Glosario - H

An H II region is an ionized nebula powered by young, massive O-type stars. Ultraviolet photons from these hot stars ionize gas in the surrounding environment, and the nebular gas shines brightly in spectral lines of hydrogen and other elements. Because O-type stars have short lifetimes (typically a few million years), the presence of an H II region indicates that massive star formation has taken place recently at that location. H II regions are often found in the arms of spiral galaxies and in star-forming irregular galaxies.

Hertzsprung–Russell diagram is a plot of luminosity versus effective temperature for a population of stars. Depending on the usage, the star's absolute magnitude may be substituted for luminosity, and its color index or spectral type for the temperature. Depending on their mass and composition, single stars follow specific tracks across this chart over the course of their evolution. Hence, knowing a star's mass and metallicity allows the age to be estimated. Stars of similar types are also found grouped together in specific regions of the chart, including the main sequence, red giant, and white dwarf stars.

Hill sphere or Hill radius is the approximate region around an astronomical object where its gravitational attraction will dominate the motions of satellites. It is computed with respect to the next most gravitationally-attractive object, such as the nearest star or the galactic core. Satellites moving outside this radius will tend to be perturbed away from the main body.[8]

Hypergalaxy refers to a system consisting of a large galaxy that is accompanied by multiple smaller satellite galaxies (often elliptical) as well as its galactic corona. The Milky Way and Andromeda galaxy systems are examples of hypergalaxies.[9]