A region around a star within which a planet can have temperatures that permit liquid water.
halo
The spherical region of a spiral galaxy containing a thin scattering of stars, star clusters, and small amounts of gas.
heliocentric universe
A model of the universe with the Sun at the center, such as the Copernical universe.
Hertzsprung-Russel (H-R) diagram
A plot of the intrinsic brightness versus the surface temperature of stars. It separates the effects of temperature and surface area on stellar luminosities. Commonly plotted as absolute magnitude versus spectral type, but also as luminosity versus surface temperature or color.
Hubble constant
A measure of the rate of expansion of the universe. The average value of velocity
of recession divided by distance. Presently believed to be between 50 and 100 km/s/Mpc.
Hubble time
The age of the universe, equivalent to 1 divided by the Hubble constant. The Hubble time is the age of the universe if it has expanded since the big bang at a constant rate.
