Galaxies
Key Points
Galaxies: Large college of stars (1011) held together by gravity
Types of galaxies:
- Spirals
- Late: more disk than bulge, high star formation rates, blue, younger
- Early: More bulge, low star formation rate
- Ellipticals: All bulge, old
- Irregulars
- Lenticular: small hint of disk
- Dwarfs: Small, most numerous, little gas, not bright because they’re sparse
Hubble sequence puts these all in order:
Old galaxies are redder
New galaxies are bluer (a result of star formation)
Collapsing galaxies create star formation
We can measure the star formation in a couple of ways:
- Hα emission lines from HII (ionized Hydrogen) regions
- Ultraviolet radiation (hot big new stars)
- Far Infrared radiation (stars that are still collapsing and forming)
ISM=Interstellar Medium: The gas between the stars.
3phases of ISM:
- Molecular: cold, dense, tightly packed, where stars form
- Neutral/Atomic: Cold, less dense, most common
- Ionized: Hot, diffuse, byproduct
Gravity, Motion, and structure are interlinked
Rotation (measured by Doppler shifts)
- Ordered: spiral galaxy disks
- Random: Elliptical orbits around 3D spherical shapes
The evidence for dark matter lies in the rotation of the outer edge of the galaxy where the velocity stays constant even though the visible mass goes down.
Dark matter is not gas, faint stars, planets, or dust (not enough IR)
Galaxies can merge (remember the movies)
Gravitational lensing: light follows the path of curved space to create lensing effect.