H0: The Hubble constant.
H I region: An interstellar region of neutral hydrogen.
H II region: An interstellar region of ionized hydrogen.
H line: The spectral line of ionized calcium at 3968 Å.
Hα: The first line of the Balmer series of hydrogen, at 6563 Å.
Half-life: The length of time for half a set of particles to decay through radioactivity or instability.
halo: Of a galaxy, the region of the galaxy that extends far above and below the plane of the galaxy, containing globular clusters.
head: Of a comet, the nucleus and coma together.
heat flow: The flow of energy from one location to another.
heavyweight stars: Stars of more than about 8 solar masses.
heliocentric: Sun-centered; using the Sun rather than the Earth as the point to which we refer. A heliocentric measurement, for example, omits the effect of the Doppler shift caused by the Earth’s orbital motion.
helium flash: The rapid onset of fusion of helium into carbon through the triple-alpha process that takes place in most red-giant stars.
Herbig-Haro objects: Blobs of gas ejected in star formation.
hertz: The measure of frequency, with units of/sec (per equivalent) vs. luminosity (or equivalent) for a group of stars.
Hertzsprung-Russell diagram: A graph of temperature (or equivalent) vs. luminosity (or equivalent) for a group of stars.
high-energy astrophysics: The study of x-rays, gamma rays, and cosmic rays, and of the processes that make them.
highlands: Regions on the Moon or elsewhere that are above the level that may have been smoothed by flowing lava.
homogenous: Uniform throughout.
horizon problem: one of the problems of cosmology solved by the inflationary theory: why the Universe is identical in all directions, even though widely separated regions could never have been in thermal equilibrium with each other since they are beyond each other’s horizon.
hot dark matter: Non-luminous matter with large speeds, like neutrinos.
hour circle: The great circles passing through the celestial poles.
H-R diagram: Hertzsprung-Russell diagram.
Hubble constant (H0): The constant of proportionality in Hubble’s law linking the speed of recession of a distant object and its distance from us.
Hubble Deep Field: A small part of the sky in Ursa Major extensively studied by the Hubble Space Telescope in December 1995 and since also studied by a wide variety of telescopes on the ground and in space; the long exposures allowed astronomers to see “deep” into space, that is to great distances and therefore far back in time. See http://www.stsci.edu/ftp/science/hdf.html. A Hubble Deep Field – South was observed subsequently
Hubble flow: The assumed uniform expansion of the Universe, on which any peculiar motions of galaxies or clusters of galaxies is superimposed.
Hubble time: The amount of time since the bug bang, assuming a constant speed for any given galaxy; the Hubble time is calculated by tracing the Universe backward in time using the current Hubble constant.
Hubble type: Hubble’s galaxy classification scheme; E0, E7, Sa, SBa, etc.
Hubble’s law: The linear relation between the speed of recession of a distant object and its distance from us, ν= H0d.
Hyperfine level: A subdivision of an energy level caused by such relatively minor effects as changes resulting from the interactions among spinning particles in an atom or molecule.
Hypothesis: The first step in the traditional formulation of the scientific method; a tentative explanation of a set of facts that is to be tested experimentally or observationally.
IC: Index Catalogue, one of the supplements to Dreyer’s New General Catalogue.
igneous: Rock cooled from lava.
inclination: Of an orbit, the angle of the plane of the orbit with respect to the ecliptic plane.
inclined: Tilted with respect to some other body, usually describing the axis of rotation or the plane of an orbit.
Index Catalogue: See IC.
inferior conjunction: An inferior planet’s reaching the same celestial longitude as the Sun’s.
inferior planet: A planet whose orbit around the Sun is within the Earth’s, namely, Mercury and Venus.
inflation: The theory that the Universe expanded extremely fast, by perhaps 10 to the 100th power, in the first fraction of a second after the big bang. The concept of inflation solves several problems in cosmology, such as the horizon problem.
inflationary universe: A model of the expanding Universe involving a brief period of extremely rapid expansion.
infrared: Radiation beyond the red, about 7000 Å to 1 mm.
interference: The property of radiation, explainable by the wave theory, in which waves in phase can add (constructive interference) and waves out of phase can subtract (destructive interference); for light, this gives alternate light and dark bands.
interferometer: A device that uses the property of interference to measure such properties of objects as their positions or structure.
interferometry: Observations using an interferometer.
intergalactic medium: Material between galaxies in a cluster.
interior: The inside of an object.
international date line: A crooked imaginary line on the Earth’s surface, roughly corresponding to 180° longitude, at which, when crossed from east to west, the date jumps forward by one day.
interplanetary medium: Gas and dust between the planets.
interstellar medium: Gas and dust between the stars.
interstellar reddening: The relatively greater extinction of blue light by interstellar matter than of red light.
inverse-square law: Decreasing with the square of increasing distance.
ion: An atom that has lost one or more electrons.
ionized: Having lost one or more electrons.
ionosphere: The highest region of the Earth’s atmosphere.
ion tail: See gas tail.
IRAS: The Infrared Astronomical Satellite (1983).
iron meteorites: Meteorites with a high iron content (about 90%); most of the rest is nickel.
irons: Iron meteorites.
irregular galaxy: A type of galaxy showing no regular shape or symmetry.
ISO (Infrared Space Observatory): A European Space Agency project, 1995-1998.
isotope: A form of chemical element with a specific number of neutrons.
isotopic: Being the same in all directions.