Imagine the Universe!
V. Glossary

Big Bang - a theory of cosmology in which the expansion of the universe started with a primeval explosion. It is supported by our current understanding of the composition and structure of the universe.

Convection - a process by which warmer, less dense gas rises, and cooler, denser gas falls. This process occurs in the outer layers of some stars.

Cosmic Ray - a nuclear particle or the nucleus of an atom which is traveling through space with very high energies (often close to the speed of light).

Deuterium - an isotope of hydrogen which consists of a proton, neutron and electron. The proton-neutron nucleus is also referred to as deuteron.

Fission - a process in which heavy atomic nuclei break apart to form two or more lighter nuclei. The excess mass of the parent nucleus compared to the resulting nuclei is converted into energy.

Fusion - a process in which atomic nuclei combine to form heavier elements. The excess mass of the colliding nuclei compared to the resulting element is converted into energy.

Heavy Element - to an astronomer, any element other than hydrogen (or possibly helium).

Nucleon - either of the particles found in an atomic nucleus, i.e. a proton or neutron.

Neutron Capture - a process in which a neutron is captured by an atomic nucleus, producing an isotope of the element. If the isotope is unstable, the nucleus will decay into a different element.

Nova - an explosion that occurs on the surface of a white dwarf, due to the accumulation and subsequent fusion of hydrogen on its surface.

Planetary Nebula - a shell of gas ejected from stars like our Sun at the end of their lifetime. This gas continues to expand away from the remaining white dwarf.

Radioactive Decay - A process by which an element is converted into a lighter element.

Red Giant - a star having a large diameter and relatively cool surface, which results after hydrogen burning has ceased in a star. A red giant has a core in which helium is fusing into carbon.

Red Supergiant - a very large star which results after helium fusion has ceased in the core of a very massive star (more than 15 times the mass of our Sun).

Stellar (or Solar) Wind - the ejection of high-energy particles, including atomic nuclei, from the surface of a star (or the Sun).

Supernova - the explosion of a star. A Type II supernova occurs when nuclear fusion can no longer occur in the core of a massive star, which occurs after the core has fused elements into iron. In a Type I supernova, a white dwarf explodes after accumulating material from a nearby companion star.

White Dwarf - the final state of a star about the same mass as our Sun after it has exhausted its nuclear fuel of hydrogen and collapsed.

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Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The Imagine Team
Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

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