chapter 15

3. Short lifetime objects like hot young stars
4. planets move around the Sun in elliptical orbits
6. The gas of the supershells can fragment into clouds which eventually fall toward the disk producing so-called
7. a flattened cloud of billions of stars
9. a hot, ionised, gaseous component in the galactic halo of the Milky Way
10. a component of disc galaxies, such as spiral galaxies like the Milky Way and lenticular galaxies
11. are a rare and extreme class of supermassive black holes that are furiously pulling material inwards
12. graph orbital velocity versus radius
13. originate from regions of star formation that have been stretched into spiral patterns by the differential rotation of the galaxy
14. spherical cloud of stars and star clusters

1. stuff in space that has gravity, but it is invisible and isn't like anything else we know about.
2. halo and bulge together
5. any of the elongated and curved spiral sections that are connected to the center of a spiral galaxy
8. a measurement of distance equal to 1,000 parsecs or 3,260 light years.
12. are a distinct class of galaxies characterized by their substantial emissions of radio waves that extend far beyond their visible structures