Chapter 5 Section 5.1 Galaxies and Star Systems Galaxies Terms: Galaxy Spiral Galaxy Elliptical Galaxy Irregular Galaxy Milky Way Galaxy Quasar Black Hole Types of Galaxies A galaxy is a huge group of single stars, star systems, star clusters, dust, and gas bound together by gravity. There are billions of galaxies in the universe. The largest galaxies have more than a trillion stars! Astronomers classify most galaxies into the following types: spiral, elliptical, and irregular. Spiral galaxies are those that appear to have a bulge in the middle and arms that spiral outward, like pinwheels. The spiral arms contain many bright, young stars as well as gas and dust. Most new stars in the spiral galaxies form in theses spiral arms. Relatively few new stars form in the central bulge. Some spiral galaxies, called barred-spiral galaxies, have a huge bar-shaped region of stars and gas that passes through their center. Not all galaxies have spiral arms. Elliptical galaxies look like round or flattened balls. These galaxies contain billions of the stars but have little gas and dust between the stars. Because there is little gas or dust, stars are no longer forming. Most elliptical galaxies contain only old stars. Some galaxies do not have regular shapes, thus they are called irregular galaxies. These galaxies are typically smaller than other types of galaxies and generally have many bright, young stars. They contain a lot of gas a dust to from new stars. The Milky Way Galaxy 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. From the side, the Milky Way would look like a narrow disk with a large bulge in the middle. From the top or bottom, the Milky Way would have a spiral, pinwheel shape. Astronomers estimate that the Milky 34
Way contains 200 to 400 billion stars. Like other spiral galaxies, our galaxy has a disk, a central bulge, and spiral arms. The disk is about 100,000 lightyears across and 3,000 light-years thick. Most of the Galaxy s gas, dust, young stars, and open clusters are in the disk. 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! 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. Just as Earth orbits the Sun, the Sun and solar system orbit the center of the 35
Galaxy. One orbit of the solar system takes about 225 to 250 million years at a speed of half a million miles per hour. The solar system has orbited 20 to 25 times since it formed 4.6 billion years ago. The center of our galaxy is located about 28,000 light-years away, beyond the constellation Sagittarius (actually just beyond the border of Sagittarius and Scorpio). So, if you can locate these two constellations in the sky, you'll be looking toward the center of our galaxy! The Milky Way is part of a set of galaxies known as the Local Group, which includes several dozen different galaxies within 3 million light-years. Only one of these, the Andromeda galaxy, is close to the size of the Milky Way. This Local Group is part of a supercluster, known as the Virgo supercluster, which has at least 5,000 member galaxies and is roughly 100 million light-years across. Beyond this level of organization, not much is known about our position in the universe. It's sad that there is so much light pollution in most cities that many people have never seen the Milky Way. On a clear night away from lights the view is of a bright white river of stars. You don't need a telescope or even binoculars to see it. The view of the Milky Way is so bright because you're looking at the stars in your own galaxy. Quasars In the late 1960s, astronomers discovered objects that are very bright but also very far away. Many of these objects are ten billion light years or more away, making them among the most distant objects in the universe. These distant, enormously bright objects looked almost like stars. Since quasi means something like in Latin, these objects were given the name quasi-stellar objects, or quasars. Astronomers have concluded that quasars are active young galaxies with giant black holes in their centers. Black holes are the remains of the most massive stars when they die. A black hole is an object with gravity so strong that nothing, not even light, can escape. Each of these black holes has a mass a billion times more as great as that of the sun. As enormous amounts of gas revolve around the black hole, the gas heats up and shines brightly. Summary: Astronomers classify most galaxies into the following types: spiral, elliptical, and irregular. A galaxy is a huge group of stars that are held together by gravity. A 36
galaxy contains single stars, star systems, star clusters, dust and gas. A spiral galaxy has a bulge in the middle and arms that curve outward. Most new stars are found in its spiral arms. An elliptical galaxy looks like a round or flattened ball. Most of these galaxies contain only old stars. An irregular galaxy does not have a certain shape. They contain many young stars. Our solar system is located in a spiral galaxy called the Milky Way. The Milky Way is part of the Local Group containing several nearby galaxies. The Local Group is part of the Virgo Supercluster, which contains thousands of galaxies. Many galaxies, including our own, have a black hole in the center. A black hole is an object with very strong gravity that does not give off any light. Nothing escapes a black hole! 37
Section 5.2 Star Systems Terms: Star Systems Binary Stars Eclipsing Binary Star Clusters Our sun is the one star in our solar system. As you move past our solar system, billions of other stars make up our galaxy. Most stars are members of groups of two or more stars, called star systems. Binary stars are pairs of stars. Scientists detect binary stars by observing the brightness and motion of nearby stars. Often one star in a binary star is much brighter and more massive than the other. Astronomers can sometimes detect a binary star even if only one of the stars can be seen from Earth. Astronomers can often tell that there is a dim star in a binary system by observing the effects of its gravity. As the dim companion revolves around a bright star, the dim star s gravity causes the bright star to wobble back and forth. Imagine watching a pair of dancers who are twirling each other around. Even if one dancer were invisible you could tell that the invisible dancer was there from watching the motion of the visible dancer. A wobble isn t the only clue that a star has a dim companion. A dim star in a binary star may pass in front of a brighter star and eclipse it. From Earth, the binary star would suddenly look much dimmer. A system in which one star periodically blocks the light from another is called an eclipsing binary. In 1995, astronomers first discovered a planet revolving around another ordinary star. They used a method similar to the one used in studying binary stars. The astronomers observed that a star was moving slightly toward and away from us. They knew that the invisible object causing the movement didn t have enough mass to be a star. They inferred that it must be a planet. Since then, astronomers have discovered many more planets and many more stars. Star clusters are larger grouping of stars. All the stars in a particular cluster 38
formed from the same nebula at about the same time and are about the same distance from Earth. There are two many types of star clusters: open clusters and globular clusters. Open clusters are loose, disorganized appearance and contain no more than a few thousand stars. They often contain many bright supergiants and a lot of gas and dust. In contrast, globular clusters are larger grouping of older stars. Globular clusters are round and densely packed with stars many contain more than a million. Summary: Most stars are members of groups of two or more stars, called star systems. A star system is a group of two or more stars. A star system that has two stars is called a double star or a binary star. A star system with three stars is called a triple star. An eclipsing binary is a star system with two stars. In an eclipsing binary, one of the stars sometimes blocks the light from the other star. Many stars belong to larger groups of stars called clusters. An open cluster is a group of star that is loose and disorganized. A globular cluster is a group of stars that is round and densely packed. 39