Survey
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* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Monday/Wednesday, November 28/30, 2005 Please come to the front of the room and take back your papers. Concept Check If you are one of the “chosen ones,” please sit next to the other members of your group. You may not use your notes, book, or un-chosen members of your group. Team A: 1. Meredith Banos 2. Talia Leeds 3. Daniel Weitz 1. Gleb Feldman 2. Jordon Helgason 3. Ariel Sims Team B: 1. Robin Barretta 2. Erik Knapp 3. Ronnie Reyes 1. Jessica Cecil 2. Georgia Fields 3. Jonathan Russo Please choose question one or two. Question One: Where would you look to find a spiral density wave? What do they cause? Answer: Spiral density waves are located in the disks of spiral galaxies. They cause gas clouds to crash into each other, squishing them and causing star formation. Question Two: What are the three major components of the Milky Way, a spiral galaxy? Answer: Halo, disk, bulge <figure 14.1> be able to reproduce these images Where are disk stars found? What are their orbits like? Disk Stars Found in the disk Spend all of time in disk Orbit in same direction, bobbing up and down Where are halo stars found? What are their orbits like? Halo Stars Found in or near the halo Spend most of time in halo Orbit in random directions Bulge Stars Found in bulge Spend all of time in bulge Orbit in random directions Measuring masses – finding dark matter! Remember how we could measure the mass of a binary system using Newton’s version of Kepler’s third law? To do so, we needed to know the period of the binary system and the distance between the two stars. In a similar way, we can determine the mass of the galaxy interior to the orbit of a star if we know the period of the star’s orbit about the galactic center and the distance between the star and the galactic center. Why is it that this only tells us the mass of the material within the Sun’s orbit? Newton’s version of Kepler’s third law is based upon gravity. (If we had derived this law from basic physical principles rather than simply presenting it to you, this would be obvious. As it is, just trust me on this one.) The force of gravity from the material outside of the orbit is in all different directions, so it practically cancels out. The force of gravity from the material within the orbit is all in the same direction, so it adds up. We can, in theory, do similar calculations for stars that are closer to the edge of the disk to find the mass of a greater portion of the galaxy. Since the bulge looks so big and bright, we expect that most of the mass of the galaxy is located in the center. However, in order for this to be true the orbital periods of the stars closer to the edge of the disk would have to be larger (i.e. their velocities would have to be smaller.) Conversely, if most of the mass is in the halo, the orbital periods of the stars closer to the edge of the disk would have to be about the same as the stars near the bulge (i.e. their velocities would have to be about the same as the stars near the bulge.) Contrary to our expectations, stars that are far from the galactic center have velocities that are comparable to stars near the bulge. We explain this by saying that the bulk of the mass in our galaxy is not in our bulge, but in our halo. We call this mass “dark matter” because we do not detect it emitting light. We’ll return to this idea next week. Galaxy Classification There are a few major types of galaxies Spiral Galaxies Galaxy has bulge (yellowish) and disk (white) What does color signify? Disk has visible spiral arms Elliptical Galaxies Round or oval shaped and redder than spiral Irregular Galaxies Any galaxy that does not fit into the above scheme is called an irregular galaxy Identify the following galaxies by type. Spiral Galaxy Elliptical Galaxy Irregular Galaxy