| 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. |