Across
1
Icy body with a tail near the sun
4
Unit of energy
5
Ball of burning gas in space
6
Element found in all living things
7
Mass per unit volume
11
Particle of light
13
Positive particle in nucleus
Down
2
Tissue that contracts for movement
3
Center of an atom or cell
8
Natural satellite of Earth
9
Precious yellow metallic element
10
Unit of power
12
Biological catalyst
14
Reddish electrical conductor
15
Form of thermal energy