Homework 5

6. What leads astronomers to believe that Sagittarius A* is a supermassive black hole?

A number of observations lead astronomers to believe that Sagittarius A* is a supermassive black hole. Among them are 1) stars orbit around Sagittarius A* while it remains unperturbed by their mass, suggesting that it is very massive, 2) an accretion disk has been observed around Sagittarius A*, and 3) it is a source of intense X-rays. See slide 13 of the "Milky Way Structure" lesson.

8. What is a spiral tracer? Which objects make good spiral tracers?

A spiral tracer is any object that is only found in spiral arms and therefore detection of the locations of these objects trace out the locations of spiral arms. Some good spiral tracers are: O&B stars, emission nebulae, young clusters and neutral hydrogen (which emits 21 cm radiation). See slide 5 of the "Milky Way Properties" lesson.

4. What are Variable stars? Why are they good distance indicators?

A variable star is a star that varies regularly in luminosity over time, and where the period of variability is related to the luminosity. The two main variable stars are Cepheid variables and RR Lyrae stars. They are useful distance indicators because we can determine their luminosities based on how their brightness varies. We can then compare their luminosities with how bright they appear to determine how far away they are. See slide 4 of the "Milky Way Structure" lesson.

3. How do astronomers detect Black Holes?

Black holes cannot be observed directly, since not even light can escape from them. We can detect them based on their gravitational influences on other objects that we can observe, or by the energy emitted from accretion disks formed as material falls towards them. See slides 6 & 7 of the "Black Holes" lesson.

5. How did Harlow Shapley determine the location of the center of our Galaxy?

Harlow Shapley was able to determine the distances to globular clusters throughout our Galaxy using variable stars. Assuming that they were distributed symmetrically throughout our Galaxy, he correctly surmised that the center of their distribution was the center of our Galaxy. See slide 6 of the "Milky Way Structure" lesson.

7. What are the main characteristics of Population I and Population II stars?

Population I stars are relatively younger stars with higher metal content. They tend to orbit in the disk of the Galaxy in near-circular orbits. Population II stars are relatively older stars with lower metal content. They tend to orbit through the halo of the Galaxy in random, eccentric orbits. See slide 2 of the "Milky Way Properties" lesson.

9. What happens when two galaxies "collide"?

The collision of two galaxies is not a physical collision in the sense that they run into each other. Instead, they will pass through one another, yet their gravities will distort the galaxies and chances are, force them to eventually merge into one galaxy. See slide 2 of the "Galaxy Collision and Dark Matter" lesson.

1. What does the term 'escape velocity' mean?

The escape velocity of an object is the speed needed to become completely free from the object's gravitational pull. See slide 2 of the "Black Holes" lesson.

10. What do the rotation curves of spiral galaxies tell us about their mass and composition?

The rotation curves of spiral galaxies tell us how mass is distributed throughout them at various distances from their centers. Based on the fact that objects at large distances are moving faster than they should, based on the mass we can "see", we can deduce the presence of dark matter in the halos of spiral galaxies. See slide 8 of the "Galaxy Collisions and Dark Matter" lesson.

2.How do astronomers define the size of a Black Hole?

The size of a black hole is defined by its event horizon, which is a sphere surrounding a black hole whose radius is defined by the distance from the black hole at which the escape velocity is exactly equal to the speed of light. See slide 2 of the "Black Holes" lesson.