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Clusters Of Galaxies
Our Milky Way galaxy has several close neighbors in space—close, that is, when we compare their distances to those of the most remote known galaxies. Our “intergalactic township” is called the Local Group . This is a bit whimsical when we are talking about millions of light-years, but compared with the whole known Universe, it is local indeed.All the galaxies in our Local Group are within a few million light-years of our galaxy. The Great Galaxy in Andromeda , also known as M31 , is almost on the other side of the group from our Milky Way. The spiral galaxy M33, in the constellation Triangulum, is another member of the Local Group. There are several smaller irregular and elliptical galaxies as well.
All the galaxies in our cluster appear tilted at different angles in space. When we look far into space, we find galaxies in every direction. Along the spiral plane of our own Milky Way, it is almost impossible to see exterior objects because of the interstellar gas and dust that obscure the view. The farther out we look, the more galaxies we find. This is to be expected because larger and larger spheres of observation must encompass more and more rapidly increasing volumes of space. Strangely enough, though, the distribution of the galaxies in the Cosmos is not altogether uniform.
Clusters of galaxies are the rule rather than the exception. Our Local Group is a comparatively small cluster. There are clusters with hundreds or even thousands of individual galaxies. Just as stars can vary greatly in their character and the galaxies can exist in many different and unique forms, so the clusters are found in differing shapes, sizes, and constitutions. The galaxies in some clusters are so close together that they collide “often” on a Cosmic time scale measured in millions upon millions of Earth years. After an encounter with another galaxy, a spiral can lose its arms because of the gravitational and electromagnetic (EM) disturbance. Some such spirals become irregular and disorganized.
One of the most dense known clusters of galaxies is found in the direction of the constellation Coma Berenices. Near the center of this rich cluster, any galaxy can be expected to collide with another member several times during its life. What would it be like if our Milky Way were currently in a collision with another galaxy? Such an event occurs over a period of millions of years. However, we certainly would be able to tell if it were happening. There would be two sources of radio noise, not just one, coming from the galactic core, and the noise level would be much greater than we currently observe. If we had the opportunity to view the invading galaxy, its nucleus would be visible with the unaided eye as a diffuse, glowing mass of starlight.
Clusters of galaxies extend as far as we can see with optical telescopes: up to several billion light-years. Such distances must be inferred indirectly; how this is done will be shown a little later in this chapter.
Superclusters, Strings, And Dark Matter
Between clusters of galaxies, space appears empty. At least there is nothing in these voids that radiates energy we can observe. However, even the clusters such as our Local Group or well-known clusters such as those that lie in the general directions of the constellations Coma Berenices or Virgo (and thus are named after those constellations) appear to exist in larger superclusters . You might think of it as the Cosmic urban structure: neighborhoods (galaxies) comprise townships (clusters), which together comprise cities (superclusters).
On a scale larger still, the superclusters are separated by voids of staggering size. When all the known galaxies are mapped using a computer program that simulates three-dimensional space, a foamlike cosmic structure becomes evident. Think of the large bubbles produced in soapy water. The superclusters exist mainly on the filmy surfaces of the bubbles. Inside the bubbles and between them is nothingness—or at least EM darkness. Some astronomers suspect that these dark regions contain some as-yet-unknown “stuff” that has mass and that has a profound effect on the evolution of the Universe. This stuff has even been given a name: dark matter . It is a topic of much interest and debate.
All galaxies emit energy at radio wavelengths, as well as in the infrared (IR), visible, ultraviolet (UV), x-ray, and gamma-ray portions of the EM spectrum. Usually, the intensity of the radio emissions from galaxies is related to their classification and their observed visual brightness. However, some galaxies emit far more energy at radio wavelengths than we would expect. These objects are called radio galaxies . The intense radio source known as Cygnus A is one such galaxy. When radio telescopes are used to map the details of Cygnus A, a double structure is found. The radio emission comes from two different regions located on either side of the visible galaxy. Other double galaxies have been observed with radio telescopes.
Several hypotheses have been put forth in an attempt to find out what is taking place in radio galaxies. One theory is that they are pairs of colliding galaxies; the magnetic and electrical fields of the two galaxies interact to produce unusual levels of radio-frequency energy. Iosif S. Shklovskii of Russia theorized that radio galaxies contain more supernovae than do normal galaxies. F. Hoyle and W. A. Fowler have suggested that the tremendous energy of the radio galaxies comes from explosions of the galactic nuclei, following or associated with a catastrophic gravitational collapse. In any case, it is believed that the nuclei of the radio galaxies are undergoing radical changes. As more becomes known about radio galaxies, astronomers hope to further unravel the puzzles of galactic formation and evolution.
When attempting to determine the distances to other galaxies and other clusters of galaxies, astronomers use tricks. These schemes rely on two assumptions: (1) the average brightnesses of the stars in all galaxies are similar, and (2) the average brightnesses of the galaxies in all clusters are similar.
Using The Cepheids
In the early-middle part of the twentieth century, when astronomers began to seriously study galaxies using the newly constructed 100-in (2.54-m) telescope on top of Mount Wilson, they found Cepheid variable stars in some of them. This resolved the question of whether or not the spiral nebulae are, in fact, “island universes.” They are—and they are farther away than anything in the Milky Way.
Two astronomers, Edwin Hubble and Milton Humason, made the assumption that the Cepheid variables in other galaxies exhibit the same brightness-versus-period relation as the Cepheids in our own galaxy. On this basis, estimates of distances to some of the spiral galaxies were made. The initial estimate of the distance to the Great Galaxy in Andromeda was 600,000 to 800,000 light-years. (More recently, this figure has been revised upward to about 2.2 million light-years.) Some of the spiral galaxies that Hubble and Humason examined appeared to be 10 million light-years distant. These were the most distant galaxies in which Cepheid variables could be individually observed.
Yet there were galaxies much smaller (in terms of angular size) and fainter than the dimmest ones in which Cepheid variables could be resolved. This, Hubble and Humason reasoned, meant that there are galaxies much farther away than 10 million light-years. Beyond the limit at which the Cepheid variables can be used, astronomers use the brightnesses of galaxies as a whole to infer their distances. This is not an exact science. Observations over great distances are complicated by the fact that what we see is an image of the distant past and not an image of things as they are “right now.”
Looking Back In Time
Visible light and all EM waves, including radio, IR, UV, x-rays, and gamma rays, travel at a finite speed through space. A galaxy 10 million light-years away appears to us as it was 10 million years ago, not as it is today. If relative brightness is used as a distance-measuring tool, there appear to be galaxies billions of light-years away. We see these as they were billions of years ago—in some cases, as they were before our Solar System existed. Do galaxies maintain the same average brightnesses over time spans this great? We do not know. If they do, then our estimates of their distances are fairly accurate. If not, then our distance estimates are not accurate.
Based on their assumptions, Hubble and Humason found an interesting correlation between the apparent distance to a galaxy and the amount by which the lines in its spectrum are shifted. In general, the farther out in inter-galactic space we look, the more the spectra of individual galaxies are red-shifted. This suggests that all the galactic clusters in the Universe are moving away from all the others. The most commonly accepted explanation for this red shift is Doppler effect caused by radial motion away from an observer.
Hubble and Humason made the assumption that the red shifts are caused by a general expansion of the Universe, and based on this, they found that the speed-versus-distance function is linear. On average, objects 200 million light-years away appear to be receding from us at twice the speed of objects 100 million light-years away, objects 400 million light-years away are receding twice as fast as those 200 million light-years away, and so on. This gives astronomers yet another tool for estimating vast distances in the Universe, but it must be based on yet another assumption: The slope of the speed-versus-distance function is constant all the way out to the limit of visibility. When this assumption is made, the conclusion follows that we will never see anything farther away than about 15 billion (1.5 × 10 10 ) light-years because such objects would be receding from us at a speed greater than the speed of light.
Does the average brilliance of galaxies remain the same over periods of billions of years? Starting in the 1960s, there were reasons to think not.
Practice problems of this concept can be found at: Galaxies and Quasars Practice Problems
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