ESO Astronomical Glossary - H
Halo
The term halo in astronomy denotes the outer region of a spiral galaxy. The halo of the Milky Way galaxy stretches as far as our nearest galactic neighbours. Halos usually contain globular clusters and are thought to house the majority of the dark matter in spiral galaxies.
HII region
HII regions are a type of emission nebulae composed of very hot gas, mainly ionised atomic hydrogen (chemically denoted as HII), heated by newly formed stars in the nebula. They are thought to originate when star formation in giant molecular clouds starts heating up the cold gas, causing it to become ionised and 'glow'.
Hubble constant
The Hubble constant, which has the symbol H0, describes the relationship between a galaxy's distance and the speed at which is it is seen to recede from am observer here on Earth. The precise value of H0 is a topic of much debate: current estimates place it at 67-70 km/s/Mpc. The constant takes its name from the famous astronomer Edwin Hubble, who formulated the law that first featured the constant in 1929.
Hypernova
A hypernova is the cataclysmic collapse and subsequent explosion of a supergiant star that eventually forms a black hole. This explosion is bigger than a supernova and is suggested to be accompanied by a gamma ray burst.