Glossary of Terms

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z #

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Accretion

The process whereby matter from a normal star or diffuse cloud is captured by a compact object such as a black hole or neutron star.
Active Galactic Nuclei (AGN)

The central regions of some galaxies that appear as point-like sources of radiation. Believed to be supermassive black holes accreting nearby matter.
 
Annihilation

The process whereby a particle and its antiparticle interact, converting their mass into energy, according to Einstein's famous formula, E = mc2. For example, the annihilation of an electron and positron results in the emission of photons with an energy of 511 keV.
Angstrom

A unit of length equal to 0.0000000001 (1 x 10-10) meters.
Anticoincidence system

A system on a gamma-ray telescope that triggers when it detects an incoming charged particle so that the telescope will not mistake the particle for a gamma ray.
Arc minute

One-sixtieth of a degree on the sky.
Arc second

One-sixtieth of a arc minute on the sky. (1/3600 of a degree)
Astronomy

The scientific study of outer space, especially the positions, dimensions, distribution, motion, composition, energy, and evolution of celestial bodies and phenomena.
Astrnomical Unit (AU)

The mean distance between the Earth and the Sun (149,597,870.691 kilometers).
Astrophysics

The subset of astronomy that deals principally with the physics of stars, stellar systems, and interstellar material.
Atmosphere

A layer of gas that surrounds a large body such as a planet, moon, or star. The Earth's atmosphere is composed mostly of nitrogen, while the Sun's atmosphere is mostly hydrogen.
Atom

The smallest unit of an element which keeps the element's characteristics. An atom can consist of a proton and neutron in a nucleus being orbited by an electron.
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BATSE

The Burst and Transient Source Experiment on board CGRO. BATSE made all-sky observations of gamma-ray bursts and flares, as well as observing many other objects between 10 keV and 5 MeV.
Big Bang

A theory of cosmology in which the expansion of the Universe is presumed to have begun with a primordial explosion.
Binary Stars

Two stars that orbit around a common center of mass. An X-ray binary is a special case where one of the stars is a collapsed object such as a white dwarf, neutron star, or black hole. In an X-ray binary, the separation between the stars is small enough so that matter is transferred from the normal star to the collapsed star, producing X-rays in the process.
Binocular Vision

Using two eyes to see an object. The ability to perceive depth comes from viewing an object with both eyes and combining the images.
Black Dwarf

A cold celestial object thought to be the remains of a dead star of low mass that is formed after a white dwarf star has radiated away most of its heat energy. Black dwarfs are extremely difficult to detect, and because white dwarfs take so long to cool down, it is possible that the Universe may not yet be old enough for any black dwarfs to develop.
Black hole

Any object with gravity so strong that not even light can escape.
Blazar

A type of AGN that often appears as a point-like source of bright, highly variable radiation.
Boson

A subatomic particle, such as a photon, a pion and certain atomic nuclei.
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Calorimeter

A detector with a component that releases electricity when a photon of light passes through it. It is used to measure a gamma ray's energy.
Compton Gamma Ray Observatory (CGRO)

A NASA gamma-ray mission that was launched in April, 1991, and which reentered in June, 2000. CGRO has four experiments: BATSE, OSSE, Comptel and EGRET, which together span the energy range 10 keV to 30 GeV.
Comptel

The Compton Telescope experiment on board CGRO. Comptel imaged gamma-rays in the energy range 100 keV to 10 MeV.
Converter

A dense material, such as lead or tungsten, used to convert a gamma ray into an electron-positron pair.
Corona (plural: coronae)

The uppermost level of a star's atmosphere. In the Sun, the corona is characterized by low densities and high temperatures (>1,000,000 K). The Sun's corona extends out to many millions of kilometers from the Sun's surface.
Coronal Mass Ejection (CME)

Huge bubbles of gas, threaded with magnetic field lines, that are ejected from the Sun over the course of several hours.
Cosmic rays

Relativistic elementary particles, such as electrons, protons or atomic nuclei, that exist throughout interstellar space.
Cosmology

The study of the origin, structure and evolution of the universe.
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Dark matter

A nonluminous gravitational component of the universe invoked to explain the internal motions of galaxies and the motions of galaxies within clusters of galaxies.
Degree

A unit of angular size. One degree is 1/360 of a full circle, or, conversely, there are 360 degrees in a circle.
Density

The ratio between the mass of an object and its volume.
Diffuse galactic emission

Non-point source gamma-ray emission from the plane of the galaxy. Mostly due to interactions of cosmic rays with interstellar material.
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EGRET

Energetic Gamma Ray Experiment Telescope on board the Compton Gamma Ray Observatory (operated from 30 MeV to 30 GeV).
Electromagnetic Spectrum

All the different colors of light, which is also called electromagnetic radiation. Only a small portion of the spectrum is visible. Names are given to broad energy bands within this spectrum: radio, infrared, visible, ultraviolet, X-rays and gamma-rays. Gamma-rays are the most energetic form of light.
Electron

An elementary particle with a single negative charge, and a mass of about 511 keV.
Electron volt (eV)

A unit of energy, sufficient to excite atoms to emit visible light.
(1 keV=1000 eV, 1 MeV=1000 keV, 1 GeV=1000 MeV)
Emit

To throw or give off.
Extragalactic

Outside of, or beyond, our own galaxy.
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Flux

A detector-independent measure of the brightness of a source.
Frequency

A property of a wave that describes how many wave patterns or cycles pass by in a period of time. Frequency is often measured in Hertz (Hz), where a wave with a frequency of 1 Hz will pass by at 1 cycle per second.
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Galaxy

A component of our Universe made up of gas and a large number (usually more than a million) of stars held together by gravity.
Gamma ray

A photon more energetic than an x-ray (more than about 50 keV).
Gamma-ray burst

Brief intense gamma-ray emission from an unknown source.
GLAST

Gamma-ray Large Area Space Telescope.
Gravity

The attractive force of an object with mass on another object. The gravitational force between two objects depends on their mass and the distance between them.
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Hertz (Hz)

The derived SI unit of frequency. A frequency of 1 Hz is equal to 1 cycle per second.
(after H. Hertz, 1857 - 1894)
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Inertia

The tendency of a body to resist acceleration; the tendency of a body at rest to remain at rest or of a body in straight line motion to stay in motion in a straight line unless acted on by an outside force. Inertia is a property of mass.
Infrared

The region of the electromagnetic spectrum with wavelengths in the range of 2.5 x 10-6 meters to 7 x 10-7 meters. Infrared photons are between optical and microwaves in the electromagnetic spectrum.
International System of Units (SI)

An internationally agreed-upon system of units with which to measure properties such as time, mass, and length.
Inverse Compton scattering

A collision between a photon and an energetic electron that transfers energy from the electron to the photon.
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Jet

A collimated stream of relativistic particles and photons which flows from a central source.
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Kelvin (K)

A temperature scale which measures an object's temperature above absolute zero, the theoretical coldest possible temperature. On the Kelvin scale, the freezing point of water is 273 K ( = 0o C = 32o F). The temperature in Kelvins can be converted to Celsius by the equation K = 273 + C and to Fahrenheit by K = 273 + 5/9 * (F-32).
(Named after Lord Kelvin)
Kilogram (kg)

A unit of mass. One kilogram is defined as the mass of a liter (1000 cubic centimeters) of water at 277 Kelvin.
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Large Magellanic Cloud

The Large Magellanic Cloud is an irregular galaxy in orbit around our own Milky Way galaxy. It is a large object, several degrees in size, and easily visible to the unaided eye from the Southern Hemisphere.
Light

Generally used to mean electromagnetic radiation that is visible to the human eye. Sometimes used to mean all wavelengths of electromagnetic radiation.
Light Curve

A graph showing how the radiation from an object varies over time. Also called a "Time Series".
Light year

The distance light travels in 1 year (6.0 x 1012 miles).
Lorentz factor

g=1/[1-(v/c)2]1/2, where v is the speed of an object, and c is the speed of light.
Luminosity

The rate at which a star or other object emits energy, usually in the form of electromagnetic radiation.
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Magnetic Field

The region of space around a magnetic body or a current-carrying body where objects can be affected by the magnetic forces due to the body or current.
Magnetosphere

The immediate region around a body with a magnetic field where particle behavior is controlled by that field.
Mass

A measure of the total amount of material in a body. The basic unit of mass is the kilogram. The mass of a body determines its gravity and its inertia.
Meter (m)

The fundamental SI unit of length, defined as the length of the path traveled by light in vacuum during a period of 1/299,792,458 s. One meter is approximately 39.4 inches.
Microwave

The region of the electromagnetic spectrum with wavelengths in the range of 2.5 x 10-6 meters to 10-4 meters. Microwave photons are between optical and radio in the electromagnetic spectrum.
Milky Way

The name of our own Galaxy, a flattened disk of stars about 100,000 light years across.
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Nanometer (nm)

A unit of length equal to one billionth of a meter.
NASA

The National Aeronautics and Space Administration, founded in 1958 as the successor to the National Advisory Committee for Aeronautics.
Nebula

A diffuse collection of interstellar dust and gas.
Neutrino

A stable elementary particle with no charge, assumed zero rest mass, and a spin of 1/2. Recent results have indicated that neutrinos may have a very small amount of mass. If so, they would be important in determining the structure and evolution of the universe.
Neutron

A particle commonly found in the nucleus of an atom with approximately the mass of a proton, but zero electrical charge.
Neutron star

A compact star with a radius of about 10 km and a mass of about 1.5 times that of our Sun. A neutron star internally supports itself against gravity by pressure from the strong nuclear force between neutrons, which are uncharged elementary particles commonly found in the nuclei of atoms.
Nova (plural: novae)

A star that experiences a sudden outburst of radiant energy, temporarily increasing its luminosity by hundreds to thousands of times before fading back to its original luminosity.
Nuclear Fusion

A physical process whereby two or more atomic nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. The small amount of mass that seems to be lost is actually converted into energy as described by Einstein's famous equation "Energy = Mass times the Speed of Light squared." This is the source of the Sun's energy.
Nucleus (plural: nuclei)
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Optical Light

The region of the electromagnetic spectrum with wavelengths in the range of 4 x 10-7 meters to 7 x 10-7 meters. Optical photons are between ultraviolet and infrared in the electromagnetic spectrum. Also called "visible light".
Orbit

The path of an object that is moving around a second object or point under the influence of gravity.
OSSE

Oriented Scintillation Spectrometer Experiment on board CGRO (operated between 50 keV and 10 MeV.)
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Pair annihilation

An interaction between a particle and its antiparticle that results in the destruction of the pair of particles and the emission of a pair of photons.
Pair production

The inverse process to pair annihilation where a particle-antiparticle pair are created from a pair of photons. This often happens when a gamma-ray passes close to an atomic nucleus.
Parallax

The apparent motion of a relatively close object compared to a more distant background as the location of the observer changes. Astronomically, it is half the angle which a star appears to move as the Earth moves from one side of the Sun to the other.
Parsec

A historical unit of distance equal to 3.26 light years. A megaparsec is one million parsecs, while a gigaparsec is one billion parsecs.
Particle Accelerator

Any machine or natural object that can accelerate charged particles, such as electrons, positrons or protons to relativistic speeds.
Photon

The fundamental quantum of light. The energy of a photon is proportional to its frequency.
Planetary Nebula

A shell of gas ejected from and expanding around a star usually seen near the end of the star's life.
Plasma

A gas in which the individual atoms are ionized (and therefore charged). In most cases, the total number of positive and negative charges is equal in a plasma, making them electrically neutral.
Pion

An unstable nuclear particle with a rest mass between that of an electron and a proton. Also known as the pi meson.
Positron

The antiparticle of the electron, it is capable of mutual annihilation with an electron. This annihilation produces two low-energy gamma-rays at 511 keV.
Proton

A positively charged particle commonly found in the nucleus of an atom.
Pulsar

A type of neutron star with a beam of emission that sweeps around as the star rotates.
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Radian (rad)

The supplementary SI unit of angular measure, defined as the central angle of a circle whose subtended arc is equal to the radius of the circle. One radian is approximately 57o. There are two pi radians in a circle.
Radiation

The process in which energy is emitted as particles or waves.
Radio

The region of the electromagnetic spectrum with wavelengths longer than 10-4 meters. Radio photons have the lowest energy and longest wavelengths in the electromagnetic spectrum.
Red Giant

A star that has a low surface temperature and a diameter that is large relative to the Sun.
Redshift

The shift of spectral lines to longer wavelengths either due to the motion of the source away from the observer or very strong gravity.
Relativistic

Approaching the speed of light.
Resolution

The size of the smallest detail visible in an image. Low resolution shows only large features, high resolution shows many small details.
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Satellite

A body that revolves around a larger body. For example, the Moon is a satellite of the Earth.
Scientific Notation

A compact format for writing very large or very small numbers, most often used in scientific fields. The notation separates a number into two parts: a decimal fraction between 1 and 10, and a power of ten. Thus 1.23 x 104 means 1.23 times 10 to the fourth power or 12,300; 5.67 x 10-8 means 5.67 divided by 10 to the eighth power or 0.0000000567.
Scintillation

The emission of light that occurs when electrons or positrons excite a substance in a transparent material they are passing through.
Second (s)

The basic unit of time in the International System of Units, equal to the duration of 9,192,631,770 cycles of radiation in a transition, or energy level change, of the cesium atom. A second is also a sixtieth part of a minute of time.
Silicon Strip Detectors (SSD)

Detectors made of tiny strips of silicon, which create voltage pulses when traversed by charged particles, such as electrons or positrons.
Solar flare

A burst-like emission of radiation from disturbances in the Sun's outer atmosphere.
Solar Mass

A unit of mass equivalent to the mass of the Sun. 1 Solar Mass = 1 Msun = 2 x 1030 kilograms.
Solar Prominence

An arc of gas that erupts from the surface of the Sun. Also known as a filament, Solar prominences are supported inside the Sun's corona by strong magnetic fields. Solar prominences often stand 100,000 km in height, and can last from several days to several months.
Spectral Line

Electromagnetic radiation given off at a specific frequency by an atom or molecule. Every different type of atom or molecule gives off light at its own unique set of frequencies; thus, astronomers can look for gas containing a particular atom or molecule by tuning the telescope detector to one of the gas's characteristic frequencies. For example, carbon monoxide (CO) has a spectral line at 115 Gigahertz which corresponds to a wavelength of 2.7 mm.
Spectrometer

The instrument connected to a telescope that separates the Electromagnetic radiation signals into different frequencies, producing a spectrum.
Spectroscopy

The study of spectral lines from different atoms and molecules. Spectroscopy is an important part of studying the chemistry that goes on in stars and in interstellar clouds.
Spectrum (plural: spectra)

A plot of the intensity of light as a function of frequencies; the distribution of wavelengths and frequencies.
Speed of Light (in a vacuum, c)

The speed at which electromagnetic radiation propagates in a vacuum; it is defined as 299,792,458 m/s (186,000 miles/second). Einstein's Theory of Relativity implies that nothing can go faster than the speed of light.
Star

A large ball of gas that creates and emits its own radiation through the process of nuclear fusion.
Structure and Evolution of the Universe (SEU)

One of four theme areas in NASA's Office of Space Science.
Strong nuclear force

A short-range nuclear force that operates within an atomic nucleus.
Subatomic Particles

Particles which are smaller than an atom. Examples include the electron, proton, and neutron.
Supernova

A violent explosion that is the endpoint of the evolution of a massive star. Often a compact object is produced such as a neutron star or black hole.
Supernova remnant

The expanding gaseous shell ejected by a supernova explosion.
Supersymmetry

An extension of the standard model of particle physics, supersymmetry hypothesizes the existence of a complete set of additional particles which complement those that are known to exist. Thus far, no supersymmetric particles have been detected. In some theories, the least massive supersymmetric particle (often called a WIMP) could be a good candidate for the dark matter in the universe.
Synchrotron emission

Electromagnetic radiation emitted by charged particles when accelerated or decelerated by a magnetic field.
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Time Series

A graph showing how the radiation from an object varies over time. Also called "Light Curve".
Tracker

The part of a high-energy gamma-ray telescope that is used to determine the trajectory of the incoming gamma-ray. For a silicon strip detector-based tracker, the trajectories of electron-positron pairs are recorded. These pairs are produced by the converter.
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Ultraviolet

The region of the electromagnetic spectrum with wavelengths in the range of 10-9 meters to 4 x 10-7 meters. Ultraviolet photons are between X-rays and optical light in the electromagnetic spectrum.
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Vacuum

A space absolutely devoid of matter. A vacuum also refers to a space partially exhausted (as to the highest degree possible) by artificial means.
Visible

Electromagnetic radiation at wavelengths which the human eye can see. We perceive this radiation as colors ranging from red (longer wavelengths; ~700 nanometers) to violet (shorter wavelengths; ~400 nanometers.) Also called "optical light".
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Wavelength

The distance between between one point on a wave and the same point in the next cycle. Also see electromagnetic spectrum.
Weak force

A short-range nuclear force responsible for radioactivity and the decay of certain atomic nuclei.
White Dwarf

The exposed core of a star after it has ejected its atmosphere as a planetary nebula. A white dwarf is approximately the size of the Earth, but has the mass of the Sun.
WIMP

A very Weakly Interacting relatively Massive elementary Particle. WIMPs have not yet been observed.
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X-ray

The region of the electromagnetic spectrum with wavelengths in the range of 10-12 meters to 10-9 meters. X-rays have a longer wavelength and lower energy than gamma rays and have a higher energy than ultraviolet light.
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