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VII. Glossary

Accretion Disk - a swirling, heated accumulation of dust and gas in orbit around a compact object such as a neutron star or black hole. Matter from this environment continues to fall onto the disk and eventually spirals into the central object.

Active Galactic Nucleus - an extremely luminous center of an otherwise normal galaxy. In many cases, it is so bright that the surrounding galactic structure cannot be seen. Supermassive black holes are the most likely source of their power.

Cygnus X-1 - a stellar black hole candidate. Cygnus X-1 was the first X-ray source found in the constellation Cygnus.

Electromagnetic Spectrum - the range of different light or radiation. Oscillating electric and magnetic fields transfer radiant energy through space. Wavelength, energy, frequency, or temperature can classify these electromagnetic waves.

Escape Velocity - the velocity needed to escape the gravitational influence of a massive body. It depends on the distance you are away from the center of the body and the mass of the body. The closer you are, the harder it is to escape.

Event Horizon - a boundary that defines the point-of-no-return for a black hole. Once this boundary is crossed, no escape or communication with the outside world is possible.

Neutron Star - the final stage of existence for stars born three to seven times more massive than our Sun. Neutron stars are produced by supernova explosions. In these objects, material is so highly compressed that all the protons, electrons, and neutrons are piled together, breaking down the normal structure of an atom.

Nuclear Fusion - an energy-generating process that occurs where the pressure and the temperature are so enormous that lighter atoms such as hydrogen can fuse together to make heavier atoms such as helium, releasing enormous amounts of energy. Nuclear fusion occurs in the cores of stars; the energy eventually emerges from the surface and we see it as sunlight/starlight.

Singularity - a geometric point with no dimensions where the laws of physics break down. It is a theoretical point of zero volume and infinite density.

Spacetime - the combination of the three spatial dimensions (length, width, and height) with time. The four together form the 4-dimensional nature of our Universe. The effects of gravity can be regarded as a result of the curving of spacetime due to the presence of massive objects.

Speed of Light - the ultimate speed limit in the Universe: 300,000 kilometers/second.

Stellar Evolution Theory - the theory of how stars evolve from birth to death. Stars are born in huge gas and dust clouds and end as a white dwarf, neutron star, or black hole, depending on initial mass.

Supernova - a dramatic explosion marking the death of stars much more massive than our Sun. Neutron stars or stellar black holes are the objects left behind.

White Dwarf - an end-stage of life for stars with masses similar to our Sun’s. A white dwarf is a stellar cinder about the size of the Earth. White dwarfs no longer have nuclear reactions taking place in their cores, but are still quite hot from their past activity. After billions of years, they will cool completely and be thought of as a black dwarf.

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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.

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

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