High Energy Astronomy Glossary

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A

absorption line
colors missing in a continuous spectrum because of absorption of those photons by some intervening material
accretion disk
a disk of material around an object. The material loses angular momentum in the disk and eventually spirals in to the object (i.e. accretes onto the object.
active galaxy
galaxies characterized by a large energy output compared to a normal galaxy
active galactic nuclei (AGN)
the tiny central engine (likely a supermassive black hole) that drives the active galaxy phenomenon
active Sun
the Sun during times when its flaring activity is strong
air shower
a cascade of particles caused by the interaction of a high-energy gamma-ray (also occurs with cosmic rays) with the atmosphere; the gamma-ray interacts to produce an electron/positron pair, this pair then interacts with the atmosphere to produce more particles, and so on until the shower reaches the Earth, at which time millions of particles have been produced as a result of that one incoming gamma-ray
anti-coincidence detector shield
a shield which is transparent to gamma rays but that detects when a cosmic ray passes through it; used to reject cosmic ray events in gamma-ray detectors
ASTRO-E
a Japanese X-ray mission which was to be launched in February 2000; unfortunately, the mission was lost due to a launch failure

B

BeppoSAX
an X-ray satellite of the Italian Space Agency built with the participation of the Netherlands Agency for Aerospace Programs. It was launched in April 1996 and is still operational.
binary star system
a system of two stars orbiting each other. Most stars in the Universe appear to be part of a binary system.
black hole
one possible end point of a star's life. A black hole's gravity is so strong that not even light can escape it's surface.

C

Chandra
X-ray mission from the NASA launched in July 1999
Chandrasekar limit
the maximum amount of mass a white dwarf can have before succombing to gravitational collapse. This is the maximum mass that can electron degeneracy pressure can hold up against gravitational collapse. The value is 1.4 solar masses.
CCD(Charged Coupled Device)
device which detects incoming photons by storing charge produced by the interaction of the photon and detector material, then transferring that charge sequentially to an amplifier and detector
Cerenkov radiation
radiation produced by particles traveling faster than the local speed of light as they slow down
coma
in astronomy, the head of a comet containing the nucleus and vaporized water
comet
a small body made mostly of frozen water and gases mixed with dust and rock
compact object
in astronomy, a compact object refers to a black hole, neutron star and/or white dwarf
CGRO (Compton Gamma-Ray Observatory)
gamma-ray observatory launched in 1991 and deorbited in 2000; observed 30 keV - 30 MeV sky
coded aperture mask
in gamma-ray astronomy, a plate of material constructed of tiles with a random half-open, half-closed pattern; this plate causes a shadow-pattern on the detector which can be used to determine the position of the source in the telescope's field-of-view
collimator
a device which collimates (makes parallel) incoming radiation; some collimators absorb the incoming radiation to ensure the detected photons are from a small angle in the sky, others deflect light to ensure that it is parallel when it hits the detector
Compton scattering
increasing the wavelength (i.e. decreasing the energy) of a photon by scattering off an electron
Compton telescope
a telescope which uses Compton scattering of incoming gamma-rays to determine position and energy of the incoming photons
core
in reference to a star, it is the very center where the temperature is the hottest and pressure is highest. This is where fusion of any element begins in the star, and then moves its way outward, if permitted by the mass (and subsequently temperature) of the star.
cosmic ray
a charged particle, usually a proton, traveling very close to the speed of light

D

dope
(1) in semiconductor physics, to add impurities to a material to change that material's ability to conduct electricity; (2) a slow or dimwitted person

E

electromagnetic radiation
a self-propagating electric and magnetic wave, such as radio, visible, ultraviolet, X- and gamma-radiation
electromagnetic spectrum
the range of all wavelengths (energies/frequencies) of electromagnetic radiation
electron
a negatively charged subatomic particle, usually found orbiting the nucleus of an atom
electron degeneracy pressure
this is a pressure created by closely-packed electrons. No two electrons can occupy the same state in a star, which creates a pressure against collapse, but only to a point. See Chandraskar limit for more.
electron-Volt (eV)
a unit of energy defined by the change in potential energy of an electron traveling across a potential difference of 1 volt
emission line
an energy peak in a continuous spectrum because of emission of those photons by the de-excitation of electrons in atoms

F

fusion
the process of bringing atoms together to form another, heavier element
frequency
the number of waves that pass a point in some time interval, usually given in Hertz (one cycle per second)

G

gamma ray
the region of the electromagnetic spectrum defined by radiation (photons) energies above 1 million eV (wavelengths less than 0.001 nanometers); this waveband has the highest energy (so shortest wavelength) in the electromagnetic spectrum
GLAST (Gamma-ray Large Area Space Telescope)
the next-generation gamma ray telescope, scheduled for launch in 2006
gravitational collpse
the collapse of a collection of material due to it's gravity on itself
grazing incidence
in terms of mirrors, this refers to shining light onto the mirror at a very shallow angle - nearly parallel to the surface of the mirror; this is the only way to focus X-rays with a mirror
ground state
the lowest energy state an atom can achieve

H

hard X-ray
X-rays on the harder, or more energetic, end of the spectrum, loosely defined as X-rays with energies above 10 keV
HEAO (High Energy Astronomy Observatory)
a series of three satellites to observe the high-energy sky; HEAO-1 surveyed the 0.2 keV to 10 MeV sky; HEAO-2, renamed the Einstein observatory after launch, was the first fully imaging X-ray telescope which looked at the 0.2 - 20 keV sky during 1979-1981; HEAO-3 surveyed the 0.2 keV to 10 MeV sky during 1978-1981
high mass X-ray binary (HMXB)
a binary star system consisting of a compact object and a high mass companion (i.e. with a mass greater than about 10 times the Sun's mass)
heat capacity
the amount of heat required to increase the temperature of a material by one degree
high energy astronomy
the study of objects in the universe with light from high energies, specifically X-ray and gamma-ray energies (the energy range from 0.01 keV and higher)

I

INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory)
an upcoming multiwavelength mission from the European Space Agency; scheduled for launch in 2002, it will carry coaligned gamma-ray, X-ray and optical telescopes
interstellar medium
simply, this is the "stuff between the stars". This includes gas and dust that can be found between stars.
ion
an atom that has either lost or gained electrons, thus possessing a postive or negative electrical charge

J

K

keV
one thousand eV

L

Large Magellanic Cloud (LMC)
this is a small galaxy that orbits our Milky Way galaxy. Along with the Small Magellenic Cloud, it is our closest neighbor
lightcurve
a graph of an object's changing brightness over time
low mass X-ray binary (LMXB)
a binary star system consisting of a compact object and a low mass companion (i.e. with a mass less than the Sun's mass)

M

medical X-ray
a picture taken of bones by shooting X-radiation through the body, the bones stop more of the X-rays than the soft tissue, creating lighter spots where the bones are
MeV
one million eV
microcalorimeter
a type of X-ray detector which measures the heat input due to an incoming X-ray
microchannel plate
a type of detector composed of layers of reactive material divided into narrow channels, incoming photons react with the material to produce an electric signal
microwave radiation
the region of the electromagnetic spectrum defined by radiation (photons) with wavelengths between 0.001 and 1 meter or energies between one millionth and one thousandth of an eV; this waveband is the step above radio in terms of energy
millisecond pulsar
a pulsar with a period of milliseconds; such a pulsar spins several tens to hundreds of times per second

N

nanometer
one billionth of a meter
neutral atom
an atom with an equal number of protons and electrons, hence no net electric charge
neutron
a subatomic particle with no charge, usually found in the nucleus of an atom
neutron star
one of the possible endpoints of a star's life. A neutron star is a star that has collapsed to a very dense soup of neutrons. They are about 10 km in diameter.
nucleus
in general, the nucleus is a central region: (1) a comet's nucleus is the ice and dirt core that orbits the sun; (2) a galaxy's nucleus is the bright central region of a galaxy (especially spiral galaxies); (3) an atom's nucleus is the central part of an atom and contains protons and neutrons

O

optical light
see visible light

P

pair production
the production of an electron/positron pair by the interaction of a photon (with energy greater than 30 MeV) with matter, another photon or magnetic field
Pauli Exclusion Principle
the physics principle that no two fermions can exist in identical quantum states. (In plain English, no two particles can occupy the same state at the same time.)
photoelectric ionization
ionization (freeing of an electron from an atom) by incident photons
photomultiplier
a device which detects dim light by producing a cascade of electrons
photon
a discrete amount of light energy
pictograph
an ancient or prehistoric image painted on a rock wall
positron
an antiparticle of an electron
proportional counter
a detector in which an incoming photon interacts with the detector material to create an electrical signal; the strength of the signal is proportional to the energy of the incoming photon
proton
a positively charged subatomic particle, usually found in the nucleus of an atom
pulsar
a neutron star which appears to emit energy in pulses. These pulses can be in any (and all) wavelengths.

Q

quiet Sun
the Sun during periods of relative inactivity; i.e. when it is not experiencing many flares or coronal mass ejections

R

radio waves
the lowest energy portion of the electromagnetic spectrum, radio waves are defines as those with wavelengths of longer than a meter or energies of below one millionth of an eV (below 10-6 eV)
red giant
the stage near the end of a star's life when it puffs up due to increased temperature in the star's core. The star may expand by 200 times it's main-sequence radius.
Roche lobe
the area around a star within which objects are gravitationally bound to the star
ROSAT
X-ray observatory developed as a collaboration between Germany, the United States, and the United Kingdom. It was launched by the United States on June 1, 1990. The mission ended after almost nine years, on February 12, 1999.
RXTE
Rossi X-ray Timing Explorer - see XTE

S

scintillator
material which produces low-energy light (usually optical light) when a high-energy photon interacts with it
semiconductor
material which can conduct electricity better than an insulator, but not as well as a conductor
Small Magellanic Cloud (SMC)
this is a small galaxy that orbits our Milky Way galaxy. Along with the Large Magellenic Cloud, it is our closest neighbor
soft gamma rays
gamma rays on the softer, or less energetic, portion of the spectrum, loosely defined as gamma rays with energies less than an MeV
soft X-rays
X-rays at the softer, or less energetic, part of the spectrum, loosely defined as X-rays with energies lower than a keV
solar wind
a stream of particles flowing from the Sun
solid state detector
detector made with a semiconducting material which is ionized by incident high-energy photons
sounding rocket
in high-energy astronomy, a rocket sent above the bulk of the Earth's atmosphere equipped with a high-energy telescope to make about 15 minutes of observations; these are used as a quick, inexpensive tool to test new detection technology
spectrum
the amount of energy given off by an object at measured energies (plural, spectra)
standard candle
in astronomy, a standard candle is an object with a known amount of energy output. By knowing the actual energy output, astronomers can use the observed output to calculate the distance to that object.
stellar wind
a stream of particles flowing from a star (like the solar wind, but around a star other than the Sun)
Supernova
a violent explosion heralding the death of a star
supernova remnant (SNR)
an expanding and slowly fading shell of gas violently ejected during a supernova explosion

T

Type 1a Supernova
a supernova caused when a white dwarf exceeds the Chandresakar mass limit
Type 2 Supernova
a supernova caused by the end of a massive star's ability to produce energy by fusing matter in its core

U

ultraviolet light
the portion of the electromagnetic spectrum from about 5 - 400 nanometers; this is the form of light responsible for a suntan

V

visible light
the part of the electromagnetic spectrum that can be picked up by human eyes, specifically the region with wavelengths ranging from 400 to 700 nanometers or with energies of 2 - 3 eV

W

wavelength
the distance between two successive peaks or troughs of a wave
white dwarf
one possible end point of a star's life. A white dwarf is a star that has collapsed to a dense soup of electrons.

X

XMM-Newton (X-ray Multi-Mirror Mission)
X-ray mission from the European Space Agency launched in December 1999
X-ray
the region of the electromagnetic spectrum defined by radiation (photons) energies between 1 thousand and 1 million eV (or wavelengths between 0.001 and 1 nanometer); this waveband is between ultraviolet and gamma-radiation in the electromagnetic spectrum
XTE (or RXTE for Rossi X-Ray Timing Explorer)
an X-ray satellite launched in December 1995 and still operating. XTE was designed to study time variability in X-ray emission.

Y

Z