GALAXIESFrom: Jim Lochner, USRA & NASA/GSFC

Solar System

What is a Galaxy ?

Distance from Earth to Sun = 93,000,000 miles = 8 light-minutes

Size of Solar System = 5.5 light-hours

What is a Galaxy?

Stellar Region

30light-years

Sun(solar systemtoo small to be

seen on this scale)

What is a Galaxy?

Our galaxy is the “Milky Way”

200,000 light-years

A galaxyIs a massivecollection ofstars, gas, and dust kept togetherby gravity

Sun’s Stellar Region

What is a Galaxy?

If our solar system was the size of a cell in the human

body, then our galaxy would still

measure over one mile

across.

What is a Galaxy?

•Smaller ones have only a few million.

•The largest galaxies contain more than a trillion stars.

•Astronomers don’t count the stars, but estimate how many stars a galaxy might have.

Types of Galaxies

Spiral disk-like appearance with arms of stars and dust forming a spiral pattern

Barred Spiral similar to spirals but with a bright bar of stars and gas through the center

Elliptical elliptically-shaped, with less gas and dust than spirals; no disk or “arms”

Irregular neither elliptical nor spiral in shape; gas and dust as in spirals but no defined “arms”

Peculiar distorted form of one of the above types, often due to collision with another galaxy or similar catastrophic event

Types of Galaxies Quiz

A B C

D E

?

Galaxy Formation

According to the Big Bang theory, galaxies began when large clouds of gas and dust collapse as a result of their gravity.

As the cloud shrinks and the debris is compacted, stars and planets form.

Contents of Galaxies

Gas Clouds

The Latin word for “cloud” is nebula. In space nebulas (or nebulae) are giant clouds of gas anddust.

Contents of Galaxies

Open Cluster

Small group of new stars that formed at the same time from a cloud of gas and dust.

Contents of Galaxies

Globular Clusters

A spherical bundle of older stars that orbit a galaxy as a satellite. They are very tightly gravitationally bound, which gives them their spherical shape and dense inner core of stars.

•.

Spirals vs. Ellipticals

Finally the type of galaxy depends on initial rate of star formation:

- If stars form quickly, then galaxy becomes elliptical. Stars form within initial distribution of gas,and follow their initial orbits.

- If stars form later, the gas has time to collapse into a disk. Most stars from within the disk. The galaxy becomes a spiral.

Formation via Galaxy Mergers

In clusters, galaxies can pass close to one another.

• Galaxies can become distorted, and often merge.

• Mergers often lead to giant elliptical galaxies at the heart of large clusters.

Spirals in Grazing Encounter

Antennae Galaxies