ASTR 103 - Astronomy

Glossary - H

H I region

Volume of interstellar space where the hydrogen remains neutral and optically dark.

H II region

Volume of interstellar space occupied by ionized hydrogen.

hadrons

Heavy class of elementary particles ranging from the mesons to the protons, neutrons, and still heavier particles up to nearly 3000 electron masses.

half-life

The length of time for half the amount of a radioactive nucleus to spontaneously disintegrate.

halo

see galactic halo.

harmonic law

 

helium burning

Fusion of helium nuclei into carbon nuclei occurring in a star's core.

helium flash

Somewhat sudden and rapid ignition of helium in the electron-degenerate core of a star, after which the star settles down to fusing helium quietly into carbon.

Helmholtz contraction

The slow contraction of a diffuse gas region in which gravitational potential energy is converted to some other form of energy, generally thermal energy.

hertz (H)

Unit of frequency of electromagnetic radiation, equivalent to one cycle per second.

Hertzsprung gap

Nearly vacant region existing between the upper portion of the main sequence and the left end of the giant branch in the H-R diagram.

Hertzsprung-Russell (H-R) diagram

Plotted positions of the stars according to their absolute magnitudes or luminosities on a vertical scale against their spectral class or temperature or color index or a horizontal scale.

horizon

The great circle on the celestial sphere marked by a plane perpendicular to the zenith-nadir axis and tangent to the Earth at the point of the observer.

Hubble constant (H)

Constant of proportionality (H) that appears in the Hubble law of recession of for galaxies. Its presently determined value is about 17km/s/megalight year.

Hubble law (of recession)

Proportional relationship between the velocities (red shift) of the galaxies and their distances.

Hubble sequence (of galaxies)

Classification of galaxies into an ordered arrangement depending on their appearance and structure.

hydrocarbon

Any chemical containing a mixture of carbon and hydrogen.

hydrogen burning

Fusion of hydrogen nuclei into helium nuclei in the core of stars.

hydrostatic equilibrium

Balance between pressure forces and gravitational forces in a star's layers.

hyperbola

Open-ended curve of a conic section formed by the intersection of a plane with a right-circular cone at any angle between the axis of the cone and its slant edge.