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habitable zone
A region around a star within which a planet can have temperatures that permit liquid water.

heliocentric
Sun-centered. The planets in our solar system are in heliocentric orbit around the sun.

heliopause
the point at which the solar wind meets the interstellar medium or solar wind from other stars. Click on the diagram to the right to see a detailed diagram of our solar system's boundaries.

heliosphere
The space within the boundary of the heliopause containing the Sun and solar system.

helium
The second lightest and second most abundant element. Helium was first discovered in our Sun. Roughly 25 percent of our Sun is helium. This gas is very light and is used to inflate balloons.

high sun
The condition of the sun being directly overhead (analogous to noon) which eliminates shadows on the surface. When planetary images are obtained in high sun conditions they are examined for albedo information rather than morphologic information (see Terminator). Areas imaged under both high and low sun conditions can be compared to link albedo and morphologic features.

hydrogen
The lightest and most abundant element. A hydrogen atom consists of one proton and one electron. Hydrogen composes about 75 percent of the Sun but only a tiny fraction of the Earth.