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NASA Spitzer Space Telescope • Jet Propulsion Laboratory
• California Institute of Technology
• Vision for Space Exploration
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Frame Frame About the Spitzer Space Telescope Frame Frame
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Fast Facts
Current Status
Spitzer History
Spitzer Technology
Spitzer Science
— Why Infrared?
— Science Overview
— Planets
— Stars
— Galaxies
— Universe
— Glossary
Lyman Spitzer, Jr.

Science Glossary

AGN - Active galactic nuclei (AGN) are the center of galaxies where black holes are thought to be the source of high luminosities. Such galaxies produce huge amounts of energy, exceeding the light emitted by all of the stars in the galaxy. A quasar is one particular type of AGN.

Astronomical Unit (AU) - The average distance between the Earth and the Sun. 1 AU is 149,597,870.691 km (about 93 million miles). The Astronomical Unit is a constant which is used to measure distances within our solar system.

Black Hole - An object whose gravity is so strong that not even light can escape from it. Black holes are the evolutionary endpoints of stars at least 10 to 15 times as massive as the Sun. If a star that massive or larger undergoes a supernova explosion, it may leave behind a fairly massive burned out stellar remnant. With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point of zero volume and infinite density, creating what is known as a "singularity." As the density increases, the path of light rays emitted from the star are bent and eventually wrapped irrevocably around the star. Any emitted photons are trapped into an orbit by the intense gravitational field; they will never leave it. Because no light escapes after the star reaches this infinite density, it is called a black hole.
(From the NASA/GSFC Imagine the Universe Dictionary.)

Brown Dwarf - A low-luminosty object with a mass that is intermediate between that of a star and a giant planet. With a mass of about 1-8% of the Sun, a brown dwarf is too small to ignite the thermonuclear fusion that defines a star. A theoretical entity until 1995, hundreds of these cool objects have been discovered in recent years.

Debris Disk - A flattened, rotating disk of dust surrounding a star during the evolutionary stage when planets are being formed. At an earlier phase, a protoplanetary disk of dust may be mixed with gas from which the future planets will form.

Early Universe - The early Universe refers to approximately the first billion years after the Big Bang, which is estimated to have taken place about 14 billion years ago. Because of the expansion of the Universe, it also corresponds to the most distant reaches of the Universe.

Gravitatonal Lens - A massive galaxy or cluster of galaxies that intervenes between us and a distant astronomical object and that gravitationally deflects the light from that distant object. Gravitational lenses can focus, distort, and split light beams in the same way that ordinary glass lenses do.

HII Region - Gaseous nebulae which contain material at a temperature of about 10,000 degrees Kelvin. At this temperature hydrogen becomes ionized as its electrons break free.

Interstellar Medium (ISM) - Gas and dust found between the stars in a galaxy.

Metallicity - A measure of the heavy element (metal) content in stars and the interstellar medium (ISM). In the astronomical context, a metal refers to any atom heavier than helium. Metals result from the thermonuclear fusion of hydrogen and helium within stars, and are ejected into the ISM during the late stages of stellar evolution. The ISM is continually enriched with heavier elements by virtue of novae and supernovae in succeeding generations of star formation.

Non-Thermal Emission - With thermal (blackbody) emission, there is a clear relationship between the distribution of energy over wavelengths and the temperature of an object. Conversely, non-thermal radiation does not obey this simple relation. The synchrotron radiation emitted at radio wavelengths, in which electrons spiral about magnetic fields at relativistic speeds, is one example of non-thermal radiation.

Nova - A star that brightens suddenly and to an unprecedented degree, creating the impression that a new star has appeared where none was before. Hence the name, from nova for "new."

Planetary Nebula - A bubble of gas surrounding a hot,dying star. The star is so hot that it makes the planetary nebula glow, which allows astronomers to see it. The star was once the core of a red giant, which ejected its outer atmosphere and created the planetary nebula. A planetary nebula has nothing to do with a planet, but through a small telescope, it looks like a planet's disk, hence the misleading name.

Quasar - Sometimes also called quasi-stellar object (QSO); A stellar-appearing object of very large redshift that is a strong source of radio waves; presumed to be extragalactic (outside of our galaxy) and highly luminous.
(From the NASA/GSFC Imagine the Universe Dictionary.)

Radio Galaxy - A galaxy that is extremely luminous at radio wavelengths. It is usually giant elliptical galaxy and a strong source of synchrotron radiation.

Redshift - In astronomy, a shift of spectral lines towards longer wavelengths (lower energies) resulting from an object moving away from the observer. This characteristic is the result of an expanding Universe and the finite speed of light.

Seyfert Galaxy - A type of spiral galaxy exhibiting a bright nucleus and with a spectrum that shows broad emission lines.

Supernova - A gigantic stellar explosion in which the star's luminosity suddenly increases by as much as a billion times. Most of the star's substance is blown off, leaving behind, at least in some cases, an extremely dense core which may be a neutron star.

Super-Planet - A planet with the mass of Jupiter, or larger. Jupiter is 318 times more massive than Earth.

Synchrotron Radiation - Electromagnetic radiation given off when very high energy electrons encounter magnetic fields.
(From the NASA/GSFC Imagine the Universe Dictionary.)

ULIRG - An ultraluminous infrared galaxy (ULIRG) emits most of its light in the infrared, and has a luminosity that is greater than a trillion Suns. Discovered by IRAS in 1983, the high luminosity of ULIRGs is created by the energy produced by widespread bursts of star formation (perhaps initiated by galaxy collisions), or by active galactic nuclei in the core of the galaxy.

The Spitzer Space Telescope is a NASA mission managed by the Jet Propulsion Laboratory. This website is maintained by the Spitzer Science Center, located on the campus of the California Institute of Technology and part of NASA's Infrared Processing and Analysis Center. Privacy Policy

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