What Are Galaxies?
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5 What Are Galaxies? s we mentioned in the last chapter, the Universe is essentially a vast collection of galaxies. The logi- cal 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. 91 92 CHAPTER 5 Welcome to the Universe! A Brief Introduction to Astronomy 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 photo- graph 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 gravi- tational 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 essen- tially 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.