10.2 Galaxies

Images from the Hubble Telescope• http://youtu.be/jhEBKr7rV18

What is a Galaxy?• From the Greek word for milk: galaktos• A galaxy is a collection of gas, dust and stars held together by

gravity• Astronomers estimate there are over 125 billion galaxies in the

universe

The Milky Way• Our solar system is contained in the Milky Way galaxy• Our sun is one of approximately 100 billion stars in this galaxy

Galaxies• Edwin Hubble in 1925 identified the first galaxy outside of our

own. Our nearest neighbouring galaxy Andromeda• Hubble initially thought he was observing a nebula• Nebula: Cloud of gas and dust often where stars are formed

Galaxy Shapes• Galaxies are classified as one of three basic shapes:• Spiral, Elliptical or Irregular

Spiral Galaxy• When viewed from above looks like a pinwheel• Many long arms spiraling out from a central core• Central bulge is made of stars formed long ago• Outer arms are made of dust and newly formed stars• Glowing halo of stars surrounds the entire galaxy• Milky Way and Andromeda are both examples of spiral

galaxies

Elliptical Galaxy• Spherical to stretched out ellipse shape• Contain some of the oldest stars in the universe• Some of the largest galaxies in the universe are elliptical• More than half of all galaxies are elliptical

Irregular Galaxy• Does not have any regular shape to categorize it as elliptical or

spiral• Contains a mixture of old and new stars

Interacting Galaxies

Features of Galaxies• Composition: Amount and type of stars, gas and dust• Size: Dwarf to Supergiants• Spin: Faster spin leads to flatter galaxies• Star Formation: Star clusters are distinct groups of stars• Globular Cluster• Open Cluster