maybe even super-clusters of galaxies.
Catalogue of the Universe p. 9 - 12
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