Galaxies

Astronomy 315Professor Lee

CarknerLecture 20

The Size of the Universe

Separate star systems like the Milky Way or just nebula in our galaxy?

In the 1920’s Edwin Hubble used the new 100 inch Mt. Wilson telescope to view Cepheids in “spiral nebula” and found they were too distant to be in our galaxy

Types of Galaxies A visual inspection reveals 3 types: Spiral

Elliptical

Composed of older stars Irregular

Look like altered spirals

Spiral Galaxies Spiral galaxies are similar to the Milky

Way

Spiral arms contain gas and dust and young stars

We see a lot of spirals (~80% of bright

galaxies) Most smaller than the Milky Way

Classifying Spiral Galaxies Spiral galaxies are classified based on two properties:

From this Hubble produced 3 categories: Sa

Sb

Sc

Loosely wound arms, small bulge

M31, The Andromeda Galaxy (Sb)

M83 (Sc)

M100 (Sc)

M51, The Whirlpool Galaxy (Sc)

NGC 891 -- Edge-on Spiral

Other Spirals Many spiral galaxies show a bar of material

across the nucleus (barred spiral) Milky Way has a bar

Some galaxies have disks and bulges, but no spiral arms (called S0 or lenticular)

Why do spirals look the way they do?

Sa to Sb to Sc is moving towards “diskier” galaxies with more star formation

M95 -- Barred Spiral (SBb)

NGC 1300 -- Barred Spiral (SBc)

M102 -- S0 Spiral

Elliptical Galaxies Elliptical galaxies have almost no structure

Classified by how elongated they look from

our point of view

Have almost no gas, dust or young stars

Have a wide range in size

M87 -- Giant Elliptical (E1)

M59 -- E5 Elliptical

Irregular Galaxies

Some galaxies have no discernable regular shape

Often show evidence of star formation

Distortion might be due to: Galaxy collisions

The Large Magellanic Cloud

Hubble’s Tuning Fork

Hubble categorized the galaxies and then placed them on a diagram

As you go from left to right in the diagram you roughly increase in gas, dust, number of young stars and star formation rates

Hubble’s Tuning Fork Diagram

Galactic Collisions Galaxies should collide fairly often

What happens when they collide? May trigger wave of star formation

One galaxy may merge with another (galactic cannibalism)

Evolution of Galaxies It is not completely clear how galaxies evolve,

but there is growing evidence for this basic picture

Burst uses up all gas and dust and star formation stops (Elliptical)

Next Time Read Chapter 24.4-24.5