What is a galaxy?

• A galaxy is a system containing millions or even billions of stars.

• We estimate that there are 50 billion to 100 billion galaxies in the observable universe.

How are galaxies identified?

• Galaxies are classified by shape.• Color is determined by the age of stars and the

amount of dust.SPIRAL ELLIPTICAL IRREGULAR

SPIRAL GALAXIES

• It has a nucleus with arms extending outward.• From the side they are lens shaped with a central

bulge.• Examples are the Milky way and Andromeda

galaxies.

The Milky Way

• Diameter = 100,000 light-years.

• Sun is 26,000 light years from the center.

• It belongs to a cluster of galaxies called the Local Group

The MThe Milky Wayilky Way

Elliptical Galaxies

• They range from nearly spherical to lens-shaped.

• Stars are concentrated in the center.

• They have very little stellar dust.

• M87 is an example of an elliptical galaxy.

Irregular Galaxies

• These can be almost any shape.

• Odd shaped galaxies are called Peculiar.

• These are usually smaller than other galaxies.

• The Large and Small Magellanic Clouds are examples.

The Cigar galaxy

Small Magallenic Cloud

New Ideas about Galaxies

• Some galaxies may have a super massive black hole in the center.

• Galaxies may be born in objects called QUASARS. These are extremely bright objects at the far edges of the universe.

                                                          

1.

                     

  

What type of galaxies are these?

Spiral Ellipticall

Irregular

What type of galaxies are these?

Spiral Irregular

Irregular

Elliptical

How did the Universe begin?

The Big Bang Model is the most commonly accepted idea.

1. All matter was condensed into a dense sphere.

2. The sphere suddenly and quickly expanded outward.

3. The superhot and superdense universe continued to expand more slowly.

4. Temperatures gradually decreased.

5. Atoms began to form and material collected into clouds of gas.

6. These clouds became galaxies.

7. Eventually stars aged and planets formed.

Evidence for the Big Bang.

Edwin Hubble discovered that all galaxies exhibit red shift. This indicates they are all still moving apart.

RED SHIFT Cosmic Background Radiation

Penzias and Wilson discovered radiation apparently left over from the Big Bang Explosion. It was measured with a satellite called the Cosmic Background Explorer or COBE.

                            

Red Shift of Galaxies

                                 

Receding Galaxy Approaching Galaxy

Hubble Deep FieldFamous photo taken by the Hubble telescope when it was pointed at a single point in space.

It shows hundreds of galaxies in an area of space which is the size of Roosevelt’s eye on a dime held at arms length.

The Known Universe