Lab7-Galaxies.pdf

Name: _________________________ Lab 7 – Galaxies and The Milky Way Pre-Lab Activity (please complete before reading notes) 1. Below is a sampling of different images of galaxies in the universe. If you were tasked with coming up with a classification system, how would you classify these different galaxies? Aim for around 3 groups. (This is graded on completion, just think about how you would do it!)

Galaxies A galaxy is a large assembly of stars, gas, dust, and dark matter, all held together by gravity. They can contain between 10 million and 1 trillion stars. Galaxies are classified by their shape, through the Hubble tuning fork discussed later. Spiral Galaxies Spiral galaxies contain a disk, a halo, and sometimes a bar. The disk contains the spiral arms, which give the name of the classification. You can think about the disk like a pizza dough: rapid rotation causes the mass to spread out into the disk shape. The spiral arms can be explained by the Lin-Shu density wave theory, which can be compared to a traffic jam. Viewing a traffic jam from above, you would be able to see a clump of cars, with no traffic in front of or behind the clump. Cars are constantly entering the clump from behind, and exiting it in the front, and the clump moves backwards over time. In the Lin-Shu density wave theory, the stars are the like the cars in the traffic jam, causing varying densities throughout the disk, but the stars themselves are still all moving. The halo is a sphere around the disk that is less dense. All galaxies are held together gravitationally by supermassive black holes located at the center. Some spirals have bars at the center. The rotation curve of a galaxy, a graph that shows the velocity with respect to radius, shows a flatness that can’t be explained by gravity, which implies the existence of dark matter. We would expect the velocity to decrease further away from the center, but the velocity flattens out. The outside

of the galaxy is spinning too fast and there needs to be more mass to hold it there. We can’t see any more mass, therefore dark matter must exist. The spiral arms are very dense and hot, with stars running through patches of gas, causing star formation. More star formation means more young, hot, massive stars, so spiral galaxies tend to have a lot of blue stars. Spiral galaxies are also isolated galaxies, found in voids rather than galaxy clusters because they’re very fragile. Questions: 2. What mechanism supports the shape of these galaxies? 3. What forms the spiral arms of this type of galaxy? 4. What is at the center of this type of galaxy? 5. How did astronomers use these galaxies to find out about the existence of dark matter? 6. Do these galaxies tend to be more blue or more red? Why? 7. Where do we see more of these galaxies: in clusters or as isolated galaxies? Why?

Elliptical Galaxies Elliptical galaxies have an ellipsoidal shape because there is no group rotation. The shape comes from the random motion of its contents. Like spiral galaxies, there are supermassive black holes at their centers. They are likely formed from spiral galaxy mergers. They have lower densities indicating less star formation and an abundance of red stars. Since elliptical galaxies are formed from mergers, we tend to see them in galaxy clusters rather than voids. Questions: 8. What mechanism supports the shape of these galaxies? 9. What is at the center of this type of galaxy? 10. How are these galaxies currently believed to be formed? 11. Do these galaxies tend to be more blue or more red? Why? 12. Where do we see more of these galaxies: in clusters or as isolated galaxies? Why? Hubble Tuning Fork The Hubble Tuning Fork is a galaxy classification system created by Edwin Hubble. On the left side of the diagram are elliptical galaxies and on the right are spiral galaxies. The top spiral galaxies are unbarred while the bottom spiral galaxies are barred. The spirals become more tightly wound from right to left, with Sa/SBa galaxies having the most tightly wound spirals and Sc/SBc galaxies having the most loosely wound spirals. The ellipses become more round from right to left, with E0 galaxies being the most round and E7 galaxies being the most elliptical. If a galaxy cannot be classified as one of the options in the tuning fork, it is considered irregular.

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