What Are Galaxies?

5 What Are Galaxies?

s we mentioned in the last chapter, the Universe is essentially a vast collection of galaxies. The logical next step in our journey then would be to understand these galaxies in a little more detail. So let’s look a little more carefully at the basic process we were just talking about. AGalaxies first began to form when the Universe was a few hundred million years old, when vast regions of hydrogen and helium began to collapse under their own gravity. Why hydrogen and helium? Well, because the Universe started out pretty simple. In order to make larger atoms you need fusion. The Universe only had about three minutes—the era of nucleosynthesis—during which fusion could occur naturally; and in that time it was only able to convert some of the hydrogen nuclei (protons) into helium nuclei. No elements heavier than helium were created. So when the galaxies were beginning to form, all the Universe had was hydrogen and helium. Each galaxy that formed as the Universe evolved out of the era of atoms took on its own unique shape. Nevertheless, many of the galaxies can be placed in one of two broad categories, based on the two predominant shapes. One shape is a disk-like structure, that often appears to have a spiral pattern swirled into it. These galaxies are generally calledspiral galaxies. Another popular shape for galaxies is a very symmetric blob. The exact shape of such galaxies can range from nearly perfect spheres to elongated footballs. Since oblongs and circles are examples of what mathematicians call ellipses, these galaxies are known as elliptical galaxies. These are the only two regular galaxy shapes. But there are also lots of other galaxies that have no particularly identifiable shape, and these are calledirregular galaxies. Irregular galaxies are not disks, they are not elliptical, they are generally not symmetric in any way. Each is just some particular, unique shape. Now, the fact that we see spiral, elliptical and irregular galaxies in our telescopes today does not mean that the galaxies necessarily formed in these shapes. Although it is possible that when the huge regions of gas in the early Universe began to collapse some of them collapsed into disks, some into ellipticals, and some into random, irregular shapes, it is also possible that all of them initially collapsed into the same shape, and only later evolved into some other shape. For example, every galaxy may have started out as a spiral galaxy, and they may all end up as irregular galaxies, passing through an elliptical phase along the way. That’s possible. Or perhaps they all started out with irregular shapes, and at some point they will all collect themselves into ellipticals and eventually evolve into spirals. The fact is that we don’t yet know how galaxies formed. However, we do have some “guesses,” if you will, as to how the process might have proceeded, and I’ll mention them in the sections below. In this chapter, which is a short one, we will take a brief look at each of the main types of galaxies, and also introduce some other, extreme types of galaxies.

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5.1 Spiral Galaxies As far as we can tell, about 70% of the large galaxies in the Universe are spiral galaxies. There are an awful lot of small galaxies that are not spirals. But of the big ones, about 70% are spirals. Figure 5.1 shows a very “spirally” looking spiral galaxy designated M101, also known as the “Pinwheel Galaxy.” Figure 5.2 shows another, particularly beautiful spiral galaxy, M104, popularly known as the “Sombrero Galaxy.”

Figure 5.1 A Face-on View of a Typical Spiral Galaxy Source: ESA/Hubble, Wikimedia Commons

Figure 5.2 The Spectacular Sombrero Galaxy, Also Known As M104 Source: ESA/Hubble, Wikimedia Commons

Spiral galaxies have three distinct regions that identify them: Thedisk , the bulge and Disk the halo. Figure 5.1 shows the disk and the bulge very clearly. The bulge is the bright Bulge region in the center, and the disk is essentially everything else. In general, it is the disk that Halo defines the overall shape of a spiral galaxy. It is round and flat, kind of like a pancake. It What Are Galaxies? 93

might have an obvious swirly pattern in it. These swirls are known as spiral arms. However, Spiral arms spiral galaxies don’t have to have spiral arms; and in those that do, the arms can vary considerably from one galaxy to the next, giving quite a variety of appearances. The halo cannot be seen in the Pinwheel Galaxy (Figure 5.1), but is clear in the Sombrero Galaxy (Figure 5.2). It is a faint spherical envelope that surrounds the entire galaxy. It is almost never visible in photographs of spiral galaxies, but we know of its exis- tence from our own galaxy. If you want to think of the disk as being like a pancake, then you can think of the halo as being like a faint spherical cloud with the pancake in the middle of it. We will look a little more closely at the halo, the disk and the bulge in Chapter 12. How big are spiral galaxies? Well, there is obviously some variation from one galaxy to the next. But the disk of a good sized spiral galaxy might be 100,000 light- years in diameter, and a few thousand light-years thick. The halo, which really defines the extent of a spiral galaxy, can be somewhat larger, up to as much as 200,000 light- years in diameter. In addition to what we might call “normal” spiral galaxies, there are a number of obviously spiral galaxies that have a different look to them because they have a straight “bar” across the center. These are calledbarred spiral galaxies; and although we won’t be Barred spiral galaxy discussing them in this book, I wanted to mention them for completeness. A photograph of a barred spiral galaxy is shown in Figure 5.3. As I said before, we don’t really know how galaxies first formed. However, here is one “guess” as to how one can make a spiral galaxy. All you need is two things: Gravity and rotation. When a cloud of gas collapses gravitationally, it pulls in on itself. If it is not rotating the natural thing for it to do is to form a perfect sphere, or ball. However, if the cloud of gas is rotating as it collapses, it will form mostly into a sphere; but some of the cloud of gas will sort of spin out into a disk around the sphere. This is a natural process. The more rapidly the cloud of gas is rotating as it collapses, the more of it will be spun out into a disk.

Figure 5.3 A Typical Barred Spiral Galaxy Barred spiral galaxies all share a straight bright region that extends through the bulge. Note how the spiral arms emanate from the ends of this bar. Source: NASA/ESA/Hubble, Wikimedia Commons 94 Chapter 5 Welcome to the Universe! A Brief Introduction to Astronomy

Why would a cloud of gas be rotating in the first place? Where does this rotation come from? This too is a perfectly natural occurrence. Think of it this way: Let’s say I have two particles that are attracted to each other by gravity. If I hold the particles perfectly still first and then release them, they will fall straight into each other—a head-on collision. But what if the particles are moving past each other when their gravitational attraction begins to kick in? Then there may not be a head-on collision. They will start moving towards each other; but because each is already moving, rather than falling directly into each other they may begin to circle around and around each other. That is, there will be some rotation going on as they come together. What applies to two particles, will apply to lots of particles. If all of the atoms in a cloud of gas all fall directly towards each other, straight-on—which is what might happen if none of them was moving very much to begin with—they may form an essentially spherical cloud. But if they are moving a lot before they fall together, then statis- tically you would expect them to kind of bypass each other as they come in, and start swirling around each other. This will produce some overall, final net rotation of the cloud of gas. For large clouds of gas that are rotating as they collapse, the shape you would expect to emerge would be a large more or less spherical region in the center surrounded by a disk; and that is the basic structure of a spiral galaxy.

5.2 Elliptical Galaxies Elliptical galaxies span a much broader range of sizes than does the spiral category. There are some, known asdwarf ellipticals, that as the name implies are extremely small; and yet the largest galaxies in the Universe are elliptical galaxies. These behe- moths are known as giant ellipticals, and the largest ones are ten times as big as any spiral galaxy. (Note: What I said earlier still applies. Most of the large galaxies in the Universe are spirals. Only a few of them are ellipticals. But those few can be larger than anything else.) As we mentioned before, the overall shape of elliptical galaxies can range from nearly perfect spheres to highly elongated symmetrical oblongs. Figure 5.4 shows a photograph of a typical one. As you might suspect from this photograph, elliptical galaxies tend to have much less structure than do spiral galaxies. There is no disk; hence, no spiral arms. There is always a bright region in the center, the counter- part to the bulge of a spiral galaxy; and this bright region is always surrounded by a highly symmetric distribution of stars that grows fainter and more diffuse as you move out from the center. Our “guess” as to how you make an elliptical galaxy is just an obvious extension of what we said about the possible formation process for a spiral galaxy. To make an elliptical galaxy all you need is a cloud of gas that will collapse with little or no rotation. If there is no rotation, then no disk will form. Elliptical galaxies in fact are sometimes described as being like spiral galaxies Figure 5.4 A Typical Elliptical Galaxy without the disk. Source: NASA, Wikimedia Commons What Are Galaxies? 95

5.3 Galaxy Interactions Before we talk about irregular galaxies, it will be worthwhile to spend a page or two just mentioning an important observational fact about galaxies. They interact a lot. You see, galaxies have a lot of mass, which means that they generate a lot of gravity. Also, galaxies are comparatively close to each other. What do I mean by “comparatively close”? Well, consider this: Take two pennies. Let one penny represent our galaxy, and let the other penny represent the Andromeda galaxy, our nearest neighbor galaxy. To represent the proper separation between the two galaxies you would put the pennies about 11 inches apart. Now let one of the pennies represent our Sun, and let the other penny represent Alpha Centauri, which is our nearest neighbor star. In order to represent the proper separation between the Sun and Alpha Centauri, you would have to hold the two pennies about 50 miles apart! In general, galaxies are much closer together for their size than are stars; which is why galaxies are often seen to be interact- ing with each other, whereas stars almost never collide. When galaxies do interact, any number of things can result. For example, if it is only a slight, grazing encounter then the two galaxies may continue on their merry ways after the encounter, with only a slight distortion in their shapes to show for it. On the other hand, they might slide straight into each other. When this happens they generally merge to form a single, larger, and generally very messed up looking galaxy. Figure 5.5 shows a photograph of two galaxies in the process of merging with each other. Eventually the two galaxies will probably look like one.

Figure 5.5 Two Galaxies Colliding Galaxies NGC 2207 and IC 2163 in the process of colliding with each other. Source: NASA/ESA/Hubble, Wikimedia Commons

5.4 Irregular Galaxies Irregular galaxies are just that—irregular. The most that can be said about their shapes is that they are not symmetrical in any way. Although each one really is completely unique so there really is no such thing as a “typical” irregular galaxy, a photograph of 96 Chapter 5 Welcome to the Universe! A Brief Introduction to Astronomy

one is shown in Figure 5.6 so that you can see the kind of thing that I’m referring to. On average, irregular galaxies tend to be rather small. The formation of irregular galaxies is not difficult to explain either, given that galaxies do sometimes interact with each other. As I mentioned in the last section, when two galaxies interact, whether they merge together or not their resulting shape or shapes can be randomly disordered.

5.5 Extreme Galaxies Whether spiral, elliptical or irregular, the vast majority of galaxies can be classified as “normal.” What “normal” means for a galaxy can only be learned by looking at lots of galaxies to see what most of them are like; and as more Figure 5.6 An Example of an Irregular Galaxy is learned, the definition of “normal” might evolve. This one is known as NGC 1427A Astronomers have now studied enough galaxies to Source: NASA/ESA/Hubble, Wikimedia Commons know that most of them contain mostly stars. So that is one characteristic of a normal galaxy: They are made up mostly of stars. Of course, stars are not static; they are Normal galaxy dynamic things, and new ones are continually being “born” as old ones “die off.” So another characteristic of a normal galaxy is that it have a “normal” star formation rate. There are other characteristics of anormal galaxy that would be of more interest to astronomers who study galaxies; but you should have enough just from this little bit to get the idea. Well, it turns out that there are some galaxies that are distinctly not normal. There are four broad categories of what might be called “extreme galaxies” that I wanted to introduce to you.

5.5a Quasars By far the most famous and in my opinion the most important category of non-normal galaxies is a type of object that can only in the loosest sense of the word be considered a “galaxy” at all. They have an interesting history that is worth a paragraph or two. Back in the 1960s Maarten Schmidt, an astronomer at Caltech in Pasadena, California, discovered an object that he thought was a star since it was small and point- like in a telescope, but which had a very strange spectrum. He didn’t recognize any of the emission or absorption lines in it, and it certainly didn’t look like the spectrum of a star. Apparently the spectrum sat on his desk for a few months until finally it dawned on him what he was looking at. The spectrumdid have all of the familiar emission lines of hydrogen and other elements, but all of the emission and absorption lines had been shifted towards longer wavelengths, or lower photon energies. In other words, the object had a large redshift. Now, stars do not have large redshifts. Stars that we can see individually are all in our own galaxy; hence they are not far enough away for their light to have experienced a significant cosmological redshift. Furthermore, they can’t be moving fast enough for their light to be significantly Doppler shifted. If they were moving fast enough for that, they would have shot out of the galaxy long ago! What Are Galaxies? 97

The only logical way to explain the redshift then was to assume that it was a cosmological redshift, and that the object was not actually a star. Rather, like the redshifts of galaxies, its distance would have to correspond with its redshift. Since this object had a very large redshift, it had to be very far away. But if it was that far away and still looked as bright as a nearby star, then it must have an extremely high luminosity. Do you see why? Imagine that you are looking at a distant light out of your bedroom window that you think is a firefly in your backyard. Then you suddenly realize that the light isn’t in your backyard, but is 10 miles away. Suddenly it’s not a firefly, it’s a searchlight! If you can see it from 10 miles away, it must be pretty luminous. Schmidt called his new discovery a “quasi-stellar object,” which later got shortened to quasar. Thousands of quasars are known today. Figure 5.7 Three Quasars They are all at very great distances and they are all extremely lumi- Source: European Southern Observatory, Wikimedia Commons nous. What makes them remarkable however is the fact that they are so luminous and they are so small. Remember, Schmidt thought his first quasar was a star. Quasars look like tiny points of light because they are very small; in fact, they are too small. And that’s what makes them interesting. You see, as we’ve already mentioned normal galaxies are made up mostly of stars; and of course, stars give off light, lots of it. But they are also pretty big. So if you put 100 billion stars together, they will take up a lot of room—which is why galaxies are so big. A typical quasar gives off as much light as a galaxy, if not more. There are quasars whose luminosities are thousands of times the luminosity of a typical normal galaxy. And yet quasars are small, much smaller than galaxies. Also, quasars generally emit much more infrared, ultraviolet, X-ray and gamma-ray light than do normal galaxies. That is, they are more luminous over a much broader range of photon energies (or wavelengths of light) than are normal galaxies. This again implies that their light is not coming primarily from stars, since stars emit mostly visible or near-visible light. Although we don’t really know where quasars come from, we do think we know where their light comes from; but I will save that discussion for later (Chapter 7).

5.5b Active Galactic Nuclei (AGN) A true, “classic” quasar looks like a single point of light when seen through a telescope. It looks like a star. It is characterized by too much light coming from too small of a region of space. Now, it turns out that the very centers of some galaxies actually look like quasars. That is, astronomers sometimes see too much light coming from too small of a region of space in the center of a galaxy. These objects are kind of like a bridge between galaxies and quasars. They look like what you might expect to get if you picked up a quasar from somewhere and stuck it in the middle of an otherwise normal spiral, elliptical or irregular galaxy. These objects are calledactive galactic nuclei, or “AGN” for short. An astronomer by the name of Carl Seyfert had a lot to do with their discovery, so they are also called Seyfert galaxies. Our explanation of AGN will be essentially the same as that for quasars, so we will save it for Chapter 7 as well. 98 Chapter 5 Welcome to the Universe! A Brief Introduction to Astronomy

5.5c Radio Galaxies Some objects have huge “jets” of material hundreds of thousands of light-years in length that give off tremendous amounts of radio light. These jets generally shoot out from a tinyradio core that is sometimes associated with a quasar, and sometimes with the core of a galaxy (see Figure 5.7). Such objects are known as radio galaxies. Whether the radio core is associated with a quasar or the center of a galaxy, radio galaxies are almost certainly closely related to other, “radio quiet” quasars.

5.5d Starburst Galaxies I mentioned a little earlier thatnormal galaxies are characterized in part by having “normal” rates of star formation. What is a normal star formation rate? Well, our own galaxy, the Milky Way, is in every respect a very normal galaxy; and astronomers Figure 5.8 Radio Galaxy estimate that the Milky Way produces maybe one new star per A visible light photograph of a “jet” emanating year, on average. So we can take that as an example of a normal from the core of the elliptical galaxy M87. This rate. However, there are some galaxies that go way beyond this, jet also emits radio light. In fact M87 is one of the brightest sources of radio light in the sky. creating as many as 100 stars per year. Such galaxies are known Source: NASA/Hubble, Wikimedia Commons charmingly as starburst galaxies. Since stars form from clouds of gas and dust that collapse due to their own gravity, high star formation rates in a galaxy simply mean either that clouds of gas and dust are collapsing at an unusually rapid rate in that galaxy, or that the galaxy has an unusually large amount of gas and dust, or both. Since the formation of a star is at root a gravitational process, it is not too surprising that starburst galaxies are sometimes associated with galaxy–galaxy interactions. In such interactions the two galaxies pull on each other gravitationally, which can naturally trigger other gravitational phenomena.

5.6 Galaxy Groups and Clusters We are almost done with this chapter, but I did want to mention one more thing that is worth knowing. Although some galaxies appear to be all by themselves, galaxies are often found bunched together in groups or clusters. In such situations the vari- ous galaxies are bound to each other by their mutual gravitational interactions. When the number of galaxies associated in this way is no more than a few dozen, we call it a group of galaxies. But in some places we find hundreds, even thousands of galaxies that are bound together in a huge gathering spread out over millions of light years. These large structures are known as galaxy clusters. Furthermore, galaxy clusters are sometimes clustered together, forming giant “clusters of clusters,” known as superclusters. So galaxies are not distributed uniformly throughout the Universe. Rather, the Universe itself has a definite structure to it, containing regions that are well populated, so to speak, side by side with regions that are vast empty voids. What Are Galaxies? 99

Summary

We still have much to learn about galaxies. But before we can do so, we’ll need to learn something about what galaxies are mostly made of: stars! So let’s move on.

Key Terms

Barred spiral galaxy, pg. 93 Halo, pg. 92 Bulge, pg. 92 Normal galaxy, pg. 96 Disk, pg. 92 Spiral arms, pg. 93