| A | B |
|---|
| comets | large masses of frozen gas and pieces of metallic and stony material |
| meteorite | a mass of stone or metal that has reached the earth from outer space |
| universe | All matter and energy, including the earth, the galaxies, and the contents of intergalactic space, regarded as a whole |
| nuclear fusion | a nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of energy |
| H-R diagram | Hertzsprung-Russell diagram of luminosity and temperature of the stars |
| sunspots | dark patches visible on the solar surface |
| light speed | 300,000 kilometers per second |
| light year | the distance light travels in one year |
| Milky Way Galaxy | band of light across the dark night sky which possesses a milky-looking appearance |
| clusters | groups of galaxies |
| spectroscope | used to study the light from distant stars and galaxies |
| spectral lines | lines caused by the different wavelengths of light due to the chemical composition of an object |
| Doppler shift | an apparent change in wavelength caused by relative motion of the source of light |
| Edwin Hubble | famed astronomer for whom the Hubble Space Telescope was named |
| red shift | the farther galaxies are away from Earth, the greater the red shift in their spectra |
| "Big Bang" | 15 billion years ago the universe began with a gigantic explosion |
