Galaxies

Mass of the Galaxy

• The orbit of clusters can be used to estimate the mass of the galaxy.

• This is the same method used for planets and binary stars.

• The Milky Way galaxy has a mass of 1 trillion (1012) M

.

• Most of the mass is not at the center.

infrared image

Mass at the Center

• The motion of stars near the galactic center gives the mass of the galactic nucleus.

• Infrared images of the stars show the motion.

• The mass is 2.6 million (106) M

.

Many Galaxies

• Any large group of stars separated by 100’s of thousands of light years from other groups is called a galaxy.

• There are over 200 billion observable galaxies.

• There are a wide variety of sizes and shapes.

• Galaxies are classified by their shape.

Spirals

• Spiral galaxies have a dense nucleus and multiple arms.

• The arms can be tight or wide apart.

• Spirals are usually large and have active star formation.

Elliptical Galaxies

• Elliptical galaxies are round or ovoid.

• They are often small compared to spiral galaxies.

• Ellipticals have little dust and new star formation is rare.

Hubble’s System

• Edwin Hubble used the shapes to label galaxy types.

• E: elliptical galaxy;

the number indicates how stretched out the shape is.

• S (or SA): regular spiral galaxy;

SB: barred spiral galaxy;

the small letter indicates how spread out the arms are.

Galaxy Formation

• Like stars, galaxies start with gas.

• The first formation was about 13 billion years ago.

• Gravity pulled small gas clouds together—with enough density, stars form.

• These first galaxies were elliptical shapes.

Close Encounters

• Galaxies are attracted to each other by gravity.

• If two ellipticals pass each other gas and stars are pulled.

• The galaxy is distorted as it rotates.

Star Ages

• Ellipticals generally have old stars (type II).

• This suggests that they were formed first.

• The center of spiral galaxies also have type II stars.

• The disk of a spiral galaxy has type I stars that are young.

New Star Regions

• As an elliptical passes through, the stars do not collide.

• The gas and dust in the two galaxies collide.

• The increased density and gravity creates new stars.

Ring Galaxies

• One galaxy may pass through another.

• The collision forms a ring of stars and gas.

• Gravity can pull the stars in, forming spiral arms.

Forming Spirals

• Computer simulations are used to see the effect of collisions.

• A near miss forms a spiral galaxy (top sequence).

• A direct hit forms a ring galaxy (bottom sequence).

History in Shapes

• Spiral galaxies are created by collisions of galaxies.

• There are many spiral galaxies, so collisions are common.

• Irregular galaxies occur during collisions.

• This lasts about 100 million years.

• The spiral will last for billions of years.