Introduction to Stellar, Galactic, & Extragalactic Astronomy
Lecture 27: Spiral Galaxies
Disk & Spheroid Components
Rotation of the Disk:
- Differential Rotation Pattern
- Measurement of Galaxy Masses
- Outlined by O&B Stars, HII Regions, & Gas
- Spiral Density Waves in the Disk
- Sites of recent star formation.
The Milky Way & Andromeda are examples of Spiral Galaxies.
All spirals share a common structure:
- Thin Disk of stars, gas, and dust.
- Thick Spheroid of stars with little gas or dust.
All Spirals have disks
Spheroids vary greatly in size.
Bulge: where inner spheroid & disk merge
- Many RR Lyrae stars
- A little gas & dust
Halo: sparse outer spheroid
- Old metal-poor stars
- Globular clusters
- RR Lyrae Stars
Thick disk of Stars (~1000 pc thick)
- Open Clusters & loose Associations of stars
- Mix of young & old stars
- Cepheid Stars in young clusters
Thin disk of Gas & Dust (~100 pc thick)
- Mostly cold atomic Hydrogen gas
- Dusty Giant Molecular Hydrogen (H2) Clouds
Rotation of the Disk
Measure using the Doppler Effect
Stars: Doppler shifts of stellar absorption lines
Ionized Gas: emission lines from HII regions
Atomic Hydrogen (HI) Gas:
- Cold H clouds emit a radio emission line at a wavelength
- Can trace nearly the entire disk beyond where the stars have
begun to thin out.
The disk rotates about the center of the galaxy
Inner Parts: Solid-Body Rotation
- speed rises with radius.
- orbit period is about constant.
Outer Parts: Differential Rotation
- speed is about constant with radius.
- orbit period increases with radius.
Inner Parts: Rise from Zero to few 100 km/sec
Outer Parts: Nearly constant at a few 100 km/sec
Example in the Milky Way:
- Sun has Vrot=220 km/sec at R=8.5 kpc
- Orbital Period: 240 Myr
Measuring Masses of Galaxies
Star or Gas cloud is held in its orbit by the mass interior
to the orbit.
Newton's Gravity gives:
- M(R) = mass interior to radius R
- Vrot = rotation speed
Example: Milky Way
- R=8.5 kpc, Vrot=220 km/sec
- Gives: M = 9.6x1010 Msun inside R=8.5 kpc
Gas Cloud in outer disk:
- R=16 kpc, Vrot=275 km/sec
- Gives: M=2.8x1011 Msun inside R=16 kpc
Provides us with a way to measure the masses of Galaxies.
Pattern of hot stars, star clusters, gas & dust crossing the
- O&B Stars
- HII Regions (star forming regions)
- Giant Molecular Clouds
- Hydrogen Gas and Dust Clouds
Sites of Active Star Formation
Recall: Sun takes 240 Myr orbit Galaxy
- Sun lives ~12 Gyr, so can make ~50 orbits
O&B Stars only live ~10 Myr
- Only move ~10-20o around before dying
- Don't move far from their birthplace.
See O&B Stars and HII Regions strung along the Spiral Arms
like "Beads on a String."
What are Spiral Arms?
Spiral Arms are Density Waves in the Disk.
Density Waves are a kind of orbital traffic jam
- Orbits crowd together in the arms, stars pile up and make
the regions look brighter.
- Gas clouds pile up, collide, fragment, and form new stars.
- O&B Stars ionize leftover gas (HII Regions), then die
before moving far from the waves.
Density waves pass through the disk like water waves pass over
- Stars move through the spiral arms.
- Gas clouds try to move through, but some are induced to form
stars (collision or compression)
We are not sure how the waves are excited:
- Tidal disturbance from a nearby companion?
- Excited by a stellar bar in the central regions?