Star Systems and Galaxies

Transcription

1 Star Systems and Galaxies Why Does the Milky Way Look Hazy? 1. Using a pencil, carefully poke at least 20 holes close together in a sheet of white paper. 2. Tape the paper to a chalkboard or dark-colored wall. 3. Go to the other side of the room and look at the paper. From the far side of the room, what do the dots look like? Can you see individual dots? Think It Over Making Models How is looking at the paper from the far side of the room like trying to see many very distant stars that are close together? What is a star system? What are the three types of galaxies? On a clear, dark summer night in the country, you can see a hazy band of light stretched across the sky. This band of stars is called the Milky Way. It looks as if the Milky Way is very far away from Earth. Actually, though, Earth is inside the Milky Way! How is this possible? Before you can understand the answer to this question, you need to know more about how stars are grouped together. Reading Tip Before you read, preview the boldfaced terms. As you read, look for a photograph or diagram that illustrates each term. Key Terms binary star eclipsing binary spiral galaxy elliptical galaxy irregular galaxy file:///d /SciExp/generic/ /ch8/ch8_s4_0.html10/24/2008 3:55:17 PM

2 Star Systems and Planets Our solar system has only one star, the sun. But more than half of all stars are members of groups of two or more stars, called star systems. If you were on a planet in one of these star systems, you might see two or more suns in the sky. Double and Triple Stars A star system with two stars is called a binary star. (The prefix bi means two. ) Those with three stars are called triple stars. Proxima Centauri is probably part of a triple-star system close to our sun. The other two stars in the system, Alpha Centauri A and Alpha Centauri B, form a double star. Scientists are not sure whether Proxima Centauri is really part of the same star system or whether it is just passing close to the other two stars temporarily. Astronomers can sometimes detect a binary star even if only one of the stars in the pair can be seen from Earth. For example, the darker star in the pair may pass in front of the other star and eclipse the other star. A system in which one star blocks the light from another is called an eclipsing binary. As the diagram below shows, the star Algol is actually part of an eclipsing binary. Eclipsing Binary Algol is an eclipsing binary star system, consisting of a bright star and a dim companion star. Each time the dimmer star passes in front of the brighter one, Algol becomes less bright. Interpreting Diagrams When does Algol become brighter? file:///d /SciExp/generic/ /ch8/ch8_s4_1.html (1 of 2)10/24/2008 3:55:29 PM

3 Often astronomers can tell that there is a second star in a system only by observing the effects of its gravity. As the second star revolves around the first star, the second star s gravity makes the first star move back and forth. Imagine you are watching a pair of dancers 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. Invisible Partner If you saw someone dancing but couldn t see a partner, you could infer that the partner was there by watching the dancer you could see. Astronomers use a similar method to detect faint stars in star systems. Planets Around Other Stars In 1995, astronomers discovered a planet revolving around a star using a method similar to the one they use to detect binary stars. The star they were observing, 51 Pegasi, moved back and forth only very slightly. Therefore, they knew the invisible object could not have enough mass to be a star. They deduced that it must be a planet. Before this discovery, there was no way to know whether stars other than the sun had planets revolving around them. Now astronomers think that our solar system is not the only one. Many of the planets found beyond our solar system so far are very large, at least half Jupiter s mass. A small planet would be difficult to detect, because it would have little gravitational effect on the star it revolved around. Astronomers are trying to find new ways to use telescopes to see planets directly. Seeing a planet around another star is like trying to see a firefly near a street light. The glare of the light makes it hard to see anything near the light. To see a planet directly, astronomers have to shield their view from the glare of the star that the planet revolves around. Some scientists hypothesize that life may exist on planets revolving around other stars. A few astronomers are using radio telescopes to search for signals that could not have come from natural sources. Such a signal might be evidence that an extraterrestrial civilization existed and was sending out radio waves. file:///d /SciExp/generic/ /ch8/ch8_s4_1.html (2 of 2)10/24/2008 3:55:29 PM

4 Galaxies Now you are ready to learn about the Milky Way. The Milky Way is the galaxy in which our solar system is located. Like other galaxies, it contains single stars, double stars, star systems, and lots of gas and dust between the stars. The Milky Way Galaxy, often just called our galaxy, looks milky or hazy because the stars are too close together for your eyes to see them individually. The dark blotches in the Milky Way are clouds of dust that block light coming from stars behind them. The Milky Way From the side, the Milky Way Galaxy appears to be a narrow disk. The spiral structure would be visible only from above the galaxy. There are billions of galaxies in the universe. Astronomers have classified most galaxies into three main categories: spiral galaxies, elliptical galaxies, and irregular galaxies. file:///d /SciExp/generic/ /ch8/ch8_s4_2.html (1 of 3)10/24/2008 3:55:46 PM

5 Spiral Galaxies Photo A below shows a galaxy that has the shape of twin spirals, called a spiral galaxy. Astronomers can see other spiral galaxies from different angles. These views show that spiral galaxies have arms that spiral outward, like pinwheels. Our galaxy has the same spiral, pinwheel shape. It is hard for us to see the spiral shape of our galaxy because our solar system is inside the galaxy, about two thirds of the way out in one of the spiral arms. The Milky Way you see in the sky is the view people on Earth get when they look toward the main part of the rest of our galaxy. The center of our galaxy is about 25,000 light-years from the sun. However, we cannot see the center of our galaxy. The center is hidden from our view by the dust associated with massive clouds of dust between the sun and the center. Galaxy Shapes (A) This spiral galaxy is similar to our galaxy. (B) An elliptical galaxy looks like a flattened ball. (C) The Large Magellanic Cloud is an irregular galaxy. file:///d /SciExp/generic/ /ch8/ch8_s4_2.html (2 of 3)10/24/2008 3:55:46 PM

6 Elliptical Galaxies Not all galaxies have spiral arms. Elliptical galaxies look like flattened balls. Look at the elliptical galaxy in photo B above. These galaxies contain billions of stars but have little gas and dust between the stars. Because of the lack of gas and dust, new stars cannot form in most elliptical galaxies. Most elliptical galaxies contain only old stars. Irregular Galaxies Some galaxies do not have regular shapes. Because of this, they are known as irregular galaxies. The Large Magellanic Cloud in photo C above is an irregular galaxy about 160,000 light-years away from our galaxy. It is one of our closest neighboring galaxies in the universe. A Spiral Galaxy You can make a model of our galaxy. 1. Using pipe cleaners, make a pinwheel with two spirals. 2. View the spirals along the surface of the table. Sketch what you see. Can you see the spiral shape? 3. Next, view the spirals from above the table and sketch them. Observing The sun is inside a flat spiral galaxy. From Earth s position on the flat surface, is it possible to get a good view of stars in the spiral arms? Why or why not? file:///d /SciExp/generic/ /ch8/ch8_s4_2.html (3 of 3)10/24/2008 3:55:46 PM

7 1. What is a star system? 2. Describe the three main types of galaxies. 3. Where is the sun in our galaxy? 4. Thinking Critically Applying Concepts Some binary stars are called eclipsing binaries. Explain why this term is appropriate. (Hint: Think about Algol as you come up with an answer.) Stargazing Plan an evening of stargazing with adult family members. Choose a dark, clear night. Use binoculars if available and the star charts in Appendix G to locate the Milky Way and some interesting stars you have learned about. Explain to your family what you know about the Milky Way and each star you observe. file:///d /SciExp/generic/ /ch8/ch8_s4_3.html10/24/2008 3:56:03 PM