 |
High-Energy Astrophysics Dictionary |
|
|
High-Energy Astrophysics Dictionary |
Please allow the whole page to load before you start searching for an entry. Otherwise, errors will occur.
AAS
American Astronomical Society
active galactic nuclei (AGN)
It is believed that these are normal galaxies with a massive black hole
accreting gas at its center, thus producing enormous amounts of energy at all
wavelengths of the electromagnetic spectrum.
angstrom
A unit of length equal to 0.00000001 centimeters. Scientists sometimes
write this as 1 × 10-8 cm (see
scientific notation).
anonymous FTP
A widely used convention for allowing users electronic access to
public data without requiring passwords or prior registration.
apastron
The point of greatest separation of two stars, as in a binary star orbit.
aphelion
The point in its orbit where a planet is farthest from the Sun.
apogee
The point in its orbit where an Earth satellite is farthest from the Earth.
Ariel V
A UK X-ray mission, also known as UK-5
ASCA
The Japanese Asuka spacecraft (formerly ASTRO-D)
ASM
All Sky Monitor. Many high-energy satellites have carried ASM
detectors, including the ASM on Vela 5B, Ariel V, and the Rossi
X-ray Timing Explorer.
astronomical unit (AU)
149,597,870 km; the average distance from the Earth to the Sun.
astrochemistry
The study of the chemical interactions between the gases and dust
interspersed between the stars.
atomic gas
Gas that is composed of individual atoms (such as hydrogen or carbon) that
are not bound to each other as molecules. Atomic gas may be ionized or mixed
with molecular gas.
Balmer series (J. Balmer; 1885)
An equation which describes the emission spectrum of hydrogen when an
electron is jumping to the second orbital. Four of the lines are in the
visible spectrum; the remainder are in the ultraviolet.
BBXRT
Broad Band X-Ray Telescope on ASTRO-1 shuttle flight (Dec. 1990)
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. Matter is stripped
from the normal star and falls onto the collapsed star, producing X-rays.
black hole
An object whose gravity is so strong that not even light can escape
from it.
black-hole dynamic laws; laws of black-hole dynamics
blackbody radiation
The radiation -- the radiance at particular frequencies all across the
spectrum -- produced by a blackbody -- that is, a perfect radiator (and
absorber) of heat. Physicists had difficulty explaining it until Planck
introduced his quantum of action.
blackbody temperature
The temperature of an object if it is re-radiating all the thermal energy
that has been added to it; if an object is not a blackbody radiator, it will not
re-radiate all the excess heat and the leftover will go toward increasing
its temperature.
Boltzmann constant; k (L. Boltzmann)
A constant which describes the relationship between temperature and kinetic
energy for molecules in an ideal gas. It is equal to
1.380622 × 10-23 J/K (see
scientific notation).
Brahe, Tycho 1546-1601
(a.k.a Tyge Ottesen) Danish astronomer whose accurate astronomical
observations formed the basis for Johannes Kepler's laws of planetary
motion.
Browse
The HEASARC's user interface/data browser
Cepheid
A pulsating variable star. This type of star undergoes a rhythmic
pulsation as indicated by its regular pattern of changing
brightness as a function of time. The period of pulsation has been
demonstrated to be directly related to a Cepheid's intrinsic brightness
making observations of these stars one of the most powerful tools for
determining distance known to modern day astronomy.
CGRO
The Compton Gamma Ray Observatory
Chandrasekhar limit (S. Chandrasekhar; 1930)
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.
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.
collecting area
The amount of area a telescope has that is capable of collecting
electromagnetic radiation. Collecting area is important for a telescope's
sensitivity: the more radiation it can collect (that is, the larger its
collecting area), the more sensitive it is to dim objects.
Compton effect (A.H. Compton; 1923)
An effect that demonstrates that photons (the quantum of electromagnetic
radiation) have momentum. A photon fired at a stationary particle, such as
an electron, will impart momentum to the electron and, since its energy has
been decreased, will experience a corresponding decrease in frequency.
Copernicus
NASA ultraviolet/X-ray mission, also known as OAO-3
Copernicus, Nicolaus 1473-1543
Polish astronomer who advanced the heliocentric theory that the Earth and
other planets revolve around the Sun. This was highly controversial at the time
as the Ptolemaic view of the universe, which was the prevailing theory for over
1000 years, was deeply ingrained in the prevailing philosophy and religion. (It
should be noted, however, that the heliocentric idea was first put forth by
Aristarcus of Samos in the 3rd century BC, a fact known to Copernicus but
long ignored.)
corona
The uppermost level of the solar atmosphere, characterized by low densities
and high temperatures (> 1,000,000 degrees K).
cosmic background radiation; primal glow
The background of radiation mostly in the frequency range
3 × 108 to 3 × 1011 Hz
(see scientific notation) discovered in space
in 1965. It is believed to be the cosmologically redshifted radiation released
by the Big Bang itself.
cosmic ray
Atomic nuclei (mostly protons) that are observed to strike the Earth's
atmosphere with exceedingly high energies.
cosmological constant; Lambda
The constant introduced to the Einstein field equation, intended to admit
static cosmological solutions. At the time the current philosophical view
was the steady-state model of the Universe, where the Universe has been
around for infinite time. Early analysis of the field equation indicated that
general relativity allowed dynamic cosmological models only (ones that are
either contracting or expanding), but no static models. Einstein introduced
the most natural abberation to the field equation that he could think of: the
addition of a term proportional to the spacetime metric tensor, g, with the
constant of proportionality being the cosmological constant:
cosmological distance
A distance far beyond the boundaries of our Galaxy. When viewing objects at
cosmological distances, the curved nature of spacetime could become
apparent. Possible cosmological effects include time dilation and red shift.
cosmological redshift
An effect where light emitted from a distant source appears redshifted
because of the expansion of spacetime itself. Compare
Doppler effect.
declination
A coordinate which, along with right ascension, may be used to locate any
position in the sky. Declination is analogous to latitude for locating
positions on the Earth.
deconvolution
An image processing technique that removes features in an image that are
caused by the telescope itself rather than from actual light coming from
the sky.
density
Measured in grams per cubic centimeter (or kilograms per liter); the
density of water is 1.0; iron is 7.9; lead is 11.3.
disk
The visible surface of the Sun (or any heavenly body) projected against the
sky.
Doppler effect (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. Compare cosmological redshift.
eccentricity
A value that defines the shape of an ellipse or planetary orbit. The
eccentricity of an ellipse (planetary orbit) is the ratio of the distance
between the foci and the major axis. Equivalently the eccentricity is
(ra-rp)/(ra+rp) where ra is the apoapsis distance and rp is the periapsis
distance.
eclipse
The cutting off, or blocking, of light from one celestial body by another.
ecliptic
The plane of Earth's orbit about the Sun
Eddington limit (Sir A. Eddington)
The theoretical limit at which the photon pressure would exceed the
gravitational attraction of a light-emitting body. That is, a body emitting
radiation at greater than the Eddington limit would break up from its own
photon pressure.
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 waves, that
characterizes light.
electromagnetic waves (radiation)
Another term for light. Light waves are fluctuations of electric and
magnetic fields in space.
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. A
keV (or kiloelectron volt) is equal to 1000 electron volts. An MeV is equal
to one million electron volts.
ellipse
Oval. That the orbits of the planets are ellipses, not circles, was first
discovered by Johannes Kepler based on the careful observations by Tycho
Brahe.
erg/sec
A form of the metric unit for power. It is equal to 10-10
kilowatts (see scientific notation).
event horizon
The radius that a spherical mass must be compressed to in order to
transform it into a black hole, or the radius at which time and space switch
responsibilities. Once inside the event horizon, it is fundamentally
impossible to escape to the outside. Furthermore, nothing can prevent a
particle from hitting the singularity in a very short amount of proper time
once it has entered the horizon. In this sense, the event horizon is
a "point of no return". See
Schwarzschild radius.
evolved star
A star near the end of its lifetime when most of its fuel has been used up.
This period of the star's life is characterized by loss of mass from its
surface in the form of a stellar wind.
EXOSAT
European Space Agency's X-ray Observatory
extragalactic
Outside of, or beyond, our own galaxy.
Fermi accelerations
The resultant accelerations of a particle which undergoes a
multi-collisional process. Based on a model developed by Enrico Fermi, it is
found that a particle which has a head-on collision is accelerated (the
"first-order" process) and that a particle decelerates from a
receding collision (the "second-order" process).
FITS
The Flexible Image Transport System format -- the IAU standard for
astronomical data.
FITSIO
A portable suite of subroutines developed to provide convenient access
to FITS files.
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.
FTOOLs
A suite of software tools developed at the OGIP for general and
mission-specific manipulation of FITS files.
FTP
File Transfer Protocol -- A widely available method for transferring
files over the Internet.
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.
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 his own version of 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.
Gamma-Ray Imaging Platform (GRIP)
A balloon-borne gamma-ray telescope made by a group at the California Institute
of Technology. It has had many successful flights.
Gamma-Ray Burst (GRB)
Plural is GRBs. A burst of gamma-rays from space lasting from a fraction of a
second to many minutes. There is no clear scientific consensus as to their
cause or even their distance.
Giant Molecular Cloud (GMC)
Massive clouds of gas in interstellar space composed primarily of hydrogen
molecules (two hydrogen atoms bound together), though also containing other
molecules observable by radio telescopes. These clouds can contain enough
mass to make several million stars like our Sun and are often the sites of
star formation.
Ginga
The third Japanese X-ray mission, also known as ASTRO-C.
globular cluster
A spherically symmetric star cluster, containing over 100,000 individual stars,
which are in a roughly spherical distribution about the main hub of a
galaxy. They form more or less a spherical halo around the main body of the
galaxy, like bees around a hive.
Grand Unified Theory (GUT)
A single theory of physics which will unite the 4 known forces of
nature -- gravity, electromagnetism, weak-interaction, and strong nuclear
forces. The pursuit of such a theory has been the focus of much effort in
the 20th century. However, a successful GUT has yet to be achieved.
gravitational radius
See event horizon.
gravity
A mutual physical force attracting two bodies.
GSFC
Goddard Space Flight Center
Hawking temperature
The temperature of a black hole caused by the emission of Hawking
radiation.
HEAO
High Energy Astrophysical Observatory
HEASARC
High Energy Astrophysics Science Archive Research Center
Herschel, Sir William (1738-1822)
Sir William Herschel was a renowned astronomer who first
detected the infrared region of the electromagnetic spectrum in 1800.
Hertz, Heinrich (1857-1894)
A German physics professor who did the first
experiments with generating and receiving electromagnetic waves, in
particular radio waves. In his honor, the units associated with measuring
the cycles per second of the waves (or the number of times the tip-tops of
the waves pass a fixed point in space in 1 second of time) is called the
hertz.
hertz; Hz (after H. Hertz, 1857-1894)
The derived SI unit of frequency, defined as a frequency of 1 cycle per s.
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 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.
Hubble constant; H_0 (E.P. Hubble; 1925)
The constant which determines the relationship between the distance to a
galaxy and its velocity of recession due to the expansion of the Universe.
Since the Universe is self-gravitating, it is not truly constant. In
cosmology, it is defined as
Hubble's law (E.P. Hubble; 1925)
A relationship discovered between distance and radial velocity. The
further away a galaxy is from us, the faster it is receding from us.
The constant of proportionality is the Hubble constant, H_0. The cause is
interpreted as the expansion of spacetime itself.
Huygens, Christiaan (1629-1695)
A Dutch physicist who was the leading proponent of the wave theory of light.
He also made important contributions to mechanics, stating that in a collision
between bodies, neither loses nor gains ``motion'' (his term for momentum).
In astronomy, he discovered Titan (Saturn's largest moon) and was the first to
correctly identify the observed elongation of Saturn as the presence of
Saturn's rings.
IDL
Interpretive Data Language: a proprietary data analysis system of Research
Systems International
IKI
Space Research Institute (Russia)
inertia
The property of matter that requires a force to act on it to change the way
it is moving; momentum is a measure of inertia.
inclination
the inclination of a planet's orbit is the angle between the plane of its
orbit and the ecliptic; the inclination of a moon's orbit is the angle between
the plane of its orbit and the plane of its primary's equator.
image
In astronomy, a picture of the sky.
interferometer
A type of telescope in which signals from two or more small telescopes are
combined to produce an image with the resolution of a much larger telescope. The
larger the separation between the individual telescopes, the higher the
resolution of the resulting image.
interstellar medium
The gas and dust that exists in the space between the stars
ionic (or ionized) gas
Gas whose atoms have lost or gained electrons, causing them to be
electrically charged. In astronomy, this term is most often used to
describe the gas around hot stars where the high temperature
causes atoms to lose electrons.
IRAF
Image Reduction and Analysis Facility -- A large astronomical
analysis system developed at Kitt Peak National Observatory (KPNO).
IUE
International Ultraviolet Explorer
Kepler, Johannes 1571-1630
German astronomer and mathematician. Considered a founder of modern
astronomy, he formulated the famous three laws of planetary motion. They
comprise a quantitative formulation of Copernicus's theory that the
planets revolve around the Sun.
Kepler's laws (J. Kepler)
Kepler's first law
A planet orbits the Sun in an ellipse with the Sun at one focus.
Kepler's second law
A ray directed from the Sun to a planet sweeps out equal areas in
equal times.
Kepler's third law
The square of the period of a planet's orbit is proportional
to the cube of that planet's semimajor axis; the constant of proportionality
is the same for all planets.
kilogram (kg)
One kilogram is equivalent to 1,000 grams or 2.2 pounds; the mass of a
liter of water. The fundamental SI unit of mass, it 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.
kilometer (km)
One kilometer is equivalent to 1,000 meters or 0.62 miles.
kiloparsec
A distance equal to 1000 parsecs.
kinematics
Refers to the calculation or description of the underlying mechanics of
motion of an astronomical object. For example, in radioastronomy,
spectral line graphs are used to determine the kinematics or relative
motions of material at the center of a galaxy or surrounding a star
as it is born.
Kirchhoff's law of radiation (G.R. Kirchhoff)
The emissivity of a body is equal to its absorbance at the same temperature.
Kirchhoff's laws (G.R. Kirchhoff)
Kirchhoff's first law
An incandescent solid or gas under high pressure will produce a continuous
spectrum.
Kirchhoff's second law
A low-density gas will radiate an emission-line spectrum with an underlying
emission continuum.
Kirchhoff's third law
Continuous radiation viewed through a low-density gas will produce an
absorption-line spectrum.
Lagrange points
Points in the vicinity of two massive bodies (such as the Earth and the
Moon) where each others' respective gravities balance. There are five,
labeled L1 through L5. L1, L2, and L3 lie along the centerline between
the centers of mass between the two masses; L1 is on the inward side of
the secondary, L2 is on the outward side of the secondary; and L3 is on
the outward side of the primary. L4 and L5, the so-called Trojan points,
lie along the orbit of the secondary around the primary, sixty degrees
ahead and behind of the secondary.
L1 through L3 are points of unstable equilibrium; any disturbance
will move a test particle there out of the Lagrange point. L4 and L5
are points of stable equilibrium, provided that the mass of the
secondary is less than about 1/25.96 the mass of the primary. These
points are stable because centrifugal pseudoforces work against
gravity to cancel it out.
LANL
Los Alamos National Laboratory
large scale structure
The largest spatial features in an image.
LHEA
Laboratory for High Energy Astrophysics (GSFC, Code 660)
light
Electromagnetic radiation that is visible to the human eye.
light year
A unit of length 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 × 1012 km,
5,880,000,000,000 miles or 63,240 AU (see
scientific notation).
limb
The outer edge of the apparent disk of a celestial body.
Lyman series
The series which describes the emission spectrum of hydrogen when electrons
are jumping to the ground state. All of the lines are in the ultraviolet.
magnetic monopole
A hypothetical particle which constitutes sources and sinks of the magnetic
field. Magnetic monopoles have never been found, but would only cause fairly
minor modifications to Maxwell's equations. They also seem to be predicted by
some grand-unified theories. If magnetic monopoles do exist, they do not seem
to be very common in our Universe.
magnetic pole
Either of two limited regions in a magnet at which the magnet's field is most
intense.
magnetosphere
the region of space in which a planet's magnetic field dominates that of
the solar wind.
magnetotail
the portion of a planetary magnetosphere which is pushed in the direction
of the solar wind.
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
visible star has magnitude 6, with the scale rule such that a decrease of
one unit represents an increase in apparent brightness by a factor of
2.512; also called apparent magnitude.
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.
megabits per second
A unit for measuring how fast data can be sent through a computer network
equal to one million bits per second.
metal
Used by astrophysicists to refer to all elements except hydrogen and
helium, as in: "the universe is composed of hydrogen, helium and
traces of metals".
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.
millibar
1/1000 of a bar. Standard sea-level pressure is about 1013 millibars.
molecular gas
Gas that is composed of atoms that are bound to each other as molecules. The
most abundant molecule in space is molecular hydrogen (two hydrogen atoms
bound to each other) followed by carbon monoxide (CO, or a carbon and oxygen
atom bound together). Molecular gas may be mixed with atomic gas.
momentum
A measure of the state of motion of a body; mathematically, it is equal to
the product of its mass and velocity.
Mosaic
A non-proprietary software tool using hypertext links to navigate
and retrieve data from the Internet -- developed by the National Center
for Supercomputing Activities.
neutrino
A fundamental particle supposedly produced in massive numbers by the
nuclear reactions in stars; they are very hard to detect because the
vast majority of them pass completely through the Earth without interacting.
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; N (after Sir I. Newton, 1642-1727)
The derived SI unit of force, defined as the force required to give a mass
of 1 kg an acceleration of 1 m/s/s.
Newton, Isaac 1642-1727
English cleric and scientist; discovered the classical laws of motion and
gravity; the bit with the apple is probably apocryphal.
Newton's law of universal gravitation (Sir I. Newton)
Two bodies attract each other with equal and opposite forces; the magnitude
of this force is proportional to the product of the two masses and is also
proportional to the inverse square of the distance between the centers of mass
of the two bodies.
Newton's laws of motion (Sir I. Newton)
Newton's first law of motion
A body continues in its state of constant velocity (which may be zero)
unless it is acted upon by an external force.
Newton's second law of motion
For an unbalanced force acting on a body, the acceleration produced is
proportional to the force impressed; the constant of proportionality is the
inertial mass of the body.
Newton's third law of motion
In a system where no external forces are present, every action force is
always opposed by an equal and opposite reaction
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
A start 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.
NRA
NASA Research Announcement
NSSDC
National Space Science Data Center (GSFC, Code 633)
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.
occultation
The blockage of light by the intervention of another object; a planet can
occult (block) the light from a distant star.
OGIP
Office of Guest Investigator Programs (GSFC, Code 668)
opacity
A property of matter that prevents light from passing through it;
non-transparent. The opacity or opaqueness of something depends on the
frequency of the light. For instance, the atmosphere of Venus is transparent to
ultra-violet light, but is opaque to visual light.
orbit
The path of an object that is moving around a second object or point.
OSO 8
Orbiting Solar Observatory 8
periapsis
The point in the orbit closest to the planet.
periastron
The point of closest approach of two stars, as in a binary star orbit.
perigee
The point in the orbit closest to the Earth.
perihelion
The point in its orbit where a planet is closest to the Sun. when referring
to objects orbiting the Earth the term perigee is used; the term periapsis is
used for orbits around other bodies. (opposite of aphelion)
perturb
To cause a planet or satellite to deviate from a theoretically regular
orbital motion.
photoelectric effect
An effect explained by A. Einstein which demonstrates that light seems to be
made up of particles, or photons. Light can excite electrons (called
photoelectrons in this context) to be ejected from a metal. Light with a
frequency below a certain threshold, at any intensity, will not cause any
photoelectrons to be emitted from the metal. Above that frequency,
photoelectrons are emitted in proportion to the intensity of incident
light.
The reason is that a photon has energy in proportion to its
wavelength, and the constant of proportionality is the Planck
constant. Below a certain frequency -- and thus below a certain
energy -- the incident photons do not have enough energy to knock the
photoelectrons out of the metal. Above that threshold energy, called
the work function, photons will knock the photoelectrons out of the
metal, in proportion to the number of photons (the intensity of the
light). At higher frequencies and energies, the photoelectrons ejected
obtain a kinetic energy corresponding to the difference between the
photon's energy and the work function.
PI
Principal Investigator
pi
The constant equal to the ratio of the circumference of a circle to its
diameter, which is approximately 3.141593.
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 × 10-34 J s (see
scientific notation).
Planck equation
The quantum mechanical equation relating the energy of a photon E to its
frequency nu:
planetary nebula
A shell of gas ejected from, and expanding about, a certain kind of extremely
hot star.
plasma
A low-density gas in which the individual atoms are ionized (and therefore
charged), even though the total number of positive and negative charges is
equal, maintaining an overall electrical neutrality.
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.
polarization
A special property of light; light has three properties, brightness, color
and polarization. Polarization is a condition in which the planes of
vibration of the various rays in a light beam are at least partially
aligned.
PROS
ROSAT (and X-ray) analysis system developed within IRAF
protostar
Very dense regions (or cores) of molecular clouds where stars are in the
process of forming.
Ptolemy (ca. 100-ca. 170)
A.k.a.Claudius Ptolemaeus. Ptolemy believed the planets and Sun to orbit the
Earth in the order Mercury, Venus, Sun, Mars, Jupiter, Saturn.
This system became known as the Ptolemaic system and predicted the
positions of the planets accurately enough for naked-eye
observations (although it made some ridiculous predictions, such as that
the distance to the moon should vary by a factor of two over its
orbit). He authored a book called Mathematical Syntaxis (widely
known as the Almagest). The Almagest included a star catalog containing
48 constellations, using the names we still use today.
pulsar
A rotating neutron star whose light appears to pulse.
These pulses are the result of beams of radiation sweeping through the
direction to the earth, much like a lighthouse beacon. Pulsars
were discovered by observations at radio wavelengths but have since been
observed at optical, x-ray, and gamma-ray energies.
Pulsars are rapidly rotating highly magnetized neutron stars. Highly energetic electrons spiraling in the magnetic field emit radio beams along the magnetic axis. The pulses are detected when the radio beam
quasi-stellar source (QSS)
Sometimes also called quasi-stellar object (QSO); A stellar-appearing object of
very large redshift that is a strong source of radio waves; presumed to be
extragalactic and highly luminous.
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.
radiation
Energy radiated in the form of waves or particles; photons.
radiation belt
Regions of charged particles in a magnetosphere.
Rayleigh criterion; resolving power
A criterion for how finely a set of optics may be able to distinguish. It
begins with the assumption that the central ring of one image should fall on
the first dark ring of another image; for an objective lens with diameter d and
employing light with a wavelength lambda (usually taken to be 560 nm), the
resolving power is approximately given by
red giant
A star that has low surface temperature and a diameter that is large
relative to the Sun.
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.
relativity principle
The principle, employed by Einstein's relativity theories, that the laws of
physics are the same, at least qualitatively, in all frames. That is, there
is no frame that is better (or qualitatively any different) from any other.
This principle, along with the constancy principle, constitute the founding
principles of special relativity.
relativity, Theory of
More accurately describes the motions of bodies in strong gravitational
fields or at near the speed of light than Newtonian mechanics. 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.
resonance
A relationship in which the orbital period of one body is related to that
of another by a simple integer fraction, such as 1/2, 2/3, 3/5.
retrograde
The rotation or orbital motion of an object in a clockwise direction when
viewed from the north pole of the ecliptic; moving in the opposite
sense from the great majority of solar system bodies.
Right Ascension
A coordinate which, along with declination, may be used to locate any
position in the sky. Right ascension is analogous to longitude for locating
positions on the Earth.
Ritter, Johann Wilhelm (1776-1810)
Ritter is credited with discovering and
investigating the ultraviolet region of the electromagnetic spectrum.
Roche limit
The smallest distance from a planet or other body at which purely
gravitational forces can hold together a satellite or secondary body of the
same mean density as the primary; at less than this distance the tidal forces
of the primary would break up the secondary.
Röntgen, Wilhelm Conrad (1845-1923)
A German scientist who fortuitously discovered X-rays in 1895.
ROSAT
Röntgen Satellite
Schwarzschild radius
The radius r of the event horizon for a Schwarzschild black hole.
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, usually between 1 and 10, and a power of ten. Thus
1.23 × 104 means 1.23 times 10 to the fourth power or 12,300;
5.67 × 10-8 means 5.67 divided by 10 to the eighth power or
0.0000000567.
second; s
The fundamental SI unit of time, defined as the period of time equal to the
duration of 9,192,631,770 periods of the radiation corresponding to the
transition between two hyperfine levels of the ground state of the
cesium-133 atom.
semimajor axis
The semimajor axis of an ellipse (e.g. a planetary orbit) is 1/2 the length
of the major axis which is a segment of a line passing thru the foci of the
ellipse with endpoints on the ellipse itself. The semimajor axis of a planetary
orbit is also the average distance from the planet to its primary. The
periapsis and apoapsis distances can be calculated from the semimajor
axis and the eccentricity by
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.
sidereal
of, relating to, or concerned with the stars. Sidereal rotation is that
measured with respect to the stars rather than with respect to the Sun or the
primary of a satellite.
singularity
The center of a black hole, where the curvature of spacetime is maximal. At
the singularity, the gravitational tides diverge; no solid object can even
theoretically survive hitting the singularity. Although singularities
generally predict inconsistencies in theory, singularities within black
holes do not necessarily imply that general relativity is incomplete so long as
singularities are always surrounded by event horizons.
A proper formulation of quantum gravity may well avoid the classical
singularity at the centers of black holes.
small scale structure
The smallest spatial features in an image.
solar flares
Violent eruptions of gas on the Sun's surface.
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 its 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.
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. Or
the distribution of wavelengths and frequencies.
speed of light (in vacuo); 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.
SQL
Structured Query Language -- The ANSI standard database language
star
A large ball of gas that creates and emits its own radiation.
star cluster
A bunch of stars (ranging in number from a few to hundreds of thousands) which
are bound to each other by their mutual gravitational attraction.
Stefan-Boltzmann constant; sigma (Stefan, L. Boltzmann)
The constant of proportionality present in the Stefan-Boltzmann law. It is
equal to 5.6697 × 10-8 Watts per square meter per degree Kelvin
to the fourth power (see scientific
notation).
Stefan-Boltzmann law (Stefan, L. Boltzmann)
The radiated power P (rate of emission of electromagnetic energy) of a hot
body is proportional to the radiating surface area, A, and the fourth power
of the thermodynamic temperature, T. The constant of proportionality is the
Stefan-Boltzmann constant.
stellar classification
Stars are given a designation consisting of a letter and a number according
to the nature of their spectral lines which corresponds roughly to surface
temperature. The classes are: O, B, A, F, G, K, and M; O stars are the
hottest; M the coolest. The numbers are simply subdivisions of the major
classes. The classes are oddly sequenced because they were assigned long
ago before we understood their relationship to temperature. O and B stars
are rare but very bright; M stars are numerous but dim. The Sun is
designated G2.
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.
steradian; sr
The supplementary SI unit of solid angle defined as the solid central angle
of a sphere that encloses a surface on the sphere equal to the square of the
sphere's radius.
supernova
The death explosion of a massive star, resulting in a sharp increase in
brightness followed by a gradual fading. At peak light output, supernova
explosions 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).
sunspots
Cooler (and thus darker) regions on the sun where the magnetic field loops
up out of the solar surface.
SXG
The Spectrum X-Gamma mission
synchronous rotation
Said of a satellite if the period of its rotation about its axis is the
same as the period of its orbit around its primary. This implies that the
satellite always keeps the same hemisphere facing its primary (e.g. the
Moon). It also implies that one hemisphere (the leading hemisphere) always
faces in the direction of the satellite's motion while the other (trailing)
one always faces backward.
Systéme Internationale d'Unités (SI)
The coherent and rationalized system of units, derived from
the MKS system (which itself is derived from the metric system),
in common use in physics today. The fundamental SI unit of length is the
meter, of time is the second, and of mass is the kilogram.
time dilation
Stretching of time produced by relativity. Time dilation is a predicted effect
of the cosmological paradigm.
ToFU
A suite of software tools developed at the OGIP to transform non-FITS
information into FITS.
21-cm Line
The spectral line given off by atomic hydrogen with a wavelength of 21 cm (
or frequency of 1.4 Gigahertz). Since hydrogen is the most abundant atom in
the universe, the 21-cm line of hydrogen is an extremely useful tool for radio
astronomers.
universal constant of gravitation; G
The constant of proportionality in Newton's law of universal gravitation
and which plays an analogous role in A. Einstein's general relativity. It is
equal to 6.664 × 10-11 newtons per square meter per kilogram
squared (see scientific notation).
Vela 5B
US Atomic Energy Commission (now the Department of Energy) satellite with an
all-sky X-ray monitor
waveband
Portions of the electromagnetic spectrum. Examples of different wavebands
include the infrared, visual, and radio wavebands.
wavelength
A property of a wave that gives the length between two peaks of the wave.
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!
Wien's displacement law
For a blackbody, the product of the wavelength corresponding to the maximum
radiancy and the thermodynamic temperature is a constant. As a result, as
the temperature rises, the maximum of the radiant energy shifts toward the
shorter wavelength (higher frequency and energy) end of the spectrum.
WORM
Write-Once-Read-Many. A CD-ROM is a typical WORM medium: the CD-ROM is written
once when it is etched and it cannot be written to again. However, it can
be read any number of times.
WWW
The World Wide Web -- a loose linkage of Internet sites which provide
data and other services from around the world.
XIMAGE
Image analysis program in XANADU
X-ray
Electromagnetic radiation of very short wavelength and very high-energy;
X-rays have shorter wavelengths than ultraviolet light but longer wavelengths
than cosmic rays.
XRONOS
Temporal analysis program in XANADU
XSELECT
A high-level tool to manage the FTOOLs
XSPEC
X-ray/gamma-ray spectral analysis package in XANADU
XTE
X-ray Timing Explorer, also known as the Rossi X-ray Timing Explorer (RXTE)
SkyView has been developed with generous support from the NASA AISR and ADP programs (P.I. Thomas A. McGlynn) under the auspices of the High Energy Astrophysics Science Archive Research Center (HEASARC) at the NASA/ GSFC Astrophysics Science Division.
We gratefully acknowledge the support of NASA and contributors of SkyView surveys.
| ||||