1. Star Systems and Galaxies
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2. Star Systems
While our system has only one star, over half all stars are in groups of two or more called star systems.
Pairs of stars are called binary stars while those with three stars are called triple systems.
The closest star (Proxima Centari) to us is believed to be a triple system.

3. Exosolar Systems
Scientists have recently found evidence that many other stars have planets.
They look for the wobble a planet would induce in a star that it is orbiting.
This method works well for large planets, but not at all for small, Earth sized ones.
Soon astronomers hope to be able to directly view exosolar planets with the new generation of space telescopes being planned.

4. How do we find exoplanets?
With our current technology, it is very hard to spot one directly.
There are however two methods today which are commonly used to find exoplanets.
1) Gravitational "tug": Basically, when a planet moves around it's parent star, the gravitational force of the planet pulls the star slightly making it wobble. Similar to our moon tugging on Earth and causing the tides. Only works for large, gas giant type planets.
2) Light dimming or transit method.: when a planet moves in front of our view of a star it blocks some of the light, therefore making the star at that time, dimmer, so we know something must be around the star to do that.
There are other methods, and new technology is being developed.

5. Galaxies
A galaxy is a huge group of stars, dust, gas, and other celestial bodies bound together by gravitational forces.
Galaxies are classified by shape.
While there are many types, we will study the three main types: spiral, elliptical, and irregular.

6. Spiral Galaxies Have spiral, pinwheel shapes.
Our galaxy, The Milky Way, is a spiral galaxy.
Resemble hurricanes viewed from above.
Have a central, flat disk containing a dense cloud of interstellar matter and young star clusters (mostly on the arms)
Also contain a central bulge (or nucleus) containing older stars

7. Elliptical Galaxies Look like flattened balls.
Are the largest galaxies we find
Found in clusters of galaxies.

8. Irregular Galaxies These galaxies have no defined shape.
Can be formed when galaxies collided or come close to each other.
Could be young galaxies that have not attained their shape.

9. Milky Way Galaxy
Our galaxy is a spiral galaxy with about billion stars. It is 100,000 t0 120,000 light years across.
Our Sun is about 2/3rds of the way from the center on the Orion Arm.
Our Sun takes between 225 and 250 million years to orbit the center of the Milky Way Galaxy one time. This is called a cosmic year.