Nature of the Universe-Chapter Eighteen

After studying the evolution of stars, we discuss objects much bigger: galaxies. They are the basic building blocks of our (visible) universe. We, including our Sun and the solar system are in one galaxy, the Milky Way Galaxy, which is a fairly typical spiral galaxy.

The Milky Way Galaxy
Almost all the stars visible by naked eyes are in our galaxy, the Milky Way Galaxy. It contains about 200 billion stars. Most of them are on the disk, which we call the galactic disk. The diameter is about 100,000 ly while the thickness is just 1,000 ly. Thus, the disk is very thin compared with the diameter. The following figure exaggerates the thickness.

At the center is the nuclear bulge with radius of about 20,000 ly. The disk and the bulge are surrounded by the galactic halo, which is spherical and even larger than the galactic disk. The halo consists of old stars, some in the globular clusters, and interstellar matter.

Another prominent feature of the Milky Way Galaxy is the spiral arms. They consist of young stars extending from the center to the edge of the disk. However, the laws of mechanics do not allow them to exist at all, because the stars near the edge will rotate much slower than the stars near the center. Hence the arms will not be able to survive.

The mostly accepted theory of the spiral arms is the density wave theory. It says that the stars indeed rotate at different speed, but the density of the interstellar medium form a wave. At the wave front, where the density is higher, star formation is enhanced. That is why we find young stars along the spiral arms and old stars elsewhere.

Our Sun is on the Orion spiral arm, about two-third from the center. It is difficult to detect the spiral arms in our galaxy because 1) we are inside the galaxy and 2)the gas and dust, which form dust lanes, block our view. In order to trace out the spiral arms, we could search for the hydrogen, which is abundant in the spiral arms. Interstellar hydrogen will emit a characteristic wave with wavelength 21cm. Most of the sources of this wave are gas clouds in the spiral arms. Second to the 21cm emission, the radio wave emitted by the molecular gas clouds in the arms are also good spiral arm tracers.

When viewed from the Earth, our galaxy appears to be a band of stars around us. The stars in the Milky Way are so many and so dense that our ancestors thought it was some kind of river. The center of our galaxy is located in the direction of Sagittarius, which is a summer constellation. Thus, the Milky Way is more spectacular in the summer. To see the Milky Way to its full beauty, one has to go to a dark site, far away from the city light. When you see the Milky Way, also look for the dust lanes, which block the stars further away.

Apart from the visible elements, we believe that our galaxy also contains some objects that do not emit any light. We call them dark matters. They reveal their existence by their gravity. How fast an object revolves depends on how much matter inside its orbit. If all the matters are visible, the orbital velocities of stars, say, near the edge of our galaxy will follow the red line below. However, we discovered that they are moving faster than expected, hence there must be more matters than we have seen. We believe dark matters are in fact, everywhere, not just in our galaxy, but we do not understand its nature yet.

Classification of Galaxies
There are about 100 billion galaxies in our universe. If we plot all the visible galaxies on a graph, we will see something like this.

In this graph, the galactic plane is horizontal and we ignore the distances. Note that the gas and dust in our galaxy not only block the stars in our galaxy, but also the galaxies far away. Taking this into consideration, the distribution of the galaxies is fairly uniform.

What do the other galaxies look like? Galaxies come with many sizes and shapes. They are classified into three types: the elliptical, the spiral and the ``none of the above'' irregular.

Elliptical galaxies: The galaxy in the photo is M87, a typical elliptical galaxy. They usually consist of old stars. Some dwarf ellipticals have as few as 10 million stars, and make them not much different from a large globular cluster. Up to a year or two ago, we believed they are the most common type of galaxies, but now, we believe the irregular galaxies are the most common.

Courtesy NASA.

Spiral galaxies: Spiral galaxies contain both young and old stars. In general, they are brighter than elliptical galaxies. Spirals are sub-divided into two classes: the normal spiral and the barred spiral. The bulge of the barred spiral is elongated like a bar, thus given the name. The galaxy above is M31, a normal spiral only 2,000,000 ly away. Photo below shows a barred spiral.

Courtesy STScI.

Irregular galaxies: They have irregular shape. The two irregular galaxies Large Magellanic Cloud and the Small Magellanic Cloud are two satellite galaxies of ours. They are visible by naked eyes, but not visible from Hong Kong.

There are sub-classes for the elliptical and spiral galaxies. They are summarized in the following figure that was first worked out by Hubble. Note that this graph is not an evolution track for galaxies, which we do not know much about at the moment.

Distance to Galaxies
Before 1920s, we did not know that galaxies are objects far away. We thought they were in our galaxy and were called spiral nebulae. Today, we mainly use two methods to measure the distances to other galaxies. The first one is the Cepheid variables that we have talked about in Chapter 13. The second one is the Type I supernova. Recall that Type I supernova is the strong explosion of a nova that even the white dwarf is destroyed. We found that the luminosities of Type I supernovae are about the same. Therefore, we can compute the distance to a galaxy by measuring the apparent brightness of a Type I supernova in that galaxy, if we can find one.

Cepheid variables are dimmer than the Type I supernova. As a result, the supernova allows us to measure the distances of galaxies further away, but it happens much less often. Each one has its advantages and disadvantages. Both of them are called the distance indicators.

Interacting and Active Galaxies
The typical distance between galaxies is only about 20-40 times the sizes of a galaxy. (The ratio of typical distance between stars to the size of a star is in the order of tens of millions.) Galaxy collisions are very common.

When galaxies collide, the stars never collide with each other. Just the distribution of the stars are distorted by the mutual gravity. Streams of stars may be ejected out to form something like antennae and the two galaxies will merge together. Some galaxies with multiple nuclei may be the end result of the merging of two galaxies long time ago. Galaxy collision will also trigger star births.

Courtesy STScI.

Galaxy collision may also lead to the formation of an active galaxy. An active galaxy is a galaxy that radiates unusual large amount of energy in the form of infrared, ultraviolet and X-rays; always from the nucleus of the galaxy. So, it is also termed the active galactic nuclei (AGN). We are not completely sure how an active galaxy is formed and how it generates such a large amount of energy. One of the most popular hypothesis is the supermassive black hole theory. When materials surrounding the black hole fall into it, large amount of energy is released. Galaxy collision may create the supermassive black hole at the center.

Another kind of interesting objects is quasar, which is the short form of the quasi-stellar object. When observed in visible light, they are small and dim, just like a star. However, near the infrared and radio, they are quite bright. Their spectra show large amount of redshift indicating that they are very far away from us (See Chapter 19.). Thus, we conclude that that they radiate enormous amount of energy, even larger than those active galaxies nearby. Quasar is probably also the supermassive black hole at the center of a young galaxy.

Courtesy NOAO/NSF.