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Glossary
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absolute zero
The zero temperature on the Kelvin scale (0 Kelvin = -459.67F = -273.15C).
The temperature at which all random molecular motion stops.
absorption
The process in which light or other electromagnetic radiation gives up its energy to an atom or molecule.
absorption line spectrum
A spectrum showing dark lines at some narrow color regions (wavelengths).
The lines are formed by atoms absorbing light, which lifts their electrons
to higher orbits. See also emission line
spectrum.
accretion
Accumulation of dust and gas onto larger bodies such as stars, planets and
moons. Accretion is a powerful source of energy when the larger body in
question is a dense object such as a black hole or a neutron star.
accretion disk
A relatively flat sheet of gas and dust surrounding a newborn star, a black
hole, or any massive object growing in mass by attracting material.
active galactic nucleus (AGN; plural: active galactic nuclei)
A class of galaxies in which the centers (the nuclei) spew massive
amount of energy. An AGN often outshines all the stars in that galaxy.
Most astronomers believe that supermassive black holes are the engines
that power the AGN.
adiabatic demagnetization refrigerator (ADR)
A specialized refrigerator that can create and maintain extremely cold
temperatures below that of normal coolants like liquid helium or solid neon.
Advanced Satellite for Cosmology and Astrophysics (ASCA)
The fourth Japanese-US X-ray astronomy satellite (1993-2001), also written
as Asuka; before launch, it was known as ASTRO-D.
Angstrom
A unit of length equal to 10-10 meters, or 0.0000000001 meters.
apogee
The point in its orbit where
an Earth satellite is farthest from the Earth.
ASTRO-E
A X-ray/gamma-ray mission built jointly by the United States and Japan. Astro E was destroyed in February 2000, when a Japanese M-5 rocket failed to lift the instrument into orbit.
Astro-E2
A replacement mission for ASTRO-E. It was
succesfully launched in July 2005, and subsequently renamed Suzaku.
astronomy
The scientific study of matter in outer space, especially the positions,
dimensions, distribution, motion, composition, energy, and evolution of
celestial bodies and phenomena.
astrophysics
The part of astronomy that deals principally with the physics of the universe, including
luminosity, density, temperature, and the chemical composition of stars, galaxies, and the interstellar medium.
atmosphere
The gas that surrounds a planet or star. The Earth's atmosphere is made up of
mostly nitrogen, while the Sun's atmosphere consists of mostly hydrogen.
AXAF
See Chandra.
BBXRT
The Broad Band X-Ray Telescope, which was flown on the Astro-1 space
shuttle flight (Dec. 1990)
Big Bang
The origin of the universe according to the leading theory of cosmology,
in which the universe was extremely hot and dense roughly 13.7 billion
years ago, and has been expanding ever since. Despite the name, the
Big Bang is not an explosion in the usual sense.
binary stars
Binary stars are 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, producing X-rays in the process
(see accretion).
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.
See also Doppler effect.
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.
bunny suit
A special garment worn in a clean room or clean tent designed to prevent
dust, hair, and skin from contaminating the environment.
calibration
The conversion factors between the raw numbers produced by a measuring
instrument (for example, "three tick marks") and physical quantities
("3 inches"); the study of instruments to derive the conversion factors,
under different environmental conditions or at different times as the
instruments age; or the act of applying such conversions to the raw
measurements, producing scientifically useful data.
calorimeter
A device that measures the heat of chemical or physical changes.
cataclysmic variable (CV)
Binary star systems with one white dwarf star and one normal star, in
close orbit about each other. As material from the normal star falls onto
the white dwarf ("accretion"), energy is released as visible light,
ultraviolet rays, and X-rays.
CGRO
The Compton Gamma Ray Observatory
Chandra X-ray Observatory (CXO)
One of NASA's Great Observatories in Earth orbit, launched in July
1999, and named after S. Chandrasekhar. It was previously named the
Advanced X-ray Astrophysics Facility (AXAF)
Chandrasekhar, S. (1910 - 1995)
Indian astrophysicist reknowned for creating theoretical models of
white dwarf stars, among other achievements. His equations explained
the underlying physics behind the creation of white dwarfs, neutron
stars and other compact objects.
Chandrasekhar limit
A limit which mandates that no white dwarf (a collapsed, degenerate star) can be more massive than about 1.4 solar masses. Any degenerate object more massive must inevitably collapse into a neutron star.
charge coupled device (CCD)
A CCD is a semiconductor device which converts light (including X-rays) into electrical charges. The charges are then read out using a complex arrangement of electrodes, which also divides the CCD into many small, discrete areas (we call these "pixels" - or picture elements).
clean room or clean tent
A special environment with a low level of pollutants such as dust,
microbes, vapors, and other contaminants. The air entering a clean
room or clean tent is filtered and constantly recirculated to remove
pollutants. This cleanliness is essential when working with equipment
that is sensitive to environmental contamination.
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.
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
degrees K).
cosmic rays
Atomic nuclei (mostly protons) and electrons that are observed to strike the
Earth's atmosphere with exceedingly high energies.
cosmology
The astrophysical study of the history, structure, and dynamics
of the universe.
cryogenics
The study of the application of low temperatures and its effects.
dark matter
Name given to the amount of mass whose existence is deduced from its
gravitational influences alone, such as the analysis of galaxy rotation
curves. So far, there is no direct detection of dark matter. There are
many theories on what dark matter could be. At the moment, none of them
is convincing enough and the question is still a mystery.
density
The ratio between the mass of an object and its volume. In the metric system, density is measured in grams per cubic centimeter (or kilograms per liter); the density of water is 1.0 gm/cm3; iron is 7.9 gm/cm3; lead is 11.3 gm/cm3.
Dewar
A container (akin to a thermos bottle) that keeps cold material cold. In astronomy, these are often used for liquid nitrogen (at 77K), but can also be used for solid neon (17K) or liquid helium (1.3K). Some astronomical detectors work better at cold temperatures.
disk
(a) A flattened, circular region of gas, dust, and/or stars. It may refer to material surrounding a newly-formed star; material accreting onto a black hole or neutron star; or the large region of a spiral galaxy containing the spiral arms. (b) The apparent circular shape of the Sun, a planet, or the moon when seen in the sky or through a telescope.
Doppler effect (after C.J. Doppler)
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.
dry ice
Solid (frozen) carbon diozide. Instead of melting like water ice,
dry ice sublimates, or goes directly form solid to carbon dioxide gas
at -78.5C (-109.3F).
dust
Not the dust one finds around the house (which is typically fine
bits of fabric, dirt, and dead skin cells). Rather, irregularly
shaped grains of carbon and/or silicates measuring a fraction of a
micron across which are found between the stars. Dust is most evident
by its absorption, causing large dark patches in regions of our Milky
Way Galaxy and dark bands across other galaxies.
eclipse
The passage of one celestial body in front of another, cutting off the
light from the second body (e.g. an eclipse of the sun by the moon, or
one star in a binary system eclipsing the other). It may also be the
passage of all or part of one body through the shadow of another (e.g.
a lunar eclipse in which the moon passes through the Earth's shadow).
Einstein, Albert (1879 - 1955)
German-American physicist; developed the Special and General Theories of Relativity which along with Quantum Mechanics is the foundation of modern physics.
electromagnetic spectrum
The full range of frequencies, from radio waves to gamma rays, that
characterizes light.
electromagnetic waves (radiation)
Another term for light in the generalized sense
(i.e., including frequencies that are invisible to the human eye).
Light waves are propagating waves made
up of oscillating electric and magnetic fields.
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 (eV)
An electron Volt (eV) is a unit of energy. Technically, it is the energy an electron or proton gains when it moves through a potential difference of 1 Volt. Potential difference is the work per unit charge done externally to move a charge from point A to point B without causing a change in potential energy.
This unit is commonly used when referring to the energy of X-rays. Often, the prefix "kilo" is put in front of it. Kilo means 1000, so a kilo-electron Volt or keV is equal to 1000 eV. A Mega-electron Volt, or MeV, is equal to one million eV. A GeV is a Giga-electron Volt - or a billion eV.
elements
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 75% and 23% by mass 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.
emission line spectrum
A spectrum consisting of bright lines at certain wavelengths separated by
darker regions in which there is much less light. The lines are formed
by atoms whose electrons drop from a higher energy orbit to a lower energy
orbit. See also absorption line spectrum.
extragalactic
Outside of, or beyond, our own galaxy.
foil
A thin, flexible sheet of metal. In the Suzaku X-Ray Telescopes,
a foil is a single reflector element in any stage of fabrication from
the flat aluminum sheet to the formed and gold-coated reflector, cut to
the final arc length.
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.
fusion
The process in which atomic nuclei collide so fast that they stick
together and emit a large amount of energy. In the center of most
stars, hydrogen fuses into helium. The energy emitted by fusion
supports the star's enormous mass from collapsing in on itself, and
causes the star to glow.
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.
Galilei, Galileo (1564 - 1642)
An Italian scientist, Galileo was renowned for his epoch making contribution to
physics, astronomy, and scientific philosophy. He is regarded as the
chief founder of modern science. He developed the telescope, with
which he found craters on the Moon and discovered the largest moons of
Jupiter. Galileo was condemned by the Catholic Church for his view of the cosmos based on the theory of Copernicus.
gamma ray
The highest energy, shortest wavelength electromagnetic radiations. Usually, they are thought of as any photons having energies greater than about 100 keV. (It's "gamma-ray" when used as an adjective.)
Gamma-ray Large Area Space Telescope (GLAST)
An international mission planned for launch in 2007, GLAST will study
the universe in the energy range 10 keV - 300 Gev.
gas
One of the three states of matter, in which atoms, molecules, or ions move
freely and are not bound to each other. Plasma, gas in which some electrons
from the atoms and molecules move independently, can be considered ionized
gas or a separate state of matter.
general relativity
The geometric theory of gravitation developed by
Albert Einstein, incorporating and extending
the theory of special relativity to
accelerated frames of reference and introducing the principle that
gravitational and inertial forces are equivalent. The theory has
consequences for the bending of light by massive objects, the nature
of black holes, and the fabric of space and time.
Ginga
The third Japanese X-ray astronomy satellite (1987-1991); before
lauch, it was known as Astro-C.
globular cluster
Approximately spherical collection of stars which shared a common
origin. The cluster may contain up to millions of stars spanning
up to 50 parsecs.
gravity
A mutual physical force attracting two bodies. It is universal
in that any body with a mass attracts all other bodies with a mass.
grazing incidence
The method of reflecting X-rays by hitting the surface at a very small
angle. X-rays cannot be reflected when the angle is large, unlike
visible light which can be reflected at or near 90 degree angle from
the surface ("normal incidence"). For the Suzaku X-Ray Telescopes,
the angle ranges from about 0.2 degrees to 0.7 degrees.
GSFC
Goddard Space Flight Center, one of the centers operated by NASA.
hard x-ray
High energy x-rays, often from about 10 keV to nearly 1000 keV. The dividing line between hard and soft x-rays is not well defined and can depend on the context.
heat capacity
The ability of matter to store heat. The heat capacity of an object
is the quantity of heat (measured in joules) required to raise its
temperature by one kelvin.
helium
The second lightest and second most abundant element. The typical
helium atom consists of a nucleus of two protons and two neutrons
surrounded by two electrons. Helium was first discovered in our Sun.
Roughly 25 percent by mass of our Sun is helium.
hertz; Hz (after H. Hertz, 1857 - 1894)
The derived SI unit of frequency, defined as a frequency of 1 cycle per
second.
high resolution spectroscopy
High resolution means the ability to distinguish small differences in the energies of light. "High" is a somewhat subjective term, and the precise meaning will vary among different areas of astronomy, and with time. Most X-ray astronomers would consider the ability to distinguish < 1% difference in photon energies to be 'high resolution'.
high throughput
High Throughput means being able to measure many X-ray photons. This is achieved by having big areas (either of the detector itself, or of the telescope in front of it) and high efficiency (being able to measure most of the X-rays that hit the instrument).
housing
The structure in each Suzaku X-ray Telescope (XRT) that provides
a rigid framework to hold the nested foil reflector elements and keep
them in alignment. There is one housing for each quadrant of each stage
(primary or secondary, for the two reflections that X-rays must undergo
to form an image).
HST
Hubble Space Telescope
Hubble, Edwin P. (1889 - 1953)
American astronomer whose observations proved that galaxies are
"island universes", not nebulae inside our own galaxy. His
greatest discovery, called "Hubble's Law", was the linear relationship
between a galaxy's distance and the speed with which it is moving. The
Hubble Space Telescope is named in his honor.
hydrogen
The lightest and most abundant element. A hydrogen atom consists of
one proton and one electron. Hydrogen composes about 75 percent of
the Sun by mass, but only a tiny fraction of the Earth.
image
In astronomy, a picture of 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). Almost none of the infrared portion of the electromagnetic spectrum can reach the surface of the Earth, although some portions can be observed by high-altitude aircraft (such as the Kuiper Observatory) or telescopes on high mountaintops (such as the peak of Mauna Kea in Hawaii).
Institute of Space and Astronautical Science (ISAS)
A space science research institute in Japan, part of the Japan Aerospace
Exploration Agency (JAXA).
interstellar medium
The gas and dust between stars, which fills the plane of the Galaxy much like air fills the world we live in. It was only in the last century that observations of interstellar material suggested that it was not even uniformly distributed through space, but that it had a unique structure.
ions
An atom with one or more electrons stripped off, giving it a net positive
charge.
jets
Beams of particles, such as those coming from an active galactic
nucleus or a pulsar. Unlike a jet airplane where the stream of gas
is in one direction, astrophysical jets come in pairs with
each jet aiming in opposite directions.
kelvin (after Lord Kelvin, 1824 - 1907)
The fundamental SI unit of thermodynamic temperature defined as 1/273.16 of
the thermodynamic temperature of the triple point of water (=0.01C). More
practically speaking, the
Kelvin temperature scale measures an object's temperature above
absolute zero, the theoretical coldest possible temperature. On the
Kelvin scale the freezing point of water is 273.15 ( = 0o C =
32o F) [ K = 273.15 + C = 273.15 + 5/9 * (F-32)]. The
Kelvin temperature scale is often used in sciences such as astronomy.
kilogram (kg)
The fundamental SI unit of mass. The kilogram is
the only SI unit still maintained by a physical artifact (a
platinum-iridium bar) kept in the International Bureau of Weights and
Measures at Sevres, France. One kilogram is equivalent to 1,000 grams
or about 2.2 pounds; the mass of a liter of water.
laser
Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. It's a device that produces a coherent beam of optical radiation by stimulating electronic, ionic, or molecular transitions to higher levels so that when they return to lower energy levels they emit energy.
light
The common term for electromagnetic waves, usually
referring to that portion visible to the human eye. However, other
bands of the electromagnetic spectrum are also often referred to as different
forms of light.
light curve
A graph showing how the brightness of an object varies over time.
light year
A unit of length sometimes used in astronomy which equals the distance light travels in a year. At the rate of 300,000 kilometers per second (671 million miles per hour), 1 light-year is equivalent to 9.46053 x 1012 km, 5,880,000,000,000 miles or 63,240 AU.
limb
The outer edge of the apparent disk of a celestial body.
magnetic field
A description of the strength of the magnetic force exerted by an object. Bar magnets have "di-polar" fields, as the force is exerted from the two ends of the bar. In simple terms, the earth, the sun, stars, pulsars all have dipolar magnetic fields.
magnitude
The degree of brightness of a celestial body designated on a numerical
scale, on which the brightest star has magnitude -1.4 and the faintest
star visible without a telescope has magnitude 6. A decrease of
one magnitude represents an increase in apparent brightness by a factor of
2.512; also called apparent magnitude.
mandrel
A round object about which something can be shaped or cast. In the
context of Suzaku X-ray Telescopes, two types of mandrels are used,
one for forming or shaping foils, and the other for surface replication.
This second type is used by depsiting gold onto it, then a foil backing
or substrate is attached with epoxy. When removed, the foil will have
a smooth gold-coated reflecting surface.
mass
A measure of the total amount of material in a body, defined either
by the inertial properties of the body or by its gravitational influence on
other bodies.
matter
A word used for any kind of stuff which contains mass.
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. A unit of
length equal to about 39 inches. A kilometer is equal to 1000
meters.
metrology
The science of making precise measurements of positions, sizes, and
orientations.
microcalorimeter
A microcalorimeter is a thermal device. When an X-ray photon strikes an
absorber, (for example HgTe), it deposits its energy, causing a rise in
temperature. The temperature rise, which is measured by a thermometer
(such as a thermistor), is approximately proportional to the energy of
the X-ray photon. So, by measuring how much the temperature changes,
we can determine the energy of the X-ray.
microwave
Electromagnetic radiation which has a longer wavelength (between 1 mm
and 30 cm) than visible light. Microwaves can be used to study the
Universe, communicate with satellites in Earth orbit, and cook
popcorn.
mirror
In the context of Suzaku X-ray Telescopes, a mirror is a fully functional
telescope optic. The terms "mirror" and "telescope" are used
interchangeably.
NASA
The National Aeronautics and Space Administration, founded in 1958
as the successor to the National Advisory Committee for Aeronautics.
nebula (pl. nebulae)
A diffuse mass of interstellar dust and gas. A reflection
nebula shines by light reflected from nearby stars. An
emission nebula shines by emitting light as electrons
recombine with protons to form hydrogen. The electrons were made
free by the ultraviolet light of a nearby star shining on a cloud
of hydrogen gas. A planetary nebula results when a sun-like star
expels much of its atmosphere near the end of its nuclear-burning life.
nesting
The system of multiple concentric reflectors used in Suzaku X-ray
Telescopes, packing (or nesting)
as many reflectors in the available space as possible. This is necessary
because the reflectors must be nearly edge-on to the incident X-rays
(see grazing incidence)
neutron
A particle with approximately the mass of a proton
but zero electric 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.) According to astronomer and author Frank
Shu, "A sugar cube of neutron-star stuff on Earth
would weigh as much as all of humanity!"
Neutron stars can be observed as pulsars.
Newton, Isaac 1642 - 1727
English cleric and scientist; discovered the classical laws of motion and gravity; the bit with the apple is probably apocryphal.
noise
The random fluctuations that are always associated with a measurement that is repeated many times over. Noise appears in astronomical images as fluctuations in the image background. These fluctuations do not represent any real sources of light in the sky, but rather are caused by the imperfections of the telescope. If the noise is too high, it may obscure the dimmest objects within the field of view.
nova (plural: novae)
In astrophysics, a nova is an explosive hydrogen fusion event
on the surface of a white dwarf in a
cataclysmic variable system.
In a less strict context, this term can be used to describe any 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 nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. The difference in mass is converted to energy by Einstein's famous equivalence "Energy = Mass times the Speed of Light squared". This is the source of the Sun's energy.
orbit
The path of an object that is moving around a second object or point.
perigee
The point in its orbit where an Earth satellite is closest to the
Earth. Opposite of apogee.
photon
Although electromagnetic radiation (visible light, IR, X-rays, ...) behaves like waves in many ways, it also comes in packets; the energy of a packet of light is uniquely determined by its wavelength. Photon is the name given to such a packet, and represents the particle-like nature of light.
Planck constant; h
The fundamental constant equal to the ratio of the energy of a quantum of energy to its frequency. It is the quantum of action. It has the value 6.626196 x 10-34 Js.
pointing
The direction in the sky to which the telescope is pointed. Pointing also describes how accurately a telescope can be pointed toward a particular direction in the sky.
pre-collimator
A section of nested, non-reflecting foils placed above the main body of the
Suzaku X-ray Telescope. It blocks X-rays from outside a prescribed viewing
angle. It is a "pre-"collimator, because it is meant to add to the natural
collimating abilities of the XRT itself.
proton
A particle with a positive charge commonly found in the nucleus of atoms.
pulsar
A rotating neutron star which generates regular pulses of radiation. Pulsars were discovered by observations at radio wavelengths but have since been observed at optical, X-ray, and gamma-ray energies.
quadrant
A quarter of a circle or a cylinder. Suzaku X-ray Telescopes are assembled
into quadrants first, then four quadrants are bolted together to form a
full telescope.
quasar
An enormously bright object millions or billions of light years away
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 active galactic nuclei.
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.
radiation
Energy emitted in the form of waves (light) or particles (photons).
radio
Electromagnetic radiation which has the lowest frequency, the longest
wavelength, and is produced by charged particles moving back and forth; the
atmosphere of the Earth is transparent to radio waves with wavelengths from
a few millimeters to about twenty meters.
redshift
An apparent shift toward longer wavelengths of spectral lines in the radiation
emitted by an object caused. Redshift can be caused when the emitting object
is moving away from the observer (see Doppler
effect), or by the gravity of a dense object, such as a black hole.
reflection law
For a wavefront intersecting a reflecting surface, the angle of incidence
is equal to the angle of reflection, in the same plane defined by the ray of
incidence and the normal.
reflector
A finished foil, in the context of building Suzaku X-ray Telescopes.
The concave surface of the reflector has a smooth,
shiny gold coating from which X-rays will reflect.
relativity principle
The principle, employed by Einstein's relativity theories, that the laws of physics are the same, at least locally, in all coordinate frames. This principle, along with the principle of the constancy of the speed of light, constitutes the founding principles of special relativity.
relativity, theory of
Theories of motion developed by Albert
Einstein, for which he is justifiably famous. Relativity describes
more accurately than Newtonian mechanics the motions of bodies in
strong gravitational fields or with velocities near the speed of light.
All experiments done to date agree with relativity's predictions to a high
degree of accuracy. (Curiously, Einstein received the Nobel prize in
1921 not for relativity but rather for his 1905 work on the
photoelectric effect.)
resolution (spatial)
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.
resolution (spectral or frequency)
Similar to spatial resolution except that it applies to frequency, spectral
resolution is the ability of the telescope to differentiate two light
signals which differ in frequency 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.
Röntgen, Wilhelm Conrad (1845 - 1923)
A German scientist who fortuitously discovered X-rays in 1895.
ROSAT
Short for Röntgen Satellite; a German-US-UK X-ray astronomy satellite
(1990-1999)
rotation
The spin of a celestial body on its own axis. In high energy astronomy, this is often measured as the "spin period."
RXTE
X-ray Timing Explorer, also known as the Rossi X-ray Timing Explorer (RXTE).
RXTE was launched in 1995.
satellite
A body that revolves around a larger body, usually a planet.
For example, the moon is a natural satellite of the earth.
A satellite can also refer to an artifitial object. For example,
Suzaku is an artifitiall satellite orbiting the earth.
sensitivity
A measure of how bright objects need to be in order for that telescope to
detect these objects. A highly sensitive telescope can detect dim objects,
while a telescope with low sensitivity can detect only bright ones.
Seyfert galaxy
A spiral galaxy whose nucleus shows bright emission lines; one of a class of
galaxies first described by C. Seyfert. Seyfert galaxies are a subclass
of active galactic nuclei.
shock wave
A strong compression wave where there is a sudden change in
gas velocity, density, pressure and temperature.
solar mass
A unit of mass equivalent to the mass of the Sun. 1 solar mass =
1 Msun = 2 x 1033 grams.
special relativity
The physical theory of space and time developed by Albert Einstein,
based on the postulates that all the laws of physics are equally valid
in all frames of reference moving at a uniform velocity and that the
speed of light from a uniformly moving source is always the same,
regardless of how fast or slow the source or its observer is moving.
The theory has as consequences the relativistic mass increase of
rapidly moving objects, time dilatation,
and the principle of mass-energy equivalence.
See also general relativity.
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).
spectrometer
The instrument connected to a telescope that separates the light signals into different frequencies, producing a spectrum.
A Dispersive Spectrometer is like a prism. It scatters
light of different energies to different places. We measure the
energy by noting where the X-rays go. A Non-Dispersive
Spectrometer measures the energy directly.
spectroscopy
Spectroscopy, the study of spectra, is the measurement of the amount of light as a function of its energy (or wavelength or frequency, all are equivalent), and the associated interpretation. Spectroscopy gives information on the physical conditions of the X-ray (etc.) emitting object, such as temperature, composition, motion, and density.
For more information about spectra, please see our Spectroscopy page.
spectrum (plural: spectra)
A plot of the intensity of light at different frequencies. Or
the distribution of wavelengths and frequencies.
speed of light (in vacuum)
The speed at which electromagnetic radiation propagates in a vacuum; it is defined as 299 792 458 m/s (186,282 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.
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.
supernova (plural: supernovae)
(a)The death explosion of a massive star, resulting in a sharp increase in brightness 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).
(b) The explosion of a white dwarf which has accumulated enough material from a companion star to achieve a mass near the Chandrasekhar limit. These types of supernovae (called Type Ia) have approximately the
same intrinsic brightness, and can be used to determine distances.
Swift
Swift is a mid-sized NASA mission whose primary goal is to study
gamma-ray bursts and address the mysteries surrounding their nature,
origin, and causes. Swift was launched in 2004 November.
Tenma
The second Japanese X-ray astronomy satellite (1983-1975). Before
its launch, it was known as Astro-B.
ultraviolet
Electromagnetic radiation at wavelengths shorter than the violet end of visible light; the atmosphere of the Earth effectively blocks the transmission of most ultraviolet light.
Universe
Everything that exists, including the Earth, planets, stars, galaxies, and all that they contain; the entire cosmos.
visible
Electromagnetic radiation at wavelengths which the human eye can see. We perceive this radiation as colors ranging from red (longer wavelengths; ~ 7000 Angstroms) to violet (shorter wavelengths; ~4000 Angstroms.)
wavelength
The distance between adjacent peaks in a series of periodic waves.
Also see electromagnetic spectrum.
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!
WMAP (Wilkinson Microwave Anistropy Probe)
A NASA satellite designed to detect fluctuations in the cosmic microwave background. From its initial results published in Feb 2003, astronomers pinpointed the age of the universe, its geometry, and when the first stars appeared.
XMM-Newton
Formerly known as "X-ray Multi-Mirror Mission", XMM-Newton was launched
in Decenber 1999 by the European Space Agency. It is an X-ray astronomy
satellite with a large collecting area and a high sensitivity.
X-ray
Electromagnetic radiation of very short wavelength and very high-energy;
X-rays have shorter wavelengths than ultraviolet light but longer wavelengths
than gamma rays.
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