heliocentric model A mode of the solar system which is centered on the Sun, with the Earth in motion about the Sun.

helioseismology The study of conditions far below the Sun's surface through the analysis of internal "sound" waves that repeatedly cross the solar interior.

helium capture The formation of heavy elements by the capture of a helium nucleus. For example, carbon can form heavier elements by fusion with other carbon nuclei, but it is much more likely to occur by helium capture, which requires less energy.

helium flash An explosive event in the post-main-sequence evolution of a low-mass star. When helium fusion begins in a dense stellar core, the burning is explosive in nature. It continues until the energy released is enough to expand the core, at which point the star achieves stable equilibrium again.

helium precipitation Mechanism responsible for the low abundance of helium of Saturn's atmosphere. Helium condenses in the upper layers to form a mist, which rains down toward Saturn's interior, just as water vapor forms into rain in the atmosphere of Earth.

Hertzsprung–Russell (H—R) diagram A plot of luminosity against temperature (or spectral class) for a group of stars.

high-energy telescope Telescope designed to detect radiation in X-rays and gamma rays.

highlands Relatively light-colored regions on the surface of the Moon which are elevated several kilometers above the maria. Also called terrae.

homogeneity Assumed property of the universe such that the number of galaxies in an imaginary large cube of the universe is the same no matter where in the universe the cube is placed.

horizon problem One of two conceptual problems with the standard Big Bang model, which is that some regions of the universe which have very similar properties are too far apart to have exchanged information in the age of the universe.

horizontal branch Region of the Hertzsprung–Russell diagram where post–main sequence stars again reach hydrostatic equilibrium. At this point, the star is burning helium in its core, and hydrogen in a shell surrounding the core.

hot dark matter A class of candidates for the dark matter in the universe, composed of lightweight particles, such as neutrinos, much less massive than the electron.

Hubble classification scheme Method of classifying galaxies according to their appearance, developed by Edwin Hubble.

Hubble's constant The constant of proportionality which gives the relation between recessional velocity and distance in Hubble's law.

Hubble's law Law that relates the observed velocity of recession of a galaxy to its distance from us. The velocity of recession of a galaxy is directly proportional to its distance away.

hydrogen envelope An invisible region engulfing the coma of a comet, usually distorted by the solar wind, and extending across millions of kilometers of space.

hydrogen shell burning Fusion of hydrogen in a shell that is driven by contraction and heating of the helium core. Once hydrogen is depleted in the core of a star, hydrogen burning stops and the core contracts due to gravity, causing the temperature to rise, heating the surrounding layers of hydrogen in the star, and increasing the burning rate there.

hydrosphere Layer of the Earth which contains the liquid oceans and accounts for roughly 70 percent of Earth's total surface area.