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:

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Rotation Curves

The disk rotates about the center of the galaxy

Inner Parts: Solid-Body Rotation

Outer Parts: Differential Rotation

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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:

Where:

M(R) = mass interior to radius R
Vrot = rotation speed

Example: Milky Way

Sun:

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.

Tracers:


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

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Density Waves

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

We are not sure how the waves are excited: