accretion disk
A relatively flat sheet of gas and dust surrounding a newborn star, a black
hole, or any massive object growing in size by attracting material.
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.
angular momentum
A quantity obtained by multiplying the mass of an
orbiting body by its velocity and the radius of its orbit. According to
the conservation laws of physics, the angular momentum of any orbiting body
must remain constant at all points in the orbit, i.e., it cannot be
created or destroyed. If the orbit is elliptical the radius will vary.
Since the mass is constant, the velocity changes. Thus planets in elliptical
orbits travel faster at periastron and more slowly at apastron.
A spinning body also possesses spin angular momentum.
apoapsis
The point in an orbit when the two objects are farthest apart. Special names
are given to this orbital point for commonly used systems. For example, the
point of greatest separation of two stars, as in a binary star orbit, is called
apastron; the point in its orbit where a planet is farthest from the Sun is
called aphelion; the point in its orbit where an Earth satellite is farthest
from the Earth is called apogee.
Ariel V
A UK X-ray mission, also known as UK-5
Astro E
A new X-ray/gamma-ray mission being built jointly by the United States and
Japan. Astro E currently has an estimated launch date of the year 2000.
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 stars, stellar
systems, and interstellar material.
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
The Advanced X-ray Astrophysics Facility. AXAF is currently scheduled to be
launched by the Space Shuttle in August, 1998.
BBXRT
Broad Band X-Ray Telescope on Astro-1 shuttle flight (Dec. 1990)
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.
blueshift
An apparent shift toward shorter wavelengths of spectral lines in the radiation
emitted by an object caused by the emitting object moving toward the
observer. See also Doppler effect.
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 x 10-23 J/K (see
scientific notation).
Brahe, Tycho 1546 - 1601
(a.k.a Tyge Ottesen) Danish astronomer whose accurate astronomical
(132 k GIF)
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.
CGRO
The Compton Gamma Ray Observatory
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.
corona (plural: coronae)
The uppermost level of the solar atmosphere, 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.
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 aberration
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 redshift.
cosmological redshift
An effect where light emitted from a distant source appears redshifted
because of the expansion of spacetime itself. Compare
Doppler effect.
cosmology
The astrophysical study of the history, structure, and constituent dynamics
of the universe.
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
The amount of mass of any substance which can be contained in one
cubic centimeter. 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 (of planet or other object)
The apparent circular shape that the Sun, a planet, or a moon displays when seen
in the sky or through a telescope.
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
(32 k GIF)
ejecta
Material that is ejected. Used mostly to describe the content of a massive
star that is propelled outward in a supernova
explosion.
electromagnetic spectrum
The full range of frequencies, from radio waves to gamma-rays, that
characterizes light.
electron
A particle commonly found in the outer layers of atoms with a negative charge.
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.
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 80and 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.
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
Fermi acceleration
In order to explain the origins of cosmic rays, Enrico Fermi (1949)
introduced a mechanism of particle acceleration, whereby charged particles
bounce off moving interstellar magnetic fields and either gain or lose energy,
depending on whether the "magnetic mirror" is approaching or receding.
In a typical environment, he argued, the probability of a head-on collision is
greater than a head-tail collision, so particles would be accelerated on
average. This random process is now called 2nd order Fermi acceleration,
because the mean energy gain per "bounce" is dependent on the
"mirror" velocity squared.
Bell (1978) and Blandford and Ostriker (1978) independently showed that
Fermi acceleration by supernova remnant (SNR) shocks is particularly
efficient, because the motions are not random. A charged particle
ahead of the shock front can pass through the shock and then
be scattered by magnetic inhomogeneities behind the shock. The particle
gains energy from this "bounce" and flies back across the shock,
where it can be scattered by magnetic inhomogeneities ahead of the shock.
This enables the particle to bounce back and forth again and
again, gaining energy each time. This process is now called 1st order Fermi
acceleration, because the mean energy gain is dependent on the
shock velocity only to the first power.
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 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.
gravitational collapse
When a massive body collapses under its own weight. (For example, interstellar
clouds collapse to become stars until the onset of
nuclear fusion stops the collapse.)
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 Imaging Spectrometer (GRIS)
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.
gravity
A mutual physical force attracting two bodies.
GSFC
Goddard Space Flight Center
guest star
The ancient Chinese term for a star that newly appears in the night sky, and
then later disappears. Later, the Europeans called this a
nova.
Hawking temperature
The temperature of a black hole caused by the emission of Hawking
radiation.
HEAO
The High Energy Astrophysical Observatory satellite series
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
second.
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
Hubble constant; Ho (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.
implosion
A violent inward collapse. An inward explosion.
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 Loa in Hawaii).
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.
ions
An atom with one or more electrons stripped off, giving it a net positive
charge.
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.
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.
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, Joseph (1736 - 1813)
A French mathematician of the eighteenth century. His work Mecanique
Analytique (Analytical Mechanics; 1788) was a mathematical masterpiece. It
contained clear, symmetrical notation and covered almost every area of pure
mathematics. Lagrange developed the calculus of variations, established the
theory of differential equations, and provided many new solutions and theorems
in number theory. His classic Theorie des fonctions analytiques laid
some of the foundations of group theory. Lagrange also invented the method of
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 pseudo-forces work against
gravity to cancel it out.
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.
LHEA
Laboratory for High Energy Astrophysics (GSFC, Code 660)
light
Electromagnetic radiation that is visible to the human eye.
light curve
A graph that displays the time variation in light or magnitude of a variable
or eclipsing star.
limb
The outer edge of the apparent disk of a celestial body.
magnetic field
A condition found in the region around a magnet or an electric current,
characterized by the existence of a detectable magnetic force at every point in
the region.
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 the magnetic field of an object (e.g., a
star or planet) dominates the radiation pressure of the stellar wind to which
it is exposed.
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.
mega-ton
A unit of energy used to describe nuclear warheads. The same amount energy as
1 million tons of TNT.
1 mega-ton = 4 x 1016 ergs = 4 x 109 joules.
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.
microwave
Electromagnetic radiation which has a long wavelength (between 1 mm and
30 cm). Microwaves can be used to study the Universe, communicate with
satellites in Earth orbit, and cook popcorn.
neutrino
A fundamental particle 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
A particle commonly found in the nucleus of atoms with approximately the mass
of a proton, but zero charge.
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'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 (plural: novae)
A star that experiences a sudden outburst of radiant energy, temporarily
increasing its luminosity by hundreds to thousands of times before fading back
to its original luminosity.
nuclear fusion
A 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.
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
ultraviolet light, but is opaque to visual light.
orbit
The path of an object that is moving around a second object or point.
OSO 3
Orbiting Solar Observatory 3
parsec
A large distance often used in astronomy, it is equal to 3.26 light years, or
3.1 x 1018 cm (see scientific
notation). A kiloparsec (kpc) is equal to 1000 parsecs. A
megaparsec (Mpc) is equal to a million (106) parsecs.
An object is at a distance of 1 parsec from us if its parallax is 1 second of arc.
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)
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
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 x 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.
positron
The antiparticle to the electron. The positron has
most of the same characteristics as an electron except it is positively
charged.
proton
A particle commonly found in the nucleus of atoms with a positive charge.
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 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.
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.
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.
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
Rayleigh-Taylor instabilities
Rayleigh-Taylor instabilities occur when a heavy (more dense) fluid is
pushed against a light fluid -- like trying to balance water on top of air by
filling a glass 1/2 full and carefully turning it over. Rayleigh-Taylor
instabilities are important in many astronomical objects, because the two
fluids trade places by sticking "fingers" into each other. These
"fingers" can drag the magnetic field lines along with them, thus
both enhancing and aligning the magnetic field. This result is evident in the
example of a supernova remnant in the diagram below,
from Chevalier (1977):
red giant
A star that has low surface temperature and a diameter that is large
relative to the Sun.
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. See also Doppler effect.
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 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
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.
Roche lobe
The volume around a star in a binary system in which, if you were to
release a particle, it would fall back onto the surface of that star.
A particle released above the Roche lobe of either star will, in general,
occupy the `circumbinary' region that surrounds both stars. The point
at which the Roche lobes of the two stars touch is called the inner
Lagrangian or L1 point. If a star in a close
binary system evolves to the point at which it `fills' its Roche lobe,
theoretical calculations predict that material from this star will overflow
both onto the companion star (via the L1 point) and into the circumbinary
environment.
Röntgen, Wilhelm Conrad (1845 - 1923)
A German scientist who fortuitously discovered X-rays in 1895.
(43 k GIF).
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 x 104 means 1.23 times 10 to the fourth power or 12,300;
5.67 x 10-8 means 5.67 divided by 10 to the eighth power or
0.0000000567.
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. A nanosecond is equal to one-billionth
(10-9) of a second.
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.
shock wave
A strong compression wave where there is a sudden change in
gas velocity, density, pressure and temperature.
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.
solar flares
Violent eruptions of gas on the Sun's surface.
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, gravitational sources
bending light, 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 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.
A Diversive Spectrometer is like a prism. It scatters the X-rays 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
The study of spectral lines from different atoms and molecules.
Spectroscopy is an important part of studying the chemistry that goes on
in stars and in interstellar clouds.
spectrum (plural: spectra)
A plot of the intensity of light at different frequencies. Or
the distribution of wavelengths and frequencies.
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 x 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 (plural: supernovae)
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).
SXG
The Spectrum X-Gamma mission
synchrotron radiation
Electromagnetic radiation given off when very high energy electrons
encounter magnetic fields.
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.
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 x 10-11 newtons per square meter per kilogram
squared (see scientific notation).
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!
WWW
The World Wide Web -- a loose linkage of Internet sites which provide
data and other services from around the world.
XSELECT
A high-level tool to manage the FTOOLs
XTE
X-ray Timing Explorer, also known as the Rossi X-ray Timing Explorer (RXTE)