Exam 21: The Milky Waya Normal Spiral Galaxy

You find a star orbiting about the center of the galaxy at a speed of 220 km/s at a distance of 15 kpc. How much mass is within the orbit of that star? You can assume the star is on a circular orbit around the center of the Milky Way.

The oldest disk stars are both __________ and __________ compared to halo stars.

If we found a star cluster at the edge of the disk of the Milky Way at a distance of 10 kpc from the center, which orbited with a velocity of 300 km/s, what would be the total mass enclosed within this radius?

Imagine you discovered a barred spiral galaxy that was a "Milky Way twin" because the size of the bulge and disk, the arrangement of spiral arms, and other characteristics were just like those in our galaxy. Then you measured its rotation curve and plotted the figure shown below. What would you conclude about this galaxy?

Cosmic rays in the Milky Way are important because:

Globular clusters have much higher stellar densities than the local solar neighborhood.

Cosmic rays come mostly from extragalactic sources and contribute little to the overall energy balance in the Milky Way disk.

If the Large Magellanic Cloud is orbiting the Milky Way in a circular orbit with a speed of 175 km/s and a distance of 50 kpc from the center, how long would it take for the Large Magellanic Cloud to complete one orbit around the Milky Way?

Stars in the Milky Way that are 12 to 13 billion years old are only found in globular clusters.

How do we know that globular cluster stars were not the first stars that formed in our galaxy? Would this still be true if some or all globular clusters came into the galaxy by accretion of small satellite galaxies?

Why is the supermassive black hole in the center of our galaxy not an AGN right now? Could it be active in the future? Why or why not?

Based on the chemical abundances and velocities of stars in the Milky Way's disk and halo, we can conclude that the protogalactic fragments that came together to form the galaxy merged when the fragments had:

The Sun has 2 percent of its mass in heavy elements (those elements that are not hydrogen and helium). What does this tell us about chemical enrichment in the galaxy?

We find that interstellar gas and the youngest disk stars are found in a very narrow distribution along the galactic plane, but that older disk stars can be found in a thicker disk. What might this mean?

What is the radius of the event horizon for the 4 *10⁶ M _$\odot$ black hole at the center of the Milky Way?

Describe the population of globular star clusters in the Milky Way: how many are there, where do they reside in the Milky Way, and how do their ages and chemical abundances compare to the Sun?