Our Galaxy: The Milky Way: Practice Problem Solutions

1. A huge system of stars, gas, dust and dark matter bound together by gravity.

2. Roughly 100 to 400 billion.

3. The disk is a flat, spinning pancake about 100,000 light-years across and 1,000 thick, with spiral arms. The bulge is a thick, old-star cluster at the center with a bar. The halo is a huge faint sphere of old stars and globular clusters around the disk.

4. A barred spiral galaxy.

5. The supermassive black hole at the center, about 4 million Suns in mass. Stars near the center orbit an invisible object at very high speeds, and the Event Horizon Telescope photographed its shadow in 2022.

6. Dust in the disk blocks visible light. Infrared and radio waves pass through it.

7. The Sun is far from the center, about 25,000 to 30,000 light-years out (now measured at about 26,000), not near the middle as Herschel had thought.