1. Galaxies are bound by .............. .

wind

gravity

water

sun

2. The size of some galaxies are measured in units called ............... .

miles

meter

parsecs

kilometer

3. One parsec is equivalent to about .............. trillion miles.

18

19

20

15

4. Light travels at nearly .............. km per second.

200000

300,000

100000

400000

5. Galaxies are usually classified according to their .............. .

type

shape

period

time

6. The Milky Way galaxy contains over 200 billion stars and has a diameter of up to ............. light years.

130000

120,000

150000

140000