Glossary -- H

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H II region cloud of ionized hydrogen around a hot, luminous star (usually O or B-type). Produced by the copious ultraviolet light from the hot star(s) causing the hydrogen to fluoresce (atoms are ionized and then when the electron recombine, they produce energy in the visible band).

half-life

the time required for one-half of a radioactive material to decay to a more stable material (it is NOT one-half the age of the rock!).

heliocentric

(universe): model of the universe with the Sun at the center and all other objects moving around it.

helioseismology

the study of the Sun's interior from observations of the Sun's pulsations on its surface.

helium flash

in low-mass red giant stars, the onset of the fusing of helium in the core can be very rapid, almost explosive.

hertz

(Hz): unit of frequency. One hertz = 1 wave peak/second.

Hertzsprung-Russell diagram (H-R diagram)

a plot of stellar luminosity vs. temperature invented by two astronomers, Hertzsprung and Russell. High temperatures are on the left side and decrease to the right. Low luminosities are on the bottom and increase vertically.

homogeneous

everywhere uniform in appearance so that there is no preferred observing position.

Hubble constant

slope of the line relating the speed of the galaxies away from each other and their distance apart from each other.

Hubble Law

the relationship between a galaxy's recession speed from other galaxies and the distance between them: the recession speed = H × distance, where H is the Hubble constant. The recession speed is derived from the redshift of the galaxy spectra and with the Hubble Law, it can be used to find the distance to the farthest galaxies.

hydrostatic equilibrium

a balance between the compression from the weight of material above a layer and the expansion of an outward-directed pressure below the layer. In normal stars and planet atmospheres, the outward-directed pressure is supplied by the thermal pressure of warm or hot gases.

hypothesis

an unproven or unverified idea or model.

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last update: 06 August 1999