Earth and Space Science / Unit 10: Stars, Galaxies and the Universe
10.5 Galaxies
Notes
A galaxy is a huge collection of stars, gas, dust and dark matter held by gravity.
- Types: spiral (arms, disk, bulge), elliptical (round, old stars), irregular.
- Our galaxy, the Milky Way, is a barred spiral about 100,000 ly across with 100-400 billion stars. The Sun is in an arm, about 26,000 ly from the center, where a supermassive black hole lies.
- The nearest large galaxy is Andromeda, about 2.5 million ly away.
- Galaxies group into clusters. There are hundreds of billions of galaxies in the observable universe.
- Dark matter is invisible matter detected by its gravity.
Practice
The Milky Way is a:
It is a barred spiral.
The Andromeda Galaxy is about 2.5 million ly away. Light we see from it left:
Light travel time equals distance in light-years.
Galaxies are held together mostly by:
Gravity binds stars and dark matter.
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