chapter 16 a universe of galaxies. introduction at the beginning of the 20th century, the nature of...

Chapter 16

A Universe of Galaxies

Introduction At the beginning of the 20th

century, the nature of the faint, fuzzy “spiral nebulae” was unknown.

In the mid-1920s, Edwin Hubble showed that they are distant galaxies like our own Milky Way Galaxy, and that the Universe is far larger than previously thought.

Galaxies are the fundamental units of the Universe, just as stars are the basic units of galaxies.

Like stars, many galaxies are found in clusters, and there are also superclusters separated by enormous voids.

By looking back in time at very distant galaxies and clusters, we can study how they formed and evolved.

Surprisingly, we now know that all these enormous structures consist largely of “dark matter” that emits little or no electromagnetic radiation.

16.1 The Discovery of Galaxies In the 1770s, the French astronomer

Charles Messier was interested in discovering comets.

To do so, he had to be able to recognize whenever a new fuzzy object (a candidate comet) appeared in the sky.

To minimize possible confusion, he thus compiled a list of about 100 diffuse objects that could always be seen, as long as the appropriate constellation was above the horizon.

Some of them are nebulae, and others are star clusters, which can appear fuzzy through a small telescope such as that used by Messier.

16.1 The Discovery of Galaxies To this day, the objects in Messier’s list

are commonly known by their Messier numbers (see figures).

They are among the brightest and most beautiful objects in the sky visible from mid-northern latitudes.

A set of photographs of all the Messier objects appears as an Appendix.

16.1 The Discovery of Galaxies

Other astronomers subsequently compiled additional lists of nebulae and star clusters.

By the early part of the 20th century, several thousand nebulae and clusters were known.

The nebulae were especially intriguing: Although some of them, such as the Orion Nebula (see figure, right), seemed clearly associated with bright stars in our Milky Way Galaxy, the nature of others was more controversial.

When examined with the largest telescopes then available, many of them showed traces of spiral structure, like pinwheels, but no obvious stars (see figure, left).

16.1a The Shapley-Curtis Debate Some astronomers thought that these so-called

spiral nebulae were merely in our own Galaxy, while others suggested that they were very far away—“island universes” in their own right, so distant that the individual stars appeared blurred together.

The distance and nature of the spiral nebulae was the subject of the well-publicized “Shapley-Curtis debate,” held in 1920 between the astronomers Harlow Shapley and Heber Curtis.

Shapley argued that the Milky Way Galaxy is larger than had been thought, and could contain such spiral-shaped clouds of gas.

Curtis, in contrast, believed that they are separate entities, far beyond the outskirts of our Galaxy.

This famous debate is an interesting example of the scientific process at work.

16.1a The Shapley-Curtis Debate

Shapley’s wrong conclusion was based on rather sound reasoning but erroneous measurements and assumptions.

For example, one distinguished astronomer thought he had detected the slight angular rotation of a spiral nebula, and Shapley correctly argued that this change would require a preposterously high physical rotation speed if the nebula were very distant.

It turns out, however, that the measurement was faulty.

16.1a The Shapley-Curtis Debate

Shapley also argued that a bright nova that had appeared in 1885 in the Andromeda Nebula, M31, the largest spiral nebula (see figures), would be far more powerful than any previously known nova if it were very distant.

Unfortunately, the existence of supernovae (this object turned out to be one), which are indeed more powerful than any known nova, was not yet known.

On the other hand, Curtis’s conclusion that the spiral nebulae were external to our Galaxy was based largely on an incorrect notion of our Galaxy’s size; his preferred value was much too small.

Moreover, he treated the nova in Andromeda as an anomaly.

16.1b Galaxies: “Island Universes”

The matter was dramatically settled in the mid-1920s, when observations made by Edwin Hubble (see figure) at the Mount Wilson Observatory in California proved that the spiral nebulae were indeed “island universes” (now called galaxies) well outside the Milky Way Galaxy.

Using the 100-inch (2.5-m) telescope, Hubble discovered very faint Cepheid variable stars in several objects, including the Andromeda Nebula.

As we saw in Chapter 11, Cepheids are named after their prototype, the variable star d (the Greek letter “delta”) Cephei.

Their light curves (brightness vs. time) have a distinctive, easily recognized shape.

Cepheids are intrinsically very luminous stars, 500 to 10,000 times as powerful as the Sun, so they can be seen at large distances, out to a few million light-years, with the 100-inch telescope used by Hubble.

16.1b Galaxies: “Island Universes” Cepheids are very special to astronomers

because measuring the period of a Cepheid’s brightness variation (using what we are calling Leavitt’s law, after Henrietta Leavitt) gives you its average luminosity.

And comparing its average luminosity with its average apparent brightness tells you its distance, using the inverse-square law of light.

The Cepheids in the spiral nebulae observed by Hubble turned out to be exceedingly distant.

The Andromeda Nebula, for example, was found to be over 1 million light-years away (the value is now known to be about 2.4 million light-years)—far beyond the measured distance of any known stars in the Milky Way Galaxy.

16.1b Galaxies: “Island Universes” From the distance and the measured angular size of

the Andromeda Nebula, its physical size was found to be enormous.

Clearly, the “spiral nebulae” were actually huge, gravitationally bound stellar systems like our own Milky Way Galaxy, not relatively small clouds of gas like the Orion Nebula (and so the Andromeda Nebula was renamed the Andromeda Galaxy).

The effective size of the Universe, as perceived by humans, increased enormously with this realization.

In essence, Hubble brought the Copernican revolution to a new level; not only is the Earth just one planet orbiting a typical star among over 100 billion stars in the Milky Way Galaxy, but also ours is just one galaxy among the myriads in the Universe!

Indeed, it is humbling to consider that the Milky Way is one of roughly 50 to 100 billion galaxies within the grasp of the world’s best telescopes such as the Keck twins and the Hubble Space Telescope.

16.2 Types of Galaxies

Galaxies come in a variety of shapes.

In 1925, Edwin Hubble set up a system of classification of galaxies, and we still use a modified form of it.

16.2a Spiral Galaxies There are two main “Hubble types” of

galaxies. We are already familiar with the first

kind—the spiral galaxies. The Milky Way Galaxy and its near-twin,

the Andromeda Galaxy (M31; see figure, left), are relatively large examples containing several hundred billion stars. (Most spiral galaxies contain a billion to a trillion stars.)

Another near-twin is NGC 7331 (see figure, below).

Spiral galaxies consist of a bulge in the center, a halo around it, and a thin rotating disk with embedded spiral arms.

There are usually two main arms, with considerable structure such as smaller appendages.

Doppler shifts indicate that spiral galaxies rotate in the sense that the arms trail.

16.2a Spiral Galaxies

In the figure, which is based on a very similar “tuning-fork diagram” drawn by Hubble, shows different types of spiral galaxies.

Relative to the disk, the bulge is large in some spiral galaxies (known as “Sa”), which also tend to have more tightly wound spiral arms.

The bulge is progressively smaller (relative to the disk) in spirals known as “Sb,” “Sc,” and “Sd,” which also tend to have more loosely wound spiral arms.

Moreover, spirals with smaller bulge-to-disk ratios generally have more gas and dust, and larger amounts of active star formation within the arms at the present time.

16.2a Spiral Galaxies Spiral galaxies viewed along, or nearly along, the plane

of the disk (that is, “edge-on”) often exhibit a dark dust lane that appears to divide the disk into two halves (see figures).

16.2a Spiral Galaxies In nearly one half of all

spirals, the arms unwind not from the nucleus, but rather from a relatively straight bar of stars, gas, and dust that extends to both sides of the nucleus (see figure, top).

These “barred spirals” are similarly classified in the Hubble scheme from “SBa” to “SBd” (the “B” stands for “barred”), in order of decreasing size of the bulge and increasing openness of the arms (see figure, bottom).

In many cases, the distinction between a barred and nonbarred spiral is subtle.

Studies show that our own Milky Way Galaxy is a barred spiral, probably of type SBbc (that is, intermediate between SBb and SBc).

16.2a Spiral Galaxies Because they contain many

massive young stars, the spiral arms appear bluish in color photographs.

Between the spiral arms, the whitish-yellow disks of spiral galaxies contain both old and relatively young stars, but not the hot, massive, blue main-sequence stars, which have already died.

Very old stars dominate the bulge, and especially the faint halo (which is difficult to see), and so the bulge is somewhat yellow/orange or even reddish in photographs (see figures).

16.2a Spiral Galaxies Since young, massive stars heat the dusty

clouds from which they formed, resulting in the emission of much infrared radiation, the current rate of star formation in a galaxy can be estimated by measuring its infrared power.

Space telescopes such as the Infrared Astronomical Satellite (IRAS, in the mid-1980s) and the Infrared Space Observatory (ISO, in the mid-1990s) were very useful for this kind of work, and it is being continued with the infrared camera on the Hubble Space Telescope and, at even longer infrared wavelengths, with the Spitzer Space Telescope.

16.2a Spiral Galaxies About half of the energy

emitted by our own Milky Way Galaxy is in the infrared, indicating that a lot of stars are being formed.

But we don’t know why the Andromeda Galaxy, which in optical radiation resembles the Milky Way, emits only 3 per cent of its energy in the infrared.

This galaxy and the Sombrero Galaxy emit infrared mostly in a ring rather than in spiral arms (see figure).

16.2b Elliptical Galaxies

Hubble recognized a second major galactic category: elliptical galaxies (see figures).

These objects have no disk and no arms, and generally very little gas and dust.

Unlike spiral galaxies, they do not rotate very much.

At the present time, nearly all of them consist almost entirely of old stars, so they appear yellow/orange or even reddish in true-color photographs.

The dearth of gas and dust is consistent with this composition: There is insufficient raw material from which new stars can form.

In many ways, then, an elliptical galaxy resembles the bulge of a spiral galaxy.

16.2b Elliptical Galaxies Elliptical galaxies can be

roughly circular in shape (which Hubble called type E0), but are usually elongated (from E1 to E7, in order of increasing elongation).

Since the classification depends on the observed appearance, rather than on the intrinsic shape, some E0 galaxies must actually be elongated, but are seen end-on (like a cigar viewed from one end).

16.2b Elliptical Galaxies

Most ellipticals are dwarfs, like the two main companions of the Andromeda Galaxy (see figure below and (b)), containing only a few million solar masses—a few per cent of the mass of our Milky Way Galaxy.

Some, however, are enormous, consisting of a few trillion stars in a volume several hundred thousand light-years in diameter (see figure (a)).

Many ellipticals may have resulted from two or more spiral galaxies colliding and merging, as we will discuss later.

16.2c Other Galaxy Types “Lenticular” galaxies (also known as “S0”

[pronounced “ess-zero”] galaxies) have a shape resembling an optical lens; they combine some of the features of spiral and elliptical galaxies.

They have a disk, like spiral galaxies. On the other hand, they lack spiral arms, and

generally contain very little gas and dust, like elliptical galaxies.

Hubble put them at the intersection between spiral and elliptical galaxies in his classification diagram (see figure on the next slide).

Though sometimes called “transition galaxies,” this designation should not be taken literally: The diagram is not meant to imply that spiral galaxies evolve with time into ellipticals (or vice versa) in a simple manner, contrary to the belief of some astronomers decades ago.

16.2c Other Galaxy Types


A few per cent of galaxies at the present time in the Universe show no clear regularity.

Examples of these “irregular galaxies” include the Small and Large Magellanic Clouds, small satellite galaxies that orbit the much larger Milky Way Galaxy (see figures).

Sometimes traces of regularity—perhaps a bar—can be seen.

Irregular galaxies generally have lots of gas and dust, and are rapidly forming stars.

Indeed, some of them emit 10 to 100 times as much infrared as optical energy, probably because the rate of star formation is greatly elevated.


Some galaxies are called “peculiar.” These often look roughly like spiral or

elliptical galaxies, but have one or more abnormalities.

For example, some peculiar galaxies look like interacting spirals (see figure, above), or like spirals without a nucleus (that is, like rings, (see figure, left)), or like ellipticals with a dark lane of dust and gas.

The ring galaxies are the result of collisions of galaxies.

16.3 Habitats of Galaxies

Most galaxies are not solitary; instead, they are generally found in gravitationally bound binary pairs, small groups, or larger clusters of galaxies.

Binary and multiple galaxies consist of several members.

An example is the Milky Way Galaxy with its two main companions (the Magellanic Clouds (see figure, top)), or Andromeda and its two main satellites (see figure, bottom).

Both Andromeda and the Milky Way have several even smaller companions.

16.3 Habitats of Galaxies Galaxies and clusters of galaxies all over the Universe are studied with the Hubble Space Telescope in the ultraviolet, visible, and near-infrared, the Spitzer Space Telescope in the infrared, and the Chandra X-ray Observatory in x-rays.

NASA’s Galaxy Evolution Explorer (GALEX), launched in 2003, is a small satellite that is studying galaxies and surveying the sky in the ultraviolet.

The Local Group is a small cluster of about 30 galaxies, some of which are binary or multiple galaxies.

Its two dominant members are the Andromeda (M31) and Milky Way Galaxies.

M33, the Triangulum Galaxy (see figures, top), is a smaller spiral.

M31 and M33, at respective distances of 2.4 and 2.6 million light-years, are the farthest objects you can see with your unaided eye.

The Local Group also contains four irregular galaxies, at least a dozen dwarf irregulars (see figure, bottom), four regular ellipticals, and the rest are dwarf ellipticals or the related “dwarf spheroidals.”

The diameter of the Local Group is about 3 million light-years.

16.3a Clusters of Galaxies


The Virgo Cluster (in the direction of the constellation Virgo, but far beyond the stars that make up the constellation), at a distance of about 50 million light-years, is the largest relatively nearby cluster (see figure, top).

It consists of at least 2000 galaxies spanning the full range of Hubble types, covering a region in the sky over 15° in diameter—about 15 million light-years.

The Coma Cluster of galaxies (in the direction of the constellation Coma Berenices) is very rich, consisting of over 10,000 galaxies at a distance of about 300 million light-years (see figure, bottom).


A majority of the galaxies in rich clusters are ellipticals, not spirals.

There is often a single, very large central elliptical galaxy (sometimes two) that is cannibalizing other galaxies in its vicinity, growing bigger with time (see figure, top).

X-ray observations of rich clusters reveal a hot intergalactic gas (10 million to 100 million K) within them, containing as much (or more) mass as the galaxies themselves (see figure, bottom).

The gas is clumped in some clusters, while in others it is spread out more smoothly with a concentration near the center.

This may be an evolutionary effect; the clumps occur in clusters that only recently formed from the gravitational attraction of their constituent galaxies and groups of galaxies.

As clusters age, the gas within them becomes more smoothly distributed and partly settles toward the center.

16.3b Superclusters of Galaxies Clusters are seen to vast distances, in

a few cases up to 8 billion light-years away.

When we survey their spatial distribution, we find that they form clusters of clusters of galaxies, appropriately called superclusters.

These vary in size, but a typical diameter is about 100 million light-years.

The Local Group, dozens of similar groupings nearby, and the Virgo Cluster form the Local Supercluster.

16.3b Superclusters of Galaxies

Superclusters often appear to be elongated and flattened.

The thickness of the Local Supercluster, for example, is only about 10 million light-years, or one tenth of its diameter.

Superclusters tend to form a network of bubbles, like the suds in a kitchen sink (see figures).

Large concentrations of galaxies (that is, several adjacent superclusters) surround relatively empty regions of the Universe, called voids, that have typical diameters of about 100 million light-years (but sometimes up to 300 million light-years).

They formed as a consequence of matter gravitationally accumulating into superclusters; the regions surrounding the superclusters were left with little matter.

16.3b Superclusters of Galaxies

Does the clustering continue in scope?

Are there clusters of clusters of clusters, and so on?

The present evidence suggests that this is not so.

Surveys of the Universe to very large distances do not reveal many obvious super-superclusters.

There are, however, a few giant structures such as the “Great Wall” that crosses the center of the slices shown in the figures.

We will discuss in Chapter 19 how the “seeds” from which these objects formed were visible within 400,000 years after the birth of the Universe.

16.4 The Dark Side of Matter There are now strong indications

that much of the matter in the Universe does not emit any detectable electromagnetic radiation, but nevertheless has a gravitational influence on its surroundings.

One of the first clues was provided by the flat (nearly constant) rotation curves of spiral galaxies.

16.4a The Rotation Curve of the Milky Way Galaxy

The rotation curve of any spinning galaxy is a plot of its orbital speed as a function of distance from its center.

For example, the rotation curve of the Milky Way Galaxy has been determined through studies of the motions of stars and clouds of gas (see figure).

It rises from zero in the center, to a value of about 200 km /sec at a radial distance of about 5000 light-years.

The rotation curve farther out is rather “flat”; the orbital speed stays constant, all the way out to distances well beyond that of the Sun.


The speed of a star at any given distance from the center is determined by the gravitational field of matter enclosed within the orbit of that star—that is, by the matter closer to the center. (It can be shown that matter at larger distances, outside the star’s orbit, does not affect the star’s speed as long as the galaxy’s disk has a smooth, symmetric distribution of matter.)

So, we can use the rotation curve to map out the distribution of mass within our Galaxy.

The speeds and distances of stars near our Galaxy’s edge, for example, are used to measure the mass in the entire Galaxy.


Specifically, Kepler’s third law can be manipulated to give an expression for the mass (M) enclosed within an orbit of radius R from the center.

A similar method is used to find the amount of mass in the Sun by studying the orbits of the planets, or the amount of a planet’s mass by observing the orbits of its moons.

In the 17th century, Newton developed this technique as part of his derivation and elaboration of Kepler’s third law of orbital motion (see Chapter 5).


If we insert the Sun’s distance from the Galactic center (26,000 light-years) and the Sun’s orbital speed (200 km /sec) into the formula, we find that the matter within the Sun’s orbit has a mass of about 100 billion (1011) solar masses!

But the mass of a typical star is about half that of the Sun.

Thus, if most of the matter in our Galaxy is in the form of stars (rather than interstellar gas and dust, black holes, etc.), we conclude that there are about 200 billion stars within the Sun’s orbit, closer to the Galaxy’s center than the Sun is.


The next thing to notice is that the flat rotation curve of the Milky Way Galaxy (see figure) is quite different from the rotation curve of the Solar System.

As discussed in Chapter 5, the orbital speeds of distant planets are slower than those of planets near the Sun, instead of being roughly independent of distance.

In the Solar System, the Sun’s mass greatly dominates all other masses; the masses of the planets are essentially negligible in comparison with the Sun.

But in the Milky Way Galaxy, the flat rotation curve implies that except in the central region (where the rotation curve isn’t flat), the mass grows with increasing distance from the center.


The growth in mass of our Galaxy continues to large distances beyond the Sun’s orbit. (The rotation curve way out from the center is usually determined from the measured speeds of clouds of hydrogen gas, which can be easily seen at radio wavelengths.)

This is very puzzling because few stars are found in those regions: The number of stars falls far short of accounting for the derived mass.

For example, at a distance of 130,000 light-years from the center, the enclosed mass is about 5 X 1011 solar masses, and the corresponding number of typical stars (each having half a solar mass) would be about a trillion—yet there are too few stars visible, by a large margin.

Indeed, studies of the outer parts of the Milky Way Galaxy throughout the electromagnetic spectrum do not reveal sufficient quantities of material to account for the derived mass.

16.4b Dark Matter Everywhere

We conclude that the Milky Way Galaxy contains large quantities of “dark matter”—material that exerts a gravitational force, but is invisible or at least very difficult to see!

This material has sometimes been called the “missing mass,” especially in older texts, but the term is not appropriate because the mass is present.

Instead, it is the light that’s missing. Many other spiral galaxies also have flat (speed roughly constant) rotation curves, as was first shown by Vera Rubin (see figure).

Estimates suggest that 80 to 90 per cent of the mass of a typical spiral galaxy consists of dark matter.

However, it has been shown that the amount of matter in the disk cannot exceed what is visible by more than a factor of 2.

Instead, the dark matter is probably concentrated in an extended, spherical, outer halo of material that extends to perhaps 200,000 light-years from the galactic center.

16.4b Dark Matter Everywhere Similar studies show that elliptical galaxies also

contain large amounts of dark matter. Gas and stars are moving so quickly that they would

escape if the visible matter alone produced the gravitational field.

There must be another, more dominant, contribution to gravity in these galaxies.

The orbital speeds of galaxies in binary pairs, groups, and clusters can be used to determine the masses of these systems.

Astronomers find that in essentially all cases, the amount of mass required to produce the observed orbital speeds is larger than that estimated from the visible light (which is assumed to come from stars and gas).

A related technique is to measure the typical speeds of particles of gas bound to a cluster of galaxies—and once again, the particles could not be gravitationally bound to the cluster if its mass consisted only of that provided by the visible matter.

16.4b Dark Matter Everywhere Decades ago, the Caltech

astronomer Fritz Zwicky was the first to point out that clusters of galaxies could not remain gravitationally bound if they contain only visible matter.

He postulated the existence of some form of dark matter.

However, this idea was largely ignored or dismissed—it was too far ahead of its time.

16.4c What Is Dark Matter? What is the physical nature of the dark matter in

single and binary galaxies, groups, and clusters? We just don’t know—this is one of the outstanding

unsolved problems in astrophysics. A tremendous number of very faint normal stars (or

even brown dwarfs) is a possibility, though it seems unlikely, extrapolating from the numbers of the faintest stars that we can study.

There is some evidence (see Section 16.5 of the following slides) that part of the dark matter consists of old white dwarfs.

If these and other corpses of dead stars (neutron stars, black holes) accounted for most of the dark matter, however, then where is the chemically enriched gas that the stars must have ejected near the ends of their lives?

Other candidates for the dark matter are small black holes, massive planets (“Jupiters”), and neutrinos.

16.4c What Is Dark Matter? We will see in Chapter 19 that certain kinds of

measurements indicate that only a small fraction of the dark matter can consist of “normal” particles such as protons, neutrons, and electrons; the rest must be exotic particles.

Most of the normal dark matter consists of tenuous, million-degree gas in galactic halos.

This gas was recently detected by the absorption spectra it produced in the radiation from background objects, and also from its emission at relatively long x-ray wavelengths.

Though no longer technically “dark” (because we have seen it!), such matter is still generally considered to be part of the “dark matter” that pervades the Universe; it is difficult to detect.

16.4c What Is Dark Matter? Probably the most likely candidate for a majority of

the dark matter (the “abnormal” part) is undiscovered subatomic particles with unusual properties, left over from the big bang, such as WIMPs—“weakly interacting massive particles.”

Physicists studying the fundamental forces of nature suggest that many WIMPs exist, though it is disconcerting that none has been unambiguously detected in a laboratory experiment.

If it is unsatisfactory to you that most of the mass in our Galaxy (and indeed, most of the mass in the Universe!) is in some unknown form, you may feel better by knowing that astronomers also find the situation unsatisfactory.

But all we can do is go out and conduct our research, and try to find out more.

Clever new techniques, such as one described on the next slides, may provide the crucial clues that we seek.

16.5 Gravitational Lensing The phenomenon of gravitational lensing of

light provides a powerful probe of the amount (and in some cases the nature) of dark matter.

About one hundred cases of gravitational lensing have been found thus far.

If the light from a distant object passes through a gravitational field, the light is bent—that is, it follows a curved path through the warped space–time.

This is analogous to (but differs in detail from) the effect that a simple glass lens has on light.

It is, perhaps, more akin to the warping of images we get from a fun-house mirror, but the terms “lens” and “lensing” have caught on.

16.5 Gravitational Lensing

We have already encountered lensing in Chapter 10: Recall that Einstein’s general theory of relativity predicts that the Sun should bend the light of stars beyond it by an amount twice that predicted with Newtonian theory, and that this effect was first measured in 1919 by Arthur Eddington and others during a total solar eclipse.

If an observer, a galaxy acting as a gravitational lens, and a more distant, very compact object are nearly perfectly aligned (colinear), the distant object will look like a circle (known as an “Einstein ring”) centered on the lens (see figures).


More usually, deviations from symmetry (for example, slight misalignment of the lens) lead to the formation of several discrete, well-separated images (see figures).

The apparent brightness of the object is magnified, in some cases by large amounts.

When a massive cluster of galaxies lenses many distant galaxies, a collection of arcs tends to be seen (see figures next slide).

The number of arcs, their magnification factors, and their distorted pattern depend on the mass of the cluster, as well as on the distribution of mass within the cluster.

This method measures the total mass (visible and dark) in the cluster, and gives results consistent with those obtained from other techniques.

Again, we conclude that dark matter dominates most clusters of galaxies.


Projects in which the apparent brightness of millions of stars in the Large and Small Magellanic Clouds are systematically monitored have revealed that occasionally, a star brightens and fades over the course of a few weeks. (These galaxies provide a nice background field of stars that are out of the Galactic plane.)

The light curve has exactly the shape expected if a compact lens were to pass between the star and us, temporarily focusing the star’s light toward us (see figure).

Moreover, the shape and height (amplitude) of the light curve is independent of the filter through which it was obtained, as predicted for gravitational lensing and unlike the case for intrinsically variable stars.

16.5 Gravitational Lensing Some of these searches for lensed stars

in the Magellanic Clouds were motivated by the opportunity of finding so-called “massive compact halo objects,” or MACHOs.

In principle, such objects would be too massive to be brown dwarfs, yet could not be normal main-sequence stars because we would see them.

White dwarfs are a possible candidate, but in this case the population of stars that produced them must have been devoid of very low-mass stars, since we do not see enough stars in the halo, where astronomers expect that MACHOs would be found.

16.5 Gravitational Lensing The searches have already detected hundreds

of lensed stars in the Magellanic Clouds. There is considerable controversy about the interpretation of these detections.

In a few especially favorable cases, the distance of the lens has been determined.

In two cases, the lens turns out to be in the Large Magellanic Cloud itself, rather than in the halo of the Milky Way Galaxy.

Another Hubble discovery of a lens in the direction of the Large Magellanic Cloud turns out to be an ordinary star in the disk of the Milky Way Galaxy.

So far, the searches have not revealed any definitive lenses in the halo of our Galaxy.

16.5 Gravitational Lensing If most of the lenses are not in our Galaxy’s

halo, then the evidence for dark, compact objects greatly decreases.

Indeed, the most recent estimates (late 2005) suggest that no more than about 10 or 20 per cent of the halo’s mass consists of MACHOs.

Thus, it appears that much of the dark matter in the halo may consist of subatomic particles like the WIMPs mentioned above.

It is exciting to think that astronomical observations may end up providing the crucial evidence for the existence of tiny, otherwise undetectable particles.

Gravitational lensing also helps us find out about dark matter by revealing how mass is distributed in galaxies.

Just how centralized mass is in galaxy cores can, in principle, be revealed by the study of gravitational lenses with multiple images.

16.6 The Birth and Life of Galaxies

It is difficult or impossible to determine what any given nearby galaxy (or our own Milky Way Galaxy) used to look like, since we can’t view it as it was long ago.

However, as we discussed in Chapter 1, the finite speed of light effectively allows us to view the past history of the Universe: We see different objects at different times in the past, depending on how long the light has been travelling toward us.

At least in a statistical manner we can explore galactic evolution by examining galaxies at progressively larger distances or lookback times, and hence progressively farther back in the past.

16.6 The Birth and Life of Galaxies An important but likely valid assumption is that we

live in a typical part of the Universe, so that nearby galaxies are representative of galaxies everywhere.

Hence, galaxies several billion light-years away, viewed as they were billions of years ago, probably resemble what today’s nearby galaxies used to look like.

The refurbished Hubble Space Telescope has led to the most progress in this field, since it provides detailed images of faint, distant galaxies.

Also, the Chandra X-ray Observatory has revealed objects that might be very primitive, distant galaxies; the seemingly uniform x-ray glow that previous x-ray telescopes had detected is actually produced by many individual discrete sources.

It is crucial to know the distances of the very distant galaxies, but they are so far away that no Cepheid variables or other normal stars can be seen and compared with nearby examples.

So, an indirect technique is used: Hubble’s law, as described below.

16.7 The Expanding Universe

Early in the 20th century, Vesto Slipher of the Lowell Observatory in Arizona noticed that the optical spectra of “spiral nebulae” (later recognized by Edwin Hubble to be separate galaxies) almost always show a redshift (recall our discussion of redshifts and blueshifts in Chapter 11).

The absorption or emission lines seen in the spectra have the same patterns as in the spectra of normal stars or emission nebulae, but these patterns are displaced (that is, shifted) to longer (redder) wavelengths (see figure).

Under the assumption that the redshift results from the Doppler effect, we can conclude that most galaxies are moving away from us, regardless of their direction in the sky. (In Chapter 18, we will see that the redshift is actually caused by the stretching of space, but the equation is the same as that for the Doppler effect, at least at low redshifts.)


In 1929, using newly derived distances to some of these galaxies (from Cepheid variable stars), Hubble discovered that the displacement of a given line (that is, the redshift) is proportional to the galaxy’s distance. (In other words, when the redshift we observe is greater by a certain factor, the distance is greater by the same factor.)

Thus, under the Doppler-shift interpretation, the recession speed, v, of a given galaxy must be proportional to its current distance, d (see figure).

This relation is known as Hubble’s law, v=H0d, where H0 (pronounced “H naught”) is the present-day value of the constant of proportionality, H (the factor by which you multiply d to get v). H0 is known as Hubble’s constant.


For various reasons, Edwin Hubble’s original data were suggestive but not conclusive.

Subsequently, Hubble’s assistant and disciple Milton Humason joined Hubble in very convincingly showing the relationship (see figure). (Interestingly, Humason had first come to Mt. Wilson as a mule-team driver, helping to bring telescope parts up the mountain. He worked his way up within the organization.)

16.7 The Expanding Universe This behavior is similar to that produced by

an explosion: Bits of shrapnel are given a wide range of speeds, and those that are moving fastest travel the largest distance in a given amount of time.

Although Edwin Hubble himself initially resisted this idea, the implication of Hubble’s law is that the Universe is expanding!

As we shall see in Chapter 18, however, there is no unique center to the expansion, so in this way it is not like an explosion.

Moreover, the expansion of the Universe marks the creation of space itself, unlike the explosion of a bomb in a preexisting space.

16.7 The Expanding Universe One of the greatest debates in 20th century

astronomy has been over the value of Hubble’s constant.

In Chapter 18 we will explain in detail how it is determined.

Measurements of the recession speeds of galaxies at known distances showed that H0=50 to 80 km /sec/Mpc; as of late 2005, the value is known to be 71 km /sec/Mpc to within 10 per cent. (We will discuss these definitive measurements, from NASA’s Wilkinson Microwave Anisotropy Probe spacecraft, in Chapter 19.)

For example, a galaxy 10 Mpc (32.6 million light-years) away recedes from us with a speed of about 710 km/sec, and a galaxy 20 Mpc away recedes with a speed of about 1420 km /sec.

16.7 The Expanding Universe Hubble’s constant is always quoted in the

strange units of km /sec/Mpc. The value H0=71 km/sec/Mpc simply means that for each megaparsec (3.26 million lightyears) of distance, galaxies are receding 71 km /sec faster.

The expansion of the Universe will be the central theme in Chapters 18 and 19; we will discuss its implications and associated phenomena.

For now, however, let us simply use Hubble’s law to determine the distances of very distant galaxies and other objects.

Knowing the value of H0, a measurement of a galaxy’s recession speed v then gives the distance d, since d=v/H0.

16.7 The Expanding Universe Note that Hubble’s law cannot be used to

find the distances of stars in our own Galaxy, or of galaxies in our Local Group; these objects are gravitationally bound to us, and hence the expansion of the intervening space is overcome.

Moreover, Hubble’s law does not imply that objects in the Solar System or in our Galaxy are themselves expanding; they are bound together by forces strong enough to overcome the tendency for empty space to expand.

Hubble’s law applies to distant galaxies and clusters of galaxies; the space between us and them is expanding.

16.8 The Search for the Most Distant Galaxies

With the Hubble Space Telescope, we obtained relatively clear images of faint galaxies that are suspected to be very distant.

The main imaging camera of the time (the Wide Field/Planetary Camera 2) exposed on a small area of the northern sky for 10 days in December 1995.

Though it covers only about one 30-millionth of the area of the sky (roughly the apparent size of a grain of sand held at arm’s length), this Hubble Deep Field contains several thousand extremely faint galaxies (see figure).

If we could photograph the entire sky with such depth and clarity, we would see about 50 to 100 billion galaxies, each of which contains billions of stars!


Three years later, the Hubble Space Telescope got very deep images of another region, this time in the southern celestial hemisphere: the Hubble Deep Field—South.

It looks similar to the northern field, even though it is nearly in the opposite direction in the sky, providing some justification for our assumption that the Universe is reasonably uniform over large scales.

Later, after the Advanced Camera for Surveys was installed on Hubble, it was used to make a Hubble Ultra Deep Field (see figure).

These regions of the deep fields and ultra deep field have since been observed by many other telescopes on the ground and in space, notably the Chandra X-ray Observatory.


Other deep surveys further strengthen our conclusion that we live in a rather typical place in the Universe. Spectra obtained with large telescopes, especially the two Keck telescopes in Hawaii, confirm that many galaxies in the Hubble Deep/Ultra Deep Fields and other deep surveys have large redshifts and hence are very far away.

Though a few of the galaxies have relatively low redshifts, typical redshifts of the faintest objects are between 1 and 4 (see figure).


Light that we observe at visible wavelengths actually corresponds to ultraviolet radiation emitted by the galaxy, but shifted redward by 100 per cent to 400 per cent!

If we convert these redshifts to “distances” (or, more precisely, lookback times—see Table 16 –1), we find that the galaxies are billions of light-years away.

We see them as they were billions of years in the past, when the Universe was much younger than it is now.

Note that when astronomers say that light from a high-redshift galaxy comes from “the distant universe,” what they really mean is “distant parts of our Universe.”

We do not receive light from other universes, even if they exist!


In the past few years, many galaxies with redshifts exceeding 5 (that is, all the spectral lines are shifted by over 500 per cent, putting them at more than 600 per cent of their original values) were discovered (see figure), and there are quite a few with redshifts over 6.

As of late 2005, at least one galaxy has been reported with a redshift of about 7.

Several objects are suspected of having even higher redshifts, though the spectra are not yet good enough to be certain. The lookback time

corresponding to redshift 6 is about 12.7 billion years (Table 16 –1); we are seeing denizens of an era shortly (a billion years) after the birth of the Universe.

They are probably newly formed galaxies.


Out of the thousands of galaxies found in images such as the Hubble Deep and Ultra Deep Fields, how do we go about choosing those that are likely to be at high redshift? (After all, with limited time for spectroscopy using large telescopes such as Keck, it is important to improve the odds if the goal is to find the highest redshifts.)

One very effective technique is to first measure the “color” of each galaxy (see figure).

For two reasons, those that are likely to be very far away look very red, and have little if any of the blue or ultraviolet light normally emitted by stars.

First, their redshift moves light to redder (longer) wavelengths.

Second, there is often another galaxy or large cloud of gas along the way, and the hydrogen gas within it completely absorbs the ultraviolet light.

16.9 The Evolution of Galaxies

Comparisons of the appearance of distant galaxies and nearby galaxies provide clues to the way in which galaxies evolve.

These comparisons must be done carefully to avoid wrong conclusions.

For example, a visible-light image of a high-redshift galaxy actually corresponds to ultraviolet radiation emitted by the galaxy and shifted into the visible band, so it would not be fair to compare the image with the visible-light appearance of a nearby (and hence essentially unshifted) galaxy.

One way around this problem is to compare the visible-light images of high-redshift galaxies with ultraviolet images of nearby galaxies, as obtained with the Hubble Space Telescope.

These images emphasize regions containing hot, massive, young stars that glow brightly in the ultraviolet (see figure).

16.9 The Evolution of Galaxies Another technique is to obtain infrared images of

the high-redshift galaxies, and compare them with the visible-light images of nearby galaxies.

Such images tend to be dominated by light from older, less massive stars that more accurately reflect the overall shape of the galaxy rather than pockets of recent, intense star formation (see figure).

So far, the clearest infrared images have been made with the Hubble Space Telescope. (Its infrared camera ran out of solid nitrogen coolant in 1998, sooner than anticipated. A new method of cooling the camera was used in equipment installed during the servicing mission in 2002, and it is now working even better than before.)

16.9 The Evolution of Galaxies These data, together with various types of

analysis such as computer simulations (for example, of what happens when two galaxies collide and merge), provide many interesting results.

One spectacular conclusion is that most spiral galaxies used to look quite peculiar; there were essentially no large galaxies with distinct, well-formed spiral arms beyond redshift 2.

By redshift 1 there were quite a few of them, but many took on their current, mature shapes more recently, in the past 5 billion years (that is, at redshifts below about 0.5).


Another conclusion is that there used to be a large number of small, blue, irregular galaxies that formed stars at an unusually high rate (see figure).

Their strange shape might be partly caused by an irregular distribution of young star clusters.

However, they appear peculiar even at infrared wavelengths, which are more sensitive to older stars, so they must be structurally disturbed.

Some of them probably later merged together to form larger galaxies, including disturbed spirals.

Perhaps others faded and are now difficult to find because they are so dim.


It appears that most elliptical galaxies formed early in the Universe, beyond redshift 2 (lookback time 10 billion years); there are many old-looking, well-formed ellipticals between redshifts of 1 and 2.

On the other hand, we also think that an elliptical galaxy can be produced by the collision and merging of two spiral galaxies.

Many new stars are created from the interstellar gas in the spirals.

Computer models of the merging process also show that long “tails” of material are sometimes temporarily formed (see figure on next slide), just as we see in nearby examples of interacting galaxies (see figures, left).

Later these tails disappear, leaving a more normal looking elliptical galaxy, but with a population of stars younger than in the really ancient ellipticals.

In fact, the Milky Way Galaxy and the Andromeda Galaxy are approaching each other and may collide (or barely miss each other) in about 5 or 6 billion years. Subsequently, they are likely to merge and become an elliptical galaxy over the course of billions of years.

16.9 The Evolution of Galaxies The total rate at which stars currently form in

the Universe is rather small compared with what it was billions of years ago.

We see that the star formation rate has decreased since a redshift of 1 (8 billion years ago) to the present time.

The rate may have been constant at still larger redshifts, up to about 4 or 5 (12 billion years ago), though we are unsure because many high-redshift galaxies are cloaked with dust.

The dust seems to have been produced by the first few generations of stars, making it difficult to detect high-redshift galaxies at visible wavelengths.

But infrared and submillimeter telescopes are finding them in progressively larger numbers, so we can expect a more accurate census in the near future.

16.9 The Evolution of Galaxies As galaxies age, they evolve chemically,

primarily because of supernovae that create many of the heavy elements through nuclear reactions and disperse them into the cosmos.

Large, massive galaxies, whose gravitational fields don’t allow much of the gas to escape, tend to become more chemically enriched than small galaxies that are not able to retain the hot gas.

So, we don’t expect to find many rocky, Earth-like planets in small (dwarf ) galaxies like the Magellanic Clouds.

The formation of massive galaxies like the Milky Way seems to be a critical step for the existence of humans.

16.10 Evolution of Large-ScaleStructure

We have also studied the evolution of large-scale structure in the Universe.

By getting the redshifts of hundreds of thousands of galaxies over large regions of the sky (see figure, right), the growth of clusters, superclusters, and voids can be traced (see figure, below).

As mentioned earlier, it appears that superclusters and giant walls of galaxies are the largest structures in the Universe; we have no clear evidence for super-superclusters of galaxies.


Galaxies generally preceded clusters, and then gravitationally assembled themselves into clusters.

Many clusters formed relatively recently, within the past 5 billion years (that is, at redshifts below 0.5), and in fact are still growing now.

However, cluster formation did begin earlier. Some very large, well-formed clusters have

been found at redshift 1 (8 billion light-years away), and evidence exists for substantial concentrations of matter (which later formed clusters) at a redshift of 4, corresponding to a lookback time of about 12 billion years.


The observed distribution of superclusters and voids (as the 2dF mapping project showed in the figure at top) can be compared with the predictions of various theoretical models using computer simulations (see figures).

One important conclusion is that dark matter pervades the Universe; otherwise, it is difficult to produce very large structures.

Galaxies and clusters seem to form at unusually dense regions (“peaks”) in the dark matter distribution, like snow on the peaks of mountains.


Agreement with observations seems best when most of the dark matter used in the simulations is “cold”—that is, moving relatively slowly compared with the speed of light.

Work is in progress to determine what specific type of cold dark matter is likely to account for most of the material.

Simulations that use primarily hot dark matter (such as neutrinos, with speeds close to that of light) do not produce galaxy distributions that resemble those observed.

Specifically, hot dark matter has a hard time clustering on small scales, like those of individual galaxies and small clusters.


Results announced in 2003 from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP; see Chapter 19) show that about 15 per cent of the matter in the Universe consists of protons, neutrons, and electrons—although only about one-quarter of this normal matter resides in stars and other visible objects, the rest being in the form of relatively hot gas, MACHOs, and other constituents.

The remaining 85 per cent is matter that does not consist of normal particles; moreover, it is dark.

This is the cold dark matter discussed above. It may be mostly WIMPs (see Section 16.4c), but

we don’t yet know this for sure, and no actual WIMPs have ever been directly detected in a laboratory.

Again, we emphasize that the nature of dark matter is one of the outstanding mysteries of modern astrophysics!

chapter 16 a universe of galaxies. introduction at the beginning of the 20th century, the nature of...

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