1. 21.4 Star systems and galaxies
Key concepts: what is a star system? What are the major types of galaxies? How do astronomers describe the scale of the universe?
Key terms: binary star, eclipsing binary, open cluster, globular cluster, galaxy, spiral galaxy, elliptical galaxy, irregular galaxy, quasar, universe, scientific notation

2. Star systems and clusters
Most stars are members of groups of two or more stars, called star systems
Multiple star systems – two or more stars – called double stars or binary stars. Alpha Centauri A and Alpha Centauri B form a double star.
Usually one star in a binary star is more bright and massive
Wobbling light – dim star’s gravity causes bright star’s light to wobble

3. Eclipsing binaries
A wobble is not the only clue that a star has a dim companion star. A dim star in a binary star may pass in front of a brighter star and eclipse it. A system in which one star periodically blocks the light from another star is called an eclipsing binary.

4. Planets around other stars
There are some ;)

5. Star clusters
Many stars belong to larger groupings called star clusters. They all formed around the same time to one another.
Open clusters – have a loose, disorganized appearance and contain no more than a few thousand stars. Bright supergiants and a lot of gas.
Globular clusters – large groupings of older stars. Round and densely packed.

6. Galaxies
A galaxy is a huge group of single stars, star systems, star clusters, dust, and gas held together by gravity. There are billions. The largest have more than 1 trillion stars.
Spiral, elliptical, irregular

7. Galaxies continued
Spiral – bugle in the middle and arms that spiral outward. Bright young stars with gas and dust. Few new stars form in the central bulge.
Elliptical – round or flattened balls. Little gas and dust, so stars are no longer forming. Old stars.
Irregular – typically smaller, no real shape. Lots of young stars and dust to form new stars.

8. Quasars
Objects that are bright and very far away. Probably active young galaxies with giant black holes at the center.

9. Milky way
Our solar system is located in a spiral galaxy called the Milky Way. It looks like a narrow disk with a bulge in the middle.
Used to be thought of as a standard spiral, now we think it is a barred-spiral

10. Scale of the universe Universe – space and everything in it
Scientific notation – powers of ten to write very large or very small numbers in shorter form.
Immensity of space – Huge. Look at pages