Galaxies

Lesson Objectives

  • Distinguish between star systems and star clusters.
  • Identify different types of galaxies.
  • Describe our own galaxy, the Milky Way Galaxy.

Vocabulary

  • binary star
  • dwarf galaxy
  • elliptical galaxy
  • galaxy
  • globular cluster
  • irregular galaxy
  • Milky Way Galaxy
  • open cluster
  • spiral arm
  • spiral galaxy
  • star cluster
  • star system

Introduction

Where do you live? Sure you live in a house or apartment, on a street, in a town or city, in a state or province, and in a country. You may not think to mention that you live on planet Earth in the solar system (as if there is no other), which is in the Milky Way Galaxy. Our galaxy is just one of many billions of galaxies in the universe. These galaxies are incomprehensible distances from each other and from Earth.

Star Systems and Star Clusters

Although constellations have stars that usually only appear to be close together, stars may be found in the same portion of space. Stars that are grouped closely together are called star systems. Larger groups of hundreds or thousands of stars are called star clusters.

Star Systems

Although the star humans know best is a single star, many stars—in fact, more than half of the bright stars in our galaxy—are star systems. A system of two stars orbiting each other is a binary star. A system with more than two stars orbiting each other is a multiple star system. The stars in a binary or multiple star system are often so close together that they appear as one and only through a telescope can the pair be distinguished.

An animation of a solar system like ours but with two suns was created by NASA: http://www.spitzer.caltech.edu/video-audio/852-ssc2007-05v1-Two-Suns-Raise-Family-of-Planetary-Bodies-

Star Clusters

Star clusters are divided into two main types, open clusters and globular clusters. Open clusters are groups of up to a few thousand stars that are loosely held together by gravity. The Pleiades, shown in Figure below, is an open cluster that is also called the Seven Sisters.

In the Pleiades, seven stars can be seen without a telescope, but the cluster has close to a thousand stars.

Open clusters tend to be blue in color and often contain glowing gas and dust. Why do you think that open clusters have these features? Open clusters are made of young stars that formed from the same nebula. The stars may eventually be pulled apart by gravitational attraction to other objects.

Globular clusters are groups of tens to hundreds of thousands of stars held tightly together by gravity. Figure below shows an example of a globular cluster. Globular clusters have a definite, spherical shape and contain mostly reddish stars. The stars are closer together, closer to the center of the cluster. Globular clusters don’t have much dust in them — the dust has already formed into stars.

Check out http://seds.org/messier/cluster.html and http://hubblesite.org/newscenter/archive/releases/star-cluster/ for more information about star clusters.

M80 is a large globular cluster containing hundreds of thousands of stars. Note that the cluster is spherical and contains mostly red stars.

Types of Galaxies

Galaxies are the biggest groups of stars and can contain anywhere from a few million stars to many billions of stars. Every star that is visible in the night sky is part of the Milky Way Galaxy. To the naked eye the closest major galaxy — the Andromeda Galaxy, shown in Figure below — looks like only a dim, fuzzy spot. But that fuzzy spot contains one trillion stars — 1,000,000,000,000 stars!

The Andromeda Galaxy is a large spiral galaxy similar to the Milky Way.

Galaxies are divided into three types according to shape: spiral galaxies, elliptical galaxies, and irregular galaxies.

Spiral Galaxies

Spiral galaxies spin, so they appear as a rotating disk of stars and dust, with a bulge in the middle, like the Sombrero Galaxy shown in Figure below. Several arms spiral outward in the Pinwheel Galaxy (seen in Figure below) and are appropriately called spiral arms. Spiral galaxies have lots of gas and dust and lots of young stars.

(a) The Sombrero Galaxy is a spiral galaxy that we see from the side so the disk and central bulge are visible. (b) The Pinwheel Galaxy is a spiral galaxy that we see face-on so we can see the spiral arms. Because they contain lots of young stars, spiral arms tend to be blue.

Elliptical Galaxies

Figure below shows a typical egg-shaped elliptical galaxy. The smallest elliptical galaxies are as small as some globular clusters. Giant elliptical galaxies, on the other hand, can contain over a trillion stars. Elliptical galaxies are reddish to yellowish in color because they contain mostly old stars.

The large, reddish-yellow object in the middle of this figure is a typical elliptical galaxy. What other types of galaxies can you find in the figure?

Most elliptical galaxies contain very little gas and dust because the gas and dust has already formed into stars. However, some elliptical galaxies, such as the one shown in Figure below, contain lots of dust. Why might some elliptical galaxies contain dust?

Astronomers believe that these dusty elliptical galaxies form when two galaxies of similar size collide.

Irregular Galaxies and Dwarf Galaxies

Is the galaxy in Figure below a spiral galaxy or an elliptical galaxy? It is neither one! Galaxies that are not clearly elliptical galaxies or spiral galaxies are irregular galaxies. How might an irregular galaxy form? Most irregular galaxies were once spiral or elliptical galaxies that were then deformed either by gravitational attraction to a larger galaxy or by a collision with another galaxy.

This galaxy, called NGC 1427A, has neither a spiral nor an elliptical shape.

Dwarf galaxies are small galaxies containing only a few million to a few billion stars. Dwarf galaxies are the most common type in the universe. However, because they are relatively small and dim, we don’t see as many dwarf galaxies from Earth. Most dwarf galaxies are irregular in shape. However, there are also dwarf elliptical galaxies and dwarf spiral galaxies.

Look back at the picture of the elliptical galaxy. In the figure, you can see two dwarf elliptical galaxies that are companions to the Andromeda Galaxy. One is a bright sphere to the left of center, and the other is a long ellipse below and to the right of center. Dwarf galaxies are often found near larger galaxies. They sometimes collide with and merge into their larger neighbors.

Images from the Hubble Space Telescope are seen in this video: http://www.space.com/common/media/video/player.php?videoRef=black_holes#playerTop.

The Milky Way Galaxy

On a dark, clear night, you will see a milky band of light stretching across the sky, as in Figure below . This band is the disk of a galaxy, the Milky Way Galaxy, which is our galaxy. The Milky Way is made of millions of stars along with a lot of gas and dust.

The Milky Way Galaxy looks different than other galaxies because we are looking along the main disk from within the galaxy.

Shape and Size

Although it is difficult to know what the shape of the Milky Way Galaxy is because we are inside of it, astronomers have identified it as a typical spiral galaxy containing about 100 billion to 400 billion stars (Figure below).

An artist’s rendition of what astronomers think the Milky Way Galaxy would look like seen from above. The Sun is located approximately where the arrow points.

Like other spiral galaxies, our galaxy has a disk, a central bulge, and spiral arms. The disk is about 100,000 light-years across and 3,000 light-years thick. Most of the Galaxy’s gas, dust, young stars, and open clusters are in the disk. What evidence do astronomers find that lets them know that the Milky Way is a spiral galaxy?

1. The shape of the galaxy as we see it (Figure below).

An infrared image of the Milky Way shows the long thin line of stars and the central bulge typical of spiral galaxies.

2. The velocities of stars and gas in the galaxy show a rotational motion.

3. The gases, color, and dust are typical of spiral galaxies.

The central bulge is about 12,000 to 16,000 light-years wide and 6,000 to 10,000 light-years thick. The central bulge contains mostly older stars and globular clusters. Some recent evidence suggests the bulge might not be spherical, but is instead shaped like a bar. The bar might be as long as 27,000 light-years long. The disk and bulge are surrounded by a faint, spherical halo, which also contains old stars and globular clusters. Astronomers have discovered that there is a gigantic black hole at the center of the galaxy.

The Milky Way Galaxy is a big place. If our solar system were the size of your fist, the Galaxy’s disk would still be wider than the entire United States!

A video closeup of the Milky Way Galaxy is seen here: http://www.space.com/common/media/video/player.php?videoRef=black_holes#playerTopjjj.

Where We Are

Our solar system, including the Sun, Earth, and all the other planets, is within one of the spiral arms in the disk of the Milky Way Galaxy. Most of the stars we see in the sky are relatively nearby stars that are also in this spiral arm. We are about 26,000 light-years from the center of the galaxy, a little more than halfway out from the center of the galaxy to the edge.

Just as Earth orbits the Sun, the Sun and solar system orbit the center of the Galaxy. One orbit of the solar system takes about 225 to 250 million years. The solar system has orbited 20 to 25 times since it formed 4.6 billion years ago. Astronomers have recently discovered that at the center of the Milky Way, and most other galaxies, is a supermassive black hole, although a black hole cannot be seen.

This video describes the solar system in which we live. It is located in an outer edge of the Milky Way galaxy, which spans 100,000 light years (2a): http://www.youtube.com/watch?v=0Rt7FevNiRc (5:10).

The Universe contains many billions of stars and there are many billions of galaxies. Our home, the Milky Way galaxy, is only one (2a, 2b): http://www.youtube.com/watch?v=eRJvB3hM7K0 (5:59).

Lesson Summary

  • Many stars are in systems of two or more stars.
  • Open clusters are groups of young stars loosely held together by gravity.
  • Globular clusters are spherical groups of old stars held tightly together by gravity.
  • Galaxies are collections of millions to many billions of stars.
  • Spiral galaxies have a rotating disk of stars and dust, a bulge in the middle, and several arms spiraling out from the center. The disk and arms contain many young, blue stars.
  • Typical elliptical galaxies are egg-shaped, reddish, and contain mostly old stars.
  • Galaxies that are not elliptical or spiral galaxies are called irregular galaxies. These galaxies were probably deformed by other galaxies.
  • The Milky Way Galaxy is a typical spiral galaxy. Our solar system is in a spiral arm of the Milky Way Galaxy, a little more than halfway from the center to the edge of the disk.

Review Questions

1. What is a binary star?

2. Compare globular clusters with open clusters.

3. Name the three main types of galaxies.

4. List three main features of a spiral galaxy.

5. Suppose you see a round galaxy that is reddish in color and contains very little dust. What kind of galaxy is it?

6. What galaxy do we live in, and what kind of galaxy is it?

7. What is the evidence that the galaxy we live in is this type of galaxy?

8. Describe the location of our solar system in our galaxy.

Further Reading / Supplemental Links

Points to Consider

  • Objects in the universe tend to be grouped together. What forces or factors do you think cause objects to form and stay in groups?
  • Some people used to call galaxies “island universes.” Are they really universes?
  • Can you think of anything, either an object or a group of objects, that is bigger than a galaxy?