Chapter 15

Galaxies

What do you think?

• Do galaxies all have spiral arms?

• Are most of the stars in a spiral galaxy in its arms?

• Are galaxies isolated objects?

• Are all galaxies moving away from the Milky Way?

Galaxies seem to take one of four different appearances

SPIRALS

Galaxies seem to take one of four different appearances

SPIRALS

Galaxies seem to take one of four different appearances

SPIRALS

The tightness of a spiral galaxy’s arms is correlated to the size of

its nuclear bulge

Type Sa Type Sb Type Sc

Variety of Spiral Arms

Flocculent spirals (fleecy)

Grand-design spirals (highly organized)

We easily see these spiral arms because they contain numerous bright O and B stars which illuminate dust in the arms.

However, stars in total seem to be evenly distributed throughout the disk.

Self-propagating star formation and spiral density waves produce spiral arms

Galaxies seem to take one of four different appearances

BARRED SPIRALS

Galaxies seem to take one of four different appearances

BARRED SPIRALS

Galaxies seem to take one of four different appearances

BARRED SPIRALS

Bars of stars run through the nuclear bulges of barred spiral galaxies

Type SBa Type SBb Type SBc

Galaxies seem to take one of four different appearances

ELLIPTICALS

Type E0 Type E3 Type E7

Galaxies seem to take one of four different appearances

ELLIPTICALS

Elliptical galaxies display a variety of sizes and masses

• Giant elliptical galaxies can be 20 times larger than the Milky Way

• Dwarf elliptical galaxies are extremely common and can contain as few as a million stars

Galaxies seem to take one of four different appearances

IRREGULAR

Galaxies seem to take one of four different appearances

• Spirals

• Barred Spirals

• Ellipticals

• Irregulars

This classification scheme is known as the Hubble Tuning Fork Scheme

Galaxies are gravitationally bound into clusters and superclusters

• Galaxies are not scattered randomly throughout the galaxy

• Galaxies are found in clusters– The Milky Way is part of the Local Group of about 35 galaxies

• Clusters of galaxies are clustered as well in groups called superclusters– Our Local Group is part of the Local Supercluster

• The majority of space is empty - called voids.

The Local Group

Virgo Cluster

Fornax Cluster

Hercules Cluster

Each dot represents a

single galaxy

Galaxies in a cluster can collide and combine

NOTE: There is so much space between stars in a galaxy that the probability of two stars crashing into each other is extremely small.

Colliding galaxies ….

• Do not involve colliding stars

• Might form hot intergalactic gas• Could initiate rapid star formation - called

STARBURST GALAXIES

• Cause galaxy mergers called “galactic cannibalism”

Three galaxies, M81 (big), M82 (medium), and

NGC 3077 (small).

Are they related to one

another?

Galactic Cannibalism Computer Simulation

NOTE: No cluster or supercluster of galaxies contains enough visible matter to provide enough gravitational “glue” to keep the spinning clusters bound together.

Non-luminous material in galactic halos may account for some dark

matter in the universe

The redshifts of remote superclusters indicate that the universe is expanding

• During the 1920s, Edwin Hubble and Milton Humanson were studying galaxy spectra

• They found that galaxy spectral lines were redshifted

• They measured the distance to galaxies by finding Cepheid variables and using the period-luminosity relationship

• They found that the most distant galaxies had the largest redshifts

• This is called the HUBBLE FLOW.

Hubble’s Law

slope = 75 km/s/Mpc called Hubble’s Constant

Astronomers now measure distances to galaxies using supernovae and the Tully-Fisher relation

• Standard candle method– If you know how bright something really is and compare that

to how bright it looks, then the distance can be determined

• Supernovae– All Type II a supernovae have the same luminosity

• Tully-Fischer Relation– The broader the spectral line, the faster it is rotating and the

more luminous the galaxy.

What did you think?• Do galaxies all have spiral arms?

No, galaxies may be either spiral, barred spiral, elliptical, or irregular. Only spirals and barred spirals have arms.

• Are most of the stars in a spiral galaxy in its arms?No, the spiral arms contain only 5% more stars than the regions between the

arms.

• Are galaxies isolated objects?No, galaxies are grouped in clusters, and clusters are grouped in superclusters.

• Are all galaxies moving away from the Milky Way?No, only the galaxies in other superclusters are necessarily receding from us.

Self-Check1: Draw and label the tuning fork diagram, showing the principal Hubble classes for

galaxies, and describe the criteria for the assignment of subclasses.

2: Discuss the differences between flocculent spirals and grand-design spirals.

3: Explain what spiral arms are and how they are sustained.

4: Compare and contrast the contents of the various Hubble classes of galaxies.

5: Describe the distribution of galaxies in space, and distinguish between regular and irregular clusters of galaxies.

6: Explain the observational basis for the dark matter problem in spiral galaxies and in clusters of galaxies.

7: Discuss the role of collisions and mergers in our understanding of galaxies.

8: State the Hubble law and describe how the Hubble constant is evaluated and how it is used to determine the distances of galaxies.