12 DiscussionWhat do you think is going to happen if two large spiral galaxies collide and merge? What happens to the stars’ orbits.Note that stars are so far apart in galaxies that they do not collide with each other.
19 DiscussionIf every mass in the Universe attracts every other mass, what is eventually going to happen to all the masses in the Universe?
20 DiscussionHow can you prevent this from happening?
21 Galaxy redshiftsSlipher – 1914 began taking spectra of a large number of spiral “nebulae” and discovered that most had velocities greater than 1000 km/s.
22 DiscussionWhy do the high velocities of spiral “nebulae” help support the hypothesis that they do not belong to our galaxy?
23 DiscussionThe proof that galaxies were actually separate from the Milky Way came in 1923 when Hubble identified Cepheid variable stars in the Andromeda galaxy taken with the 100 inch telescope on Mount Wilson. Why is this important?
24 DiscussionIf you had to figure out the distances to galaxies, what are some things you might try?
47 Rotation curves of spiral galaxies In spiral galaxies, atomic hydrogen extends to a radius about twice that of the visible stars.By using 21 cm radio emission, we can follow the rotation curve beyond the visible disk of a galaxy.
59 DiscussionWhat are some of the objects we’ve talked about that do not emit a lot of light that might make up this dark matter in galaxies.
60 MACHO project Massive Compact Halo Objects If the object is compact and massive it will bend and focus background star light. Thus, if one of these happens to pass in front of a background star, that star will appear to brighten and then dim back to normal.
63 DiscussionHow would you go about looking of these things?
64 The resultsThere are not enough MACHO events to explain the missing mass in our galaxy.WIMPs? Weakly Interacting Massive Particles.
65 DiscussionCan we use the 21 cm emission for neutral hydrogen to determine the total mass of galaxies? Why or why not?
66 The Local GroupThe Local Group contains about 3 dozen galaxies within 1 Mpc. 90% of the mass is in two large spirals, the Andromeda galaxy, the Milky Way and a dwarf spiral M33. The rest of the galaxies are dwarf galaxies.
74 Local Group dynamicsWhile it is easy to get radial velocities of Local Group galaxies, plotting their orbits is impossible. Proper motions are simply too small to measure.
75 DiscussionHow can we use the radial velocities of the galaxies in the Local Group to estimate the mass?
76 Magellanic StreamWe can estimate the orbit of the Magellanic Clouds because they appear to be leaving behind a trail of HI. This gas is most likely tidally pulled from the clouds as the orbit the Milky Way.