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H-R (Hertzsprung-Russell) diagram A plot of luminosity versus temperature for a group of stars that can be used to classify the evolutionary state of stars.

helium capture The formation of heavy elements by the capture of a helium nucleus. For example, carbon can form heavier elements by fusion with other carbon nuclei, but it is much more likely to occur by helium capture, which requires less energy.

helium flash An explosive event in the post-main-sequence evolution of a low-mass star. When helium fusion begins in a dense stellar core, the burning is explosive in nature. It continues until the energy released is enough to expand the core, at which point the star achieves stable equilibrium again.

Herbig-Haro objects Clouds of dust and gas that are either part of high-speed jets of gas streaming away from very young stars, or clouds of gas that have been hit by such jets.

high-mass star: A star that is about 8 or more times the mass of the Sun.

homogeneity Assumed property of the universe such that the number of galaxies in an imaginary large cube of the universe is the same no matter where in the universe the cube is placed.

horizon problem One of two conceptual problems with the standard Big Bang model, which is that some regions of the universe which have very similar properties are too far apart to have exchanged information in the age of the universe.

horizontal branch Region of the H-R diagram where post-main sequence stars again reach hydrostatic equilibrium. At this point, the star is burning helium in its core, and hydrogen in a shell surrounding the core.

hot dark matter A class of candidates for the dark matter in the universe, composed of lightweight, rapidly moving particles, such as neutrinos, much less massive than the electron.

Hubble Classification scheme Method of classifying galaxies according to their appearance, developed by Edwin Hubble.

Hubble's Constant The constant of proportionality which gives the relation between recessional velocity and distance in Hubble's Law. [More Info: Photo Album]

Hubble's Law Law that relates the observed velocity of recession of a galaxy to its distance from us. The velocity of recession of a galaxy is proportional to its distance. [More Info: Photo Album]

Hubble Space Telescope The first large optical telescope launched above the Earth's atmosphere and carrying instruments sensitive to visible and ultraviolet light. The telescope was built by NASA with major contributions from the European Space Agency, and was launched in April, 1990.

hydrogen shell burning Fusion of hydrogen in a shell that is driven by contraction and heating of the helium core. Once hydrogen is depleted in the core of a star, hydrogen burning stops and the core contracts due to gravity, causing the temperature to rise, heating the surrounding layers of hydrogen in the star, and increasing the burning rate there.

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