When telescopes became powerful enough to resolve nebulae into definite shapes, one type of nebula presented a conundrum. Many of the spiral nebulae, which looked like whirlpools of glowing gas, had spectral lines whose wavelengths were much longer than they ought to be. This phenomenon, called red shift, suggests that an object is receding. Red shifts were seen commonly for spiral nebulae, but blue shifts—foreshortening of the waves when an object is approaching—were almost never seen, and when they were observed, they were minimal. Some astronomers thought that the spiral nebulae actually were huge congregations of stars at immense distances and that our own Milky Way was just one such congregation. Until individual stars could be resolved within the spiral nebulae, however, this idea remained an unproved hypothesis.
Today we know that the spiral nebulae do consist of stars, and we call them spiral galaxies. Spirals are not the only type of galaxy. Other objects, previously thought to be emission nebulae or globular star clusters within our Milky Way, turned out to be irregular galaxies or elliptical galaxies. Some of these are billions (units of 1 billion or 109) of light-years away from us. Many contain hundreds of billions of individual stars.
Some of the brightest galaxies, containing the greatest numbers of stars, are ellipsoidal or spherical in shape. These galaxies are classified according to their eccentricity, the extent to which they are elongated from a perfectly spherical shape. (Actually, the sphere or ellipsoid represents the median boundary, the two-dimensional region such that half the stars are inside and half are outside.) Eccentricity zero (E0) represents a perfect sphere; E1 and E2 are egg-shaped. The E3 and E4 elliptical galaxies resemble elongated eggs. When we get to E5, the median boundary is football-shaped. Elliptical galaxies of E6 and E7 classification are even more elongated. Figure 15-1 shows approximate representations for E0 through E7 elliptical galaxies. This scheme was devised by astronomer Edwin Hubble in the 1930s.
Elliptical galaxies contain comparatively little gas and dust. It is believed that this is so because most of the interstellar material has developed into stars. This suggests that elliptical galaxies are old. There are numerous red giants in these galaxies, and this is consistent with the theory that they are old. Some of these must be supergiants, with diameters hundreds of times that of our Sun, and thousands of times brighter. Despite the vast distances separating other galaxies from ours, some of the stars in elliptical galaxies resolve into points of light in large telescopes. Were it not for the red shift, these giant galaxies could be mistaken for globular star clusters within our own galaxy.
Figure 15-1. Classifications of elliptical galaxies made by Edwin Hubble in the 1930s. Type E0 has a spherical shape. Type E7 has the greatest eccentricity.
Irregular galaxies have no defined shape or apparent structure. Our Milky Way has two small irregular galaxies near it. These are the Magellanic Clouds, named after the famous explorer who sailed around the world. They can be seen with the unaided eye, but only from the Earth’s southern hemisphere. They look like faintly glowing clouds on a moonless night. With a good telescope, it is easy to tell that they are made up of stars.
Some irregular galaxies show signs of coordinated motions among their stars, such as slow rotation around a central axis. In some cases, it is difficult to tell whether such apparent organization is real or an artifact of the expectation phenomenon: Sometimes we think we see something only because we expect to see it. In other instances, there is clear evidence of rotation. If the rotation is significant enough, the galaxy can be classified as a spiral.
The most stunning galaxies, from the standpoint of the visual observer, are the spirals. Their variety is almost infinite. Some spirals appear broadside to us, some appear at a slant, and still others present themselves edgewise. The spiral arms can have many different shapes.
There are two major types of spiral galaxies: the normal spiral and the barred spiral. There are subclassifications within these two major categories. These are rather subjective and must be judged based on what we see. It is easy to classify spirals when they present themselves nearly broadside to us but difficult when they present themselves edgewise or nearly edgewise. Normal spirals are classified S0, Sa, Sb, and Sc (Fig. 15-2) depending on how tightly their bands of stars are wound around the nucleus. The barred spirals are classified as S0, SBa, SBb, and SBc (Fig. 15-3). The S0 galaxies are shaped like oblate (flattened) spheres, or like donuts with golf balls stuck in their centers. Both the normal spirals and the barred spirals branch off from this common root type. These classifications, like the classifications for the elliptical galaxies, were devised by Hubble.
Figure 15-2. Hubble classification of spiral galaxies. Type S0 is shaped like an oblate sphere. Type Sa has tight spiral arms; types Sb and Sc have more open spiral arms.
Figure 15-3. Hubble classification of barred spiral galaxies. Type S0 is shaped like an oblate sphere. Type SBa has tight spiral arms and a short bar; types SBb and SBc have more open spiral arms and longer bars.
In a normal spiral galaxy, the arms extend from the bright nucleus and are coiled in a more or less uniform fashion throughout the disk. Some spirals have prominent arms, whereas others have almost invisible arms. In a barred spiral, the central region is rod-shaped. The spiral arms trail off from the ends of the rod, in some cases prominently and in other cases almost invisibly. The barred spirals are especially interesting because the rod-shaped region appears to rotate with constant angular speed at all points along its length, as if it were solid. According to one theory, the nuclei of such galaxies are undergoing catastrophic explosions, and the bars are streamers of gas and dust ejected from the core at high speeds.
The appearance of a spiral galaxy gives the illusion that it is a uniformly rotating system. This is basically true, but the motions of the individual stars within a spiral galaxy are varied, and the overall pattern of motion is quite complicated. The fact that these galaxies rotate is verified by spectral examination of different regions when the disk of the spiral presents itself nearly edgewise to us. On one side of the galaxy, the light is shifted toward the blue end of the spectrum compared with the light from the nucleus. On the other side of the galaxy, the light is shifted toward the red end compared with the light from the nucleus. This indicates that the two sides of the galaxy have different radial speeds, and this can be explained only by rotation around the center. These determinations must be made independently of the overall motion of the galaxy with respect to us; in general, most galaxies are moving away.
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.
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.
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.”
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 × 1010) 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.
In 1960, the position of a strong radio source was defined with great accuracy, and its angular diameter was found to be less than a second of arc. Comparing the position of this radio source with various visible objects in its vicinity, this “radio star” was found to be a faint blue star in photographs. There was something especially odd about this star: The astronomers J. L. Greenstein and A. Sandage could not identify the absorption lines in its spectrum. It did not take them long to find the problem. The red shift in the spectrum of this cosmic energy source is so great that the lines are greatly altered, suggesting that the object is receding from us at a sizable fraction of the speed of light.
Soon after the discovery of this “radio star,” several other similar objects were found, and they also had very large red shifts in their spectral lines. The objects, because of their visual resemblance to stars and because of their strong radio emissions, were called quasi-stellar radio sources. This name has since been shortened to the more palatable term quasar.
After the first few quasars were found, many others were discovered and observed. Some quasars had been photographed previously, but in the photographs they had been dismissed as ordinary stars. In one case, when several photographs having been taken over a period of decades were examined, it was found that large changes in brightness had occurred within periods of a few months. This implied that the quasar is a fraction of a light-year in diameter. However, if its red shift is a correct indicator of its distance, its energy output is many times that of a normal galaxy! Quasars are concentrated, as well as intense, sources of energy.
When observed with radio telescopes having high resolution—less than 1 second of arc in some cases—some quasars still appear as point sources. This is also true of the nuclei of certain radio galaxies. Optically, many of the quasars look like point sources of light, and therefore, they resemble stars, until the extreme magnification and resolving power of the Hubble Space Telescope (HST) is put to work on them. Then some quasars show evidence of glowing matter around a central, intense core.
Some quasars can be resolved into components by radio telescopes, but this requires the use of multiple antennas and a baseline of hundreds or even thousands of kilometers. Antennas in diverse locations on Earth are linked by satellite communications systems, and their outputs are combined by computer programs in order to accomplish this. This provides the equivalent resolving power of an antenna much larger than any single structure that could be constructed. The angular resolution goes down to less than 0.001 second of arc. With such sophisticated apparatus, radio galaxies and quasars have been probed in detail.
There is another, quite different way to estimate the angular diameter of an object that emits energy at radio wavelengths: observing and measuring changes in intensity, called scintillations, that occur as the radio waves pass through turbulent ionized clouds of particles in space.
Everyone has noticed the twinkling of the stars, while the planets appear to shine almost without blinking. The reason for this difference is that the planets have a much greater angular diameter than any star. Small telescopes show the planets as disks, but even the nearest stars resolve only as points of light, even at high magnification. Turbulence in the air, such as that produced on summer evenings as the warm land heats the atmosphere and causes convection currents, make a point of light seem to twinkle because the light rays are refracted more and then less, and then more again by parcels of air having variable density. The charged subatomic particles of the solar wind, as they stream outward from the Sun, have a similar effect on radio waves coming from far away in space. Other stars produce “winds” too, making interstellar space a turbulent sea of charged subatomic particles. A source of radio waves with a small angular diameter therefore scintillates.
By observing quasars with a single radio telescope antenna to avoid diversity effect (averaging out of the strengths of radio signals as received at different locations), and by carefully recording the intensity of the waves reaching the antenna, it is possible to get an accurate idea of the angular size of a radio object. Quasars always appear as small sources of radio energy, at most a few light-years in diameter. Galaxies, in contrast, are many thousands of light-years across. Other observed properties of the quasars, such as curvature in the spectral lines, have led astronomers to believe that they are small compared with galaxies, even though they emit fantastic amounts of energy.
The sizes of the quasars, as well as estimates of their energy output, have been determined according to the Hubble relation between red shifts and distances. All quasars show significant red shifts in the absorption lines of their spectra. This has led most astronomers to surmise that they are billions of light-years away from us.
Suppose, however, that the red shifts are being misinterpreted? Are quasars actually local objects of modest size that are thrust outward from the nucleus of our galaxy at tremendous speeds? This is an interesting theory, but it is not widely accepted. If quasars are being ejected from our galaxy, then it is reasonable to suppose that they are ejected from other galaxies too. In such a case, some of the quasars ejected from other galaxies should be observed as approaching us. This would give such objects a pronounced blue Doppler shift. However, no quasar has ever been found that exhibits a blue shift in its spectral lines.
Another attempt has been made to prove that quasars are “local.” Albert Einstein showed, in the formulation of his general theory of relativity, that a powerful gravitational field can produce a red shift in the spectrum of the light coming from the source of the gravitation. This effect has been observed and measured, so scientists know that Einstein’s theory is correct. Can the red shifts in the spectral lines of quasars be explained in terms of the relativistic effect of gravitation? A super dense object with extreme gravitation near its surface could produce a large red shift. This remains an open question. Still, an affirmative answer would not constitute conclusive proof that quasars are “local.”
Recent observations of quasars using the HST have begun to resolve the riddle. Evidence is accumulating to support the theory that quasars are among the most distant objects we can see in the Cosmos and that they therefore present a picture of the Universe as it was when it was much younger than it is now. This gives astronomers a way to look back in time to whatever extent they want simply by observing galaxies and quasar objects at various distances as indicated by their red shifts.
A severe blow was dealt to the local quasar theory when scientists calculated that the first quasar that was discovered, called 3C4S, would have to have a mass the same as the Sun, be only 10 km (6 mi) in diameter, and be in the Earth’s atmosphere in order to account for the radiation intensity it possesses. Even if 3C4S has thousands of times the mass of the Sun, calculations show that it still must reside within our Solar System, and this obviously is not the case. The derivations in these terms for other quasars give similar results.
The determination of the distances to quasars represents a good example of the devil’s-advocate method of lending support to a theory by discrediting all its plausible refutations. The quasars, even after attack by the devil’s-advocate scientific method of inquiry, appear to be distant and energetic cosmic phenomena.
The internal anatomy of quasars is still largely a mystery. It seems that the quasars are very distant and also very powerful sources of energy. Suppose that we accept this hypothesis without further question. If we are willing to do this, then certain things can be deduced about quasars.
The quasars are much farther away than all the nearby galaxies. (In this sense, nearby means within several hundred million light-years.) Quasars are not only distant, but they are extremely distant. Without exception, they show large red shifts in their spectral lines. In the Cosmos, distance is time; when we look at something 2 billion light-years away, we are looking 2 billion years into the past. Whenever we look at a quasar, we gaze into a past so remote that the Earth itself was much different than we know it today. Suppose that the quasars are—or were—a common phenomenon of the Universe in its younger age? This is a tempting proposition.
The present estimate of the age of our Universe is on the order of 12 billion to 15 billion years. Some of the quasars, at distances approaching 10 billion light-years, are thus images of the Universe at less than half its present age. Many stars have lifespans of much less than 10 billion years. Suppose that the quasars are young galaxies?
Observations of quasars and radio galaxies often reveal striking similarities, so some astronomers believe that quasars and radio galaxies are in fact the same sort of object. The nuclei of radio galaxies have diameters much less than that of a typical galaxy, but they put out vast amounts of energy. They share this characteristic with quasars. When we look at the most distant known radio galaxies with visual apparatus, we see only the brilliant nuclei; the peripheral glow is washed out by the light from the core.
When large amounts of matter are concentrated into a small volume of space, the gravitation can have profound effects. Do dense congregations of stars, such as exist in the centers of spiral galaxies, gravitationally seal themselves off from the rest of the Universe? That is, do they become black holes? The stars near the periphery of the congregation will, in this case, orbit the central region at great speeds before being pulled forever into the mass. Their high velocity and the accompanying magnetic fields would produce large amounts of EM energy at visible and radio wavelengths. Are quasars active black holes, like cosmic tornadoes?
Yet another theory concerning the origin and anatomy of the quasars suggests that they are points in space through which new matter is entering from some other space-time continuum. Such objects, in the vernacular of the cosmologist, are called white holes. As matter bursts into our space-time continuum, having been pulled from another Universe by overwhelming gravitational forces, the flash of radiant energy would outshine any typical galaxy. Direct evidence to support this theory is lacking, but it is one of the most fascinating in all cosmology. It implies that there exist other universes with space-time singularities connecting them with ours.
In recent years, the idea that black holes exist at the centers of many, if not most, spiral galaxies has been gaining acceptance. This is so in part because of an interesting twist in the formula for the Schwarzchild radius as a function of mass.
The radius that an object must attain in order to become a black hole is directly proportional to the mass. From elementary solid geometry, you will recall that the volume of an object is proportional to the cube of the radius. If an object’s mass is doubled, the size of its Schwarzchild radius doubles too. However, its volume becomes eight times as great. This means that the more massive a black hole happens to be, the less dense it is. In fact, the density of a black hole is inversely proportional to the square of its mass (Fig. 15-4).
Suppose that before there were any stars in the Universe, but only hydrogen and helium gas, vast clouds congealed because of gravitation? Suppose that this took place on a much bigger scale than the process of star formation? If such a cloud were large enough, it could become a black hole before it attained a density anywhere near enough to start nuclear fusion. The black hole would continue to pull matter in, becoming larger still and yet less dense. As the atoms approached the event horizon, they would be accelerated to nearly the speed of light. This would give them tremendous kinetic energy, and the result would be an object of brilliance greater than that of any nuclear fusion engine of comparable size. If the cloud had any spin to begin with, that spin would be exaggerated as the gas atoms fell into the black hole, in much the same way as the air circulation around a hurricane gets faster and faster as the molecules are drawn into the eye of the storm.
Figure 15-4. The density of a black hole is inversely proportional to the square of the mass.
In the nuclei of spiral galaxies such as ours, the concentration of stars is highest. This is to say, there are the most stars per cubic light-year in and near the center of a galaxy. In the spiral arms, the concentration of stars is lower. The concentration is lower still in regions above and below the plane of the spiral disk and in between the spiral arms. Our Sun is near the plane of the Milky Way’s disk, in one of the spiral arms, and approximately halfway from the center to the edge.
The appearance of spiral galaxies, some of which bear remarkable resemblance to satellite photographs of hurricanes and typhoons, makes it tempting to think that they spin around and around. They do, and all the stars move in the direction intuitively suggested by the sense of the pinwheel. However, some stars stay near the plane of the disk, whereas others dip below it and rise above it during each orbit around the center (Fig. 15-5). Stars in the central bulge, which resembles a gigantic globular cluster or a small elliptical galaxy having low eccentricity, orbit in planes that are tilted every which way. Near the center of the bulge, the density of stars increases. If our Sun were one of the stars in this region, our nighttime sky would be filled with many more stars than we see now. Moonless, clear nights would be as bright as a gloomy day.
However, what if the Sun were located at or very near the exact center of the galactic core? Many astronomers think that if that were the case, there would be no life on Earth. The Sun and all the stars in its vicinity would have passed through the event horizon of a black hole and would be “on the inside looking out.” This black hole is thought to contain millions, if not billions, of solar masses. If the black hole is big enough, stars can fall through the event horizon and still remain intact. According to this theory, the centers of some, if not most, spiral and elliptical galaxies are “island universes” of a special sort, for they are closed off from the rest of the Cosmos by a one-way gate in time-space. Every time another star falls in, the mass of the black hole increases, and its density goes down a little more. Given sufficient time, measured in trillions of years, is it possible that these black holes might swallow whole galaxies and then clusters of galaxies?
Figure 15-5. Structure of a spiral galaxy as seen edgewise.
At this point, we enter the realm of pure speculation. This is a good place to shift our attention to the theory that gave rise to notions of spatial curvature, time warps, and other esoteric aspects of latter-day cosmology.
QuizRefer to the text if necessary. A good score is 8 correct. Answers are in the back of the book.
1. As a black hole pulls more and more matter in,
(a) the Schwarzchild radius increases.
(b) the Schwarzchild radius decreases.
(c) the density increases.
(d) it gets darker and darker.
2. Quasars with large blue shifts in their spectra
(a) are receding from us at nearly the speed of light.
(b) are approaching us at nearly the speed of light.
(c) are never seen.
(d) emit large amounts of x-rays compared with other quasars.
3. When a celestial object scintillates, we can surmise that it
(a) is extremely luminous.
(b) has a small angular diameter.
(c) is a great distance from us.
(d) emits energy mainly at short wavelengths.
4. Radio galaxies
(a) emit far more energy at radio wavelengths than typical galaxies.
(b) emit energy only at radio wavelengths.
(c) have been observed only with radio telescopes.
(d) are those galaxies toward which we have sent radio signals in an attempt to communicate with alien civilizations.
5. If the spectrum of an object is red-shifted, this means that the emission or absorption lines
(a) look red in color when examined visually.
(b) appear at longer wavelengths than normal.
(c) appear at higher frequencies than normal.
(d) are most prominent in the red part of the spectrum.
6. A certain galaxy is observed, and its distance is estimated at 10 Mpc. We see this object as it appeared approximately
(a) 10,000 years ago.
(b) 10 million years ago.
(c) 32,600 years ago.
(d) 32.6 million years ago.
7. Within a cluster of galaxies,
(a) all the galaxies are of the same type.
(b) all the galaxies spin in the same direction.
(c) all the galaxies are approximately the same size.
(d) None of the above
8. Quasars are believed to be much smaller than typical galaxies based on the observation that
(a) they emit large amounts of energy at radio wavelengths.
(b) their spectra are red-shifted.
(c) they are nearby, in the Milky Way galaxy.
(d) they can grow dimmer or brighter in short periods of time.
9. The spiral nebulae are large, distant congregations of stars rather than smaller objects within the Milky Way. In order to determine this, astronomers observed
(a) the colors of the gases comprising them.
(b) the radio emissions produced by them.
(c) the rate at which they spin.
(d) the brightnesses and periods of Cepheid variables within them.
10. A football-shaped galaxy might be classified as
(a) S0.
(b) S2.
(c) E5.
(d) SBc.
