Astronomy Today

active actually amount appear astronomers atmosphere atoms become bright called cause Chapter close cloud clusters color Compare contain continuous core craters dark density described determine direction discussed disk distance dust early Earth effect electrons elements emission emitted energy eventually example explain fact field Figure force formation Galactic galaxies gravity greater halo helium hole hydrogen important increases interstellar Jupiter known larger less light lines luminosity magnetic Mars mass material matter measure million Moon motion move nebula nuclei object observed occur orbit particles period photograph physical planet present produced properties radiation radio regions relative result ring rotation scale seen shown shows similar solar system space spectrum speed spiral stage stars stellar structure surface telescope temperature theory tion units universe Venus visible wave wavelength