| A | B |
|---|
| constellation | a group of stars that form an imaginary picture |
| number of constellations | 88 |
| star system | a group of two or more stars |
| binary star system | a group of two stars |
| eclipsing binary star system | a group of two stars where the dimmer star blocks the light of the other star |
| number of planets discovered around other stars | more than 100 |
| two types of star clusters | open and globular |
| open cluster | loosely, disorganized group of stars that are newer and number no more than a few thousand |
| globular cluster | a densely packed group of stars that contain older stars and number more than a million |
| galaxy | a large group of stars, star systems, star clusters, gas, and dust bound together by gravity |
| 3 types of galaxies | spiral, elliptical, and irregular |
| spiral | a galaxy that has a bulge in the middle with spiral arms. the spiral arms contain lots of gas and dust and new stars. has the most stars---numbers in the billions |
| elliptical | looks like a large flattened ball. contains little gas and dust, mainly older stars---billions of stars |
| irregular | lots of gas and dust, very young stars, least amount of stars of all the galaxies |
| quasars | young active galaxies, that have enormous black holes in the center |
| location of quasars | at least 10 billion light-years away from us |
| name of our galaxy | Milky Way |
| type of galaxy our galaxy | spiral |
| distance from center of our galaxy | 25,000 light-years |
| width of our galaxy | 100,000 light-years |
| we are we located in our galaxy | on one of the spiral arms |
| use of constellations by astronomers | to locate objects in the night sky and to find directions |
| how astronomers found planets around other stars | by noticing the wobble of the star as it rotates, and by noticing the star moving back and forth |
