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H or Ho  See Hubble constant.
H I region  Region of neutral hydrogen in interstellar space. 
H 11 region  Region of ionized hydrogen in interstellar space. 
half-life  The time required for half of the radioactive atoms in a sample to disintegrate. 
halo (of galaxy)  The outermost extent of our Galaxy or another, containing a sparse distribution of stars and globular clusters in a more or less spherical distribution. 
heavy elements  in astronomy, usually those elements of greater atomic number than helium.
helio-  Prefix referring to the Sun. 
heliocentric  Centered on the Sun. 
helium flash  The nearly explosive ignition of helium in the triple-alpha process in the dense core of a red giant star. 
Herbig-Haro (HH) object  Luminous knots of gas in an area of star formation, which are set to glow by jets of material from a protostar. 
hertz  A unit of frequency: one cycle per second. Named for Heinrich Hertz, who first produced radio radiation.
Hertzsprung-Russell (H-R) diagram  A plot of luminosity against surface temperature (or spectral type) for a group of stars. 
highlands (lunar)  The older, heavily cratered crust of the Moon, covering 83 percent of its surface and composed in large part of anorthositic breccias. 
homogeneous  Having a consistent and even distribution of matter that is the same everywhere. 
horizon (astronomical)  A great circle on the celestial sphere 90 from the zenith; more popularly, the circle around us where the dome of the sky meets the Earth. 
Hubble constant  Constant of proportionality between the velocities of remote galaxies and their distances. The Hubble constant is thought to lie in the range of 15 to 30 km/s per million LY. 
Hubble law (or the law of the redshifts The radial velocities of remote galaxies are proportional to their distances from us. 
hydrostatic equilibrium  A balance between the weights of various layers, as in a star or the Earth's atmosphere, and the pressures that support them. 
hypothesis A tentative theory or supposition, advanced to explain certain facts or phenomena, which is subject to further tests and verification. 
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