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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.- Active Galaxy
A galaxy with an unusually large amount of energy emittied from the nucleus.
(see active galactic nuclei)- 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.
- 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.
- 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 k
eV
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.
- 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.
- 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.
- 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.
- 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.
- 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)
- 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.
- Jet
A collimated stream of relativistic particles
and photons which flows from a central source.
- 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.
- 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.
- 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.
- 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)
- The positively charged central portion of an atom that
comprises nearly all of the atomic mass and that consists of protons and (except
for the simplest form of hydrogen) neutrons.
- In general, the central point, group, or mass about which gathering, concentration,
or accretion takes place.
- 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.)
- 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
The shift of spectral lines to longer wavelengths
either due to the motion of the source away from the observer or very strong
gravity.- Redshift
A star that has a low surface temperature and a diameter that is large relative
to the Sun.- 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.
- 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.
- 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.
- 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.
- 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".
- 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.
- 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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