Astronomy 162:
Introduction to Stellar, Galactic, & Extragalactic Astronomy

Lecture 27: Spiral Galaxies

Key Ideas:

Disk & Spheroid Components

Rotation of the Disk:

Spiral Arms:

Spiral Galaxies

The Milky Way & Andromeda are examples of Spiral Galaxies.

All spirals share a common structure:

All Spirals have disks

Spheroids vary greatly in size.

Spheroid Structure

Bulge: where inner spheroid & disk merge

Halo: sparse outer spheroid

Disk Structure

Thick disk of Stars (~1000 pc thick)

Thin disk of Gas & Dust (~100 pc thick)

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:


Rotation Curves

The disk rotates about the center of the galaxy

Inner Parts: Solid-Body Rotation

Outer Parts: Differential Rotation


Rotation Speeds

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:

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


Gas Cloud in outer disk:

Provides us with a way to measure the masses of Galaxies.

Spiral Arms

Pattern of hot stars, star clusters, gas & dust crossing the disk.


Sites of Active Star Formation

Recall: Sun takes 240 Myr orbit Galaxy

O&B Stars only live ~10 Myr

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


Density Waves

Density waves pass through the disk like water waves pass over the ocean.

We are not sure how the waves are excited: