H

H I region

A region in space of neutral Hydrogen

 

H II region

A region in space of ionized Hydrogen

 

Halo

A region that surrounds the nucleus of a galaxy and contains globular clusters and stars.

 

Harmonic laws

Kepler's third law of planetary motion, which states that the squares of the sidereal periods of planets are proportional to the cubes of the semimajor axes of their orbits: P² = a³.

 

Harvest moon

The full moon nearest to the time of the autumnal equinox, which rises after sunset on successive  night with the minimum delay.

 

Hayashi lines

The theoretical evolutionary track on the Hertzsprung-Russel diagram along which a star contracts in its early stages of evolution.

 

Head of comet

The principal part of a comet, which contains the nucleus and coma.

 

Heavy elements

In astronomy, (usually) the elements whose atomic numbers are greater than that of helium.

 

Heliocentric system

In models of the planetary system, any system that is centered around the sun.

 

Helium flash

An explosion-like ignition of the helium in the core of a red giant star, which starts the triple-alpha nuclear process.

 

Hertzsprung-Russel (H-R) diagram

A plot of the absolute magnitudes of stars against their spectral class, color index, color, or temperature.

 

Horizon system

A system in which the coordinates azimuth and altitude are used to establish the position of a celestial body.

 

Hour circle

A great circle on the celestial sphere that passes through the celestial poles and crosses the celestial equator at 90^o.

 

Hubble law

The velocity-distance relation for extragalactic objects, arising from the expansion of the universe. (V = dH, where H is the Hubble constant).

 

Hubble constant

A number that relates the rate of recession of a galaxy to its distance. Its reciprocal measures the age of the universe.