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H

Heliopause
The point at which the solar wind meets the interstellar medium or solar wind from other stars.

Heliosphere
The space within the boundary of the heliopause containing the Sun and the Solar System.

Hydrogen
An element consisting of one electron and one proton. Hydrogen is the lightest of the elements and is the building block of the universe. Stars form from massive clouds of hydrogen gas.

Hubble's Law
The law of physics that states that the farther a galaxy is from us, the faster it is moving away from us.

Hydrostatic equilibrium
A state that occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. Hydrostatic equillibrium is responsible for keeping stars from imploding and for giving planets their spherical shape.

Hypergalaxy
A system consisting of a spiral galaxy surrounded by several dwarf white galaxies, often ellipticals. Our galaxy and the Andromeda galaxy are examples of hypergalaxies.

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