GALAXIES

A GALAXY IS

A massive group of stars held together by their mutual gravity.

Our sun is in the MILKY WAY galaxy

Profile of the MW

The milky way is:

• 100,000 LY across• 1000 LY thick• The sun is approximately

28,000 LY from the center

The milky way contains

100-400 billion stars

100 000 000 000 stars?

• If you counted one star a second, 24 hours a day, it would take 3171 years to count to 100 billion.

• (this is about 40 human lifetimes)

A billion dollar bills?

• Stacked on top of each other would make a stack 67.7 miles high.

If the stack tipped over, it would reach from Mentor to Canton.

Surrounding the center and arms

Is a “halo” of stars and smaller galaxies (millions of stars, not billions) called “globular clusters)

Milky Way

Globular cluster

Galaxies also contain

Large amounts of gas and dust (nebulae)

Gas and Dust clouds

Are the material from which future stars will form, but….

Make it difficult to see into the center of the galaxy.

Gas and dust create dark areas in the night sky.

Thanks to new technologies, however,

new information continues to pour in. The picture that follows was taken by the Hubble space telescope. The circles identify new planets outside of our solar system.

Other orbiters detect

radio waves, infrared, x-rays and gamma rays, which pass though

nebulae.

Evidence from radio waves

• Indicates that there is an intense source of radio waves at the center of the galaxy, perhaps 10 AUs in diameter.

(930,000,000 miles)

The object is named

Sagittarius A* (saggitarius a star)(Shown as the red dot in the following photo)

Looking toward the center of the MW

Stars near saggitarius A*

• Appear to be moving 100s of thousands of km per second (the speed of light is 300km/s)

• An object moving this fast should be able to easily break away from the galaxy. Since they don’t there must be something MASSIVE holding them in place.

It is assumed that Sagg A* is a black hole of 4,000,000 solar masses

Saggitarius A*?

The MW belongs to a group of galaxiescalled the local group

The local group

As is the case with all of the stars in the milky way….

All of the GALAXIES in the LOCAL GROUP orbit a gravitational center.

The local group in turn

• Is a member of a group of groups, called the local supercluster.

Supercluster of galaxies