Glossary

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A

absorption
the process in which light or other electromagnetic radiation gives up its energy to an atom or molecule
accretion
Accumulation of dust and gas onto larger bodies such as stars, planets and moons.
active galaxy
a class of galaxies that emit massive amounts of energy from their centers, far more than ordinary galaxies; they are believed to be powered by supermassive black holes at the center of these galaxies
ASCA
the Japanese Asuka spacecraft (formerly Astro-D), an X-ray mission
ASM
The All-Sky Monitor on Ginga
Astro-H
a joint Japanese/US X-ray mission that is scheduled to launch in 2014.

B

binary star system
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, and the separation between the stars is small enough so that matter is transferred from the normal star to the compact star star, producing X-rays in the process.
black hole
an object whose gravity is so strong that not even light can escape from it
blueshift
an apparent shift toward shorter wavelengths of spectral lines in the radiation emitted by an object caused by motion between the object and the observer which decreases the distance between them
bremsstrahlung
“Braking radiation”, the main way very fast charged particles lose energy when traveling through matter. Radiation is emitted when charged particles are accelerated. In this case, the acceleration is caused by the electromagnetic fields of the atomic nuclei of the medium

C

calorimeter
an instrument that measures the energy of a particle or photon through the amount of heat the particle or photon deposits in a material
CCD
CCD stands for charge-coupled device, a semi-conductor radiation detector with rows of pixels that store charge and are read out sequentially, row-by row.
cluster of galaxies
a system of galaxies containing from a few to a few thousand member galaxies which are all gravitationally bound to each other
Compton scattering
a type of scattering where a photon collides with an electron causing the photon to gain or lose energy and the electron to change its velocity
Compton telescope
a telescope that uses the Compton scatter interaction to detect the energy and location of gamma rays that enter the telescope
continuum emission
emission that produces light at many different energies; physical processes that produce continuum emission have distinctive signatures in their spectra
corona
the uppermost level of a star’s atmosphere. In the sun, the corona is characterized by low densities and high temperatures (> 1,000,000 degrees K)
cosmological redshift
an effect where light emitted from a distant source appears redshifted because of the expansion of spacetime itself
cosmology
the astrophysical study of the history, structure, and dynamics of the universe

D

dark energy
the name given to a hypothesized form of energy in space that exerts a negative pressure. It is deduced from the fact that the expansion of the Universe is speeding up rather than slowing down, as had been expected. Dark energy makes up about 75% of the “stuff” in the Universe.
dark matter
name given to the amount of mass whose existence is deduced from the analysis of galaxy rotation curves but which until now, has escaped all detections. There are many theories on what dark matter could be, but its nature is still a mystery.
Doppler effect
the apparent change in wavelength of sound or light caused by the motion of the source, observer or both. Waves emitted by a moving object as received by an observer will be blueshifted (compressed) if approaching, redshifted (elongated) if receding. It occurs both in sound and light. How much the frequency changes depends on how fast the object is moving toward or away from the receiver.

E

electromagnetic spectrum
the full range of frequencies (or wavelengths or energies), from radio waves to gamma rays, that characterizes light
electron
a negatively charged particle commonly found in the outer layers of atoms; the electron has only 0.0005 the mass of the proton
electron volt
the change of potential energy experienced by an electron moving from a place where the potential has a value of V to a place where it has a value of (V+1 volt). This is a convenient energy unit when dealing with the motions of electrons and ions in electric fields; the unit is also the one used to describe the energy of X-rays and gamma rays. A keV (or kiloelectron volt) is equal to 1000 electron volts. An MeV is equal to one million electron volts. A GeV is equal to one billion (109) electron volts. A TeV is equal to a million million (1012) electron volts.
element
the fundamental kinds of atoms that make up the building blocks of matter, which are each shown on the periodic table of the elements. The most abundant elements in the universe are hydrogen and helium. These two elements make up about 80% and 20% of all the matter in the universe respectively. Despite comprising only a very small fraction the universe, the remaining heavy elements can greatly influence astronomical phenomena. About 2% of the Milky Way’s disk is comprised of heavy elements
emission
the production of light, or more generally, electromagnetic radiation by an atom or other object

F

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

G

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; when capitalized, Galaxy refers to our own Milky Way Galaxy
gamma ray
electromagnetic radiation with the highest energy, shortest wavelength; usually, they are thought of as any photons having energies greater than about 100 keV
gamma-ray burst
a burst of gamma rays from space lasting from a fraction of a second to many minutes. They probably result from the formation of a black hole, most likely either from the collapse of a massive star in a supernova or from the merger of two neutron stars.
GBD
The Gamma-ray Burst Detector on Ginga
Ginga
the third Japanese X-ray mission, also known as Astro-C
GIS
the Gas Imaging Spectrometer on ASCA
GRB
a gamma-ray burst; plural is GRBs

H

HXI
the Hard X-ray Imager on Astro-H
HXT
the Hard X-ray Telescope on Suzaku

I

image
in astronomy, a picture of the sky or of an object in the sky
infrared
electromagnetic radiation at wavelengths longer than the red end of visible light and shorter than microwaves (roughly between 1 and 100 microns)
interstellar medium
the gas and dust between stars, which fills the plane of the Galaxy much like air fills the world we live in
ISAS
Institute of Space and Astronautical Sciences is a Japanese national research organization of astrophysics. In 2003 it merged with two other agencies to become the Japan Aerospace Exploration Agency (JAXA).

J

JAXA
Japan Aerospace Exploration Agency; the Japanese space agency

K

keV
kiloelectron volt. An electron volt is the change of potential energy experienced by an electron moving from a place where the potential has a value of V to a place where it has a value of (V+1 volt). This is a convenient energy unit when dealing with the motions of electrons and ions in electric fields; the unit is also the one used to describe the energy of X-rays and gamma rays. A keV (or kiloelectron volt) is equal to 1000 electron volts. An MeV is equal to one million electron volts. A GeV is equal to one billion (109) electron volts. A TeV is equal to a million million (1012) electron volts.

L

LAC
The Large Area Proportional Counter on Ginga
light
the common term for electromagnetic radiation; often it refers to the portion visible to the human eye, but all forms of electromagnetic radiation are referred to as different forms of light
light curve
a graph showing how the radiation from an object varies over time

M

matter
a word used for any kind of stuff which contains mass
microwave
electromagnetic radiation with a longer wavelength (shorter frequency) than visible light

N

neutron
a particle with approximately the mass of a proton, but zero charge, commonly found in the nucleus of atoms
neutron star
the imploded core of a massive star produced by a supernova explosion (typical mass of 1.4 times the mass of the Sun, radius of about 5 miles, density of a neutron)

P

photon
the smallest (quantum) unit of light/electromagnetic energy; photons are generally regarded as particles with zero mass and no electric charge
Planck constant
the fundamental constant equal to the ratio of the energy of a quantum of energy to its frequency; it has the value 6.626196 x 10-34 J s
proportional counter
a radiation detector that detects incoming photons when they interact with a gas, causing a cascade of electrons which are read by the detector’s electronics
proton
a particle with a positive charge commonly found in the nucleus of atoms
pulsar
A rotating neutron star that generates regular pulses of radiation. Pulsars were discovered by observations at radio wavelengths but have since been observed at nearly every part of the electromagnetic spectrum including optical, X-ray, and gamma-ray energies.

Q

quasar
an enormously bright object at the edge of our universe which emits massive amounts of energy. In an optical telescope, they appear point-like, similar to stars, from which they derive their name (quasar = quasi-stellar). Current theories hold that quasars are one type of AGN.

R

radio
electromagnetic radiation with the lowest frequency (or the longest wavelength)
redshift
an apparent shift toward longer wavelengths of spectral lines in the radiation emitted by an object caused by the emitting object moving away from the observer

S

satellite
a body that revolves around a larger body; for example, the moon is a satellite of the earth
scintillation detector
a radiation detector that detects incoming non-visible light when it interacts with the detector material (either a solid crystal or a gas), emitting optical light as a result of the interaction. Photomultiplier tubes detect the flash of optical light.
SGD
the Soft Gamma-ray Detector on Astro-H
SIS
the Solid-state Imaging Spectrometer on ASCA
solid state detector
a radiation detector that detects incoming photons when they interact with a semiconductor, creating a cascade of electron/”hole” pairs
spatial resolution
in astronomy, the ability of a telescope to differentiate between two objects in the sky which are separated by a small angular distance; the closer two objects can be while still allowing the telescope to see them as two distinct objects, the higher the resolution of the telescope
spectral line
light 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 to one of the gas’s characteristic frequencies. For example, carbon monoxide (CO) has a spectral line at 115 Gigahertz (or a wavelength of 2.7 mm).
spectral resolution
in astronomy, the ability of the telescope to differentiate two light signals which differ in frequency (or energy) by a small amount; the closer the two signals are in frequency while still allowing the telescope to separate them as two distinct components, the higher the spectral resolution of the telescope
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
a plot of the intensity of light at different frequencies; the distribution of wavelengths and frequencies.
speed of light
the speed at which electromagnetic radiation propagates in a vacuum; it is defined as 299,792,458 m/s (186,282 miles/second) and is denoted by the letter “c”. 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
stellar wind
the ejection of gas off the surface of a star. Many different types of stars, including our Sun, have stellar winds; however, a star’s wind is strongest near the end of its life when it has consumed most of its fuel
supermassive black hole
the largest type of black hole with a mass of between millions to billions of times the mass of our Sun; it is believed that most, if not all, large galaxies have a supermassive black hole at their center
supernova
the explosion of a star that is extremely luminous. There are two main processes that result in a supernova, either the death of a massive star or the explosion of a white dwarf star.

In the death explosion of a massive star, the resulting sharp increase in brightness is followed by a gradual fading. At peak light output, these type of supernova explosions (called Type II supernovae) can outshine a galaxy. The outer layers of the exploding star are blasted out in a radioactive cloud. This expanding cloud, visible long after the initial explosion fades from view, forms a supernova remnant (SNR).

When a white dwarf has accumulated enough material from a companion star to achieve a mass equal to the Chandrasekhar limit, it will explode. All of these types of supernovae (called Type Ia) have approximate the same intrinsic brightness, and can be used to determine distances.

supernova remnant
the remains of a supernova explosion
Suzaku
a Japanese X-ray satellite observatory for which NASA provided X-ray mirrors and an X-ray Spectrometer using a calorimeter design; Suzaku (formerly known as Astro-E2) was successfully launched in July 2005
SXI
the Soft X-ray Imager on Astro-H
SXS
the Soft X-ray Spectrometer on Astro-H
synchrotron radiation
electromagnetic radiation given off when very high energy electrons encounter magnetic fields

U

ultraviolet
electromagnetic radiation at wavelengths shorter than the violet end of visible light
universe
everything that exists, including the Earth, planets, stars, galaxies, and all that they contain; the entire cosmos

V

visible light
electromagnetic radiation at wavelengths which the human eye can see; this light is perceived as colors ranging from red (longer wavelengths; ~ 700 nanometers) to violet (shorter wavelengths; ~400 nanometers.)

W

white dwarf
a star that has exhausted most or all of its nuclear fuel and has collapsed to a very small size. Typically, a white dwarf has a radius equal to about 0.01 times that of the Sun, but it has a mass roughly equal to the Sun’s. This gives a white dwarf a density about 1 million times that of water.

X

X-ray
Light with an a very short wavelength and very high energy. X-rays lie between ultraviolet light and gamma rays on the electromagnetic spectrum.
X-ray binary
a binary star system where one of the stars is a collapsed object such as a white dwarf, neutron star, or black hole, and the separation between the stars is small enough so that matter is transferred from the normal star to the compact star star, producing X-rays in the process
XIS
the X-ray Imaging Spectrometers on Suzaku
XMM-Newton
the X-ray Multi-Mirror Mission, launched by the European Space Agency in 1999. Observation targets include quasars, gamma-ray bursts, galaxy clusters and comets. The telescope’s field of view is 30 arcmin, in the energy range from 0.15 to 15 keV
XRS
the X-ray Spectrometer on Suzaku
XRT
the X-ray Telescopes on Suzaku