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Glossary

This page is meant to make it easier for you to understand the content of this web site. The glossary contains both astronomical and art terms.


H

Halo: An extension of a galaxy. Halos around spiral galaxies are often spherical, and contain mostly old globular clusters

Heliocentric: A model of the solar system, where the sun is at the centre and the planets orbit it.

Helioseismology: The science which studies the interior of the sun.

Helium: The second most common element in the Universe. Only hydrogen is more abundant. In stars hydrogen is fused to become helium.

Herschel, William (1738 - 1822): The astronomer who discovered Uranus in 1781. He is also known to have made great improvements on the size of telescopes.

Hertzsprung-Russell Diagram (H-R Diagram) : A diagram which plots the stars' luminosity versus their surface temperature. Some notable groups appear when many stars have been plotted. The most apparent group, a diagonal band ranges from upper left to lower right. When stars are on this band, the "Main sequence" they fuse hydrogen to helium in their cores. Other notable groups are giants, super giants and white dwarfs.
The diagram is logarithmic and the luminosity is often plotted on the vertical axis. At the upper left are the most luminous and hottest stars (class O) while at bottom right are the dimmest and 'coolest' stars (class M).
Stars in on this diagram are classified from O to M class stars (O, B, A, F, G, K, M). Link: H-R diagram.

HII region: Hot emission nebulae. The temperatureis often between 10 000K and 20 000 degrees Kelvin. They are nebulae that have been ionized by nearby hot class O-B stars. The ultraviolet light from these stars strips the hydrogen atom of an electron. When the electron recombines with a hydrogen atom (the hydrogen atom now has two electrons) a characteristic light is emitted. The Orion nebula is a famous example of a HII region where nearby hot stars have ionized the gas. Link: HII-regions.

Hubble, Edwin (1889 - 1953): The astronomer who proved the true nature of the galaxies. Prior to him they were thought to be a part of our own galaxy: The Milky Way galaxy.

Hubble Space Telescope: A telescope which lies in orbit around the earth and studies the universe. It is a very popular telescope known to have taken many breathe-taking images. It is scheduled to be terminated soon in the future.

Hydrogen: The most abundant element in the universe. About 3/4 of the universe is thought to be hydrogen. It is a very volatile element and is thus rarely found on earth. The element is much more abundant in stars where it is fused to become helium, among other elements. Link: Fusion processes.

Hydrostatic equilibrium: In stars there are two very strong forces working against each other: The gravity, which wants to compress the star, and the internal gas pressure, which strives to expand the gas.
When these two forces are equal, at equilibrium, there is a hydrostatic equilibrium, meaning that the star is stable. If this stability is lost the stronger force will make the star contract or expand. Link: Stellar formation.







 




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