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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 26: Spiral Galaxies

Readings: Ch 25, Section 25-4 & 25-5

Key Ideas

Disk & Spheroid Components

Rotation of the Disk
Differential Rotation Pattern
Measurement of Galaxy Masses

Spiral Arms:
Outlined by O&B Stars, HII Regions, & Gas
Spiral Density Waves in the Disk
Sites of recent star formation.

Spiral Galaxies

The Milky Way & Andromeda are examples of Spiral Galaxies.

All spiral galaxiess share a common structure:

All spiral galaxies have disks of varying sizes.

The spheroids of spirals, however, can vary greatly in size.

Spheroid Structure


The bulge is the center of the galaxy where the inner spheroid & the disk merge together. In the bulge we find


The halo is the low-density outer regions of the spheroid, where we find:

Disk Structure

Thick disk of Stars

This is the main body of the disk, about 1000pc thick, composed of:

Thin disk of Gas & Dust

This is a region about 100 pc thick embedded within the stellar disk composed of

This gas and dust is the raw material that fuels the on-going formation of new stars in the disks of spiral galaxies.

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:

Rotating Disk

Rotation Curves

The disk rotates about the center of the galaxy

Inner Parts: Solid-Body Rotation

Outer Parts: Differential Rotation

Galaxy Rotation Curve
Differential Rotation

Typical Rotation Speeds:

Inner Parts:
Rise from Zero to few 100 km/sec

Outer Parts:
Nearly constant at a few 100 km/sec
How fast the speed are depends on the mass of the galaxy.

Measuring Masses of Galaxies

Star or Gas cloud is held in its orbit by the mass interior to its orbit (the matter outside makes a small contribution).

Newton's Gravity predicts:

Galaxy Mass-Rotation Formula
M(R) = mass interior to radius R
Vrot = rotation speed
This measures the enclosed mass within an orbit. As you go further out, you expect the enclosed mass, M(R), to increase (more of the galaxy is inside your orbit).

Example: Milky Way

At the Orbit of the Sun:

Gas Cloud in Outer Disk:

Measuring the rotation speeds in spiral galaxies provides us with a good way to measure the masses of spiral galaxies, and measure how that mass is distributed within the galaxies. As we'll see in future lectures, those measurements held some surprises.

Spiral Arms

The spiral arms are regular, spiral-shaped patterns of hot stars, star clusters, gas & dust that cross face of the disk.

Spiral Arm Tracers:

These are rarely if ever found outside of spiral arms.

Sites of Active Star Formation

Recall that the Sun takes about 200 Myr to complete one orbit around the center of our Galaxy

By comparison, O&B Stars only live for ~10 Myr

When look at deep images of spiral galaxies, we see O&B Stars and HII Regions strung out along the Spiral Arms like "Beads on a String." (in the words of Walter Baade).

What are Spiral Arms?

Spiral Arms are Density Waves that pass through the general disk of stars and gas.

Density Waves are a kind of orbital traffic jam

Kalnaj Spiral Schematic

Density Waves

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

We are not sure how the waves are excited:

Both of these have been implicated, and there are strong (but not conclusive) arguments for each. Some computer simulations suggest that spiral structure may be transitory, but given how many disk-shaped galaxies show at least some spiral pattern, it must recur frequently.
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Updated: 2006 February 8
Copyright Richard W. Pogge, All Rights Reserved.