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Imagine the Universe! Dictionary |
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[A B C D
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I J K L
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(Note - Greek letters are written out by name - alpha, beta etc.)
accretion
Accumulation of dust and gas onto larger bodies such as stars, planets and
moons.
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)
A class of galaxies which spew massive amounts of energy from their
centers, far more than ordinary galaxies. Many astronomers believe
supermassive black holes may lie at the center of these galaxies and
power their explosive energy output.
angstrom
A unit of length equal to 0.00000001 centimeters. This may also be
written as 1 x 10-8 cm (see
scientific notation).
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 perihelion and more slowly at aphelion.
A spinning body also possesses spin angular momentum.
apastron
The point of greatest
separation between two stars which are in orbit around each other.
See binary stars. Opposite of
periastron.
aphelion
The point in its orbit where
a planet is farthest from the Sun. Opposite of
perihelion.
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: see
apastron, aphelion, and
apogee.
apogee
The point in its orbit where
an Earth satellite is farthest from the Earth. Opposite of
perigee.
Ariel V
A UK X-ray mission, also known as UK-5
Tell me more about Ariel V. (http://heasarc.gsfc.nasa.gov/docs/ariel5/ariel5.html)
ASCA
The Japanese Asuka spacecraft (formerly Astro-D), an X-ray mission
ASM
All Sky Monitor. An instrument designed to observe large areas of the
sky for interesting astronomical phenomena. An ASM measures the
intensity of many sources across the sky and looks for new sources.
Many high-energy satellites have carried ASM detectors,
including the ASM on Vela 5B,
Ariel V, and the Rossi X-ray Timing
Explorer.
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. A replacement mission,
Astro-E2, is planned for 2005.
Tell me more about Astro E. (http://heasarc.gsfc.nasa.gov/docs/astroe/astroe.html)
astronomical unit (AU)
149,597,870 km; the average distance from the Earth to the Sun.
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
The Advanced X-ray Astrophysics Facility. AXAF was renamed Chandra
X-ray Observatory, CXO, and launched in July 1999.
Tell me more about AXAF. (http://chandra.harvard.edu/about/axaf_mission.html)
Balmer lines (J. Balmer)
Emission or absorption lines in the spectrum of hydrogen that arise from
transitions between the second (or first excited) state and higher energy
states of the hydrogen atom. They were discovered by Swiss physicist
J. J. Balmer.
baryon
Any of the subatomic particles which interact via the strong nuclear force. Most commonly, these are protons and neutrons. Their presence in the universe is determined through their gravitational and electromagnetic interactions.
BATSE
BATSE (Burst and Transient Source Experiment) was an instrument aboard the
Compton Gamma Ray Observatory that detected and
located gamma-ray bursts in the sky.
BBXRT
The Broad Band X-Ray Telescope, which was flown on the Astro-1 space
shuttle flight (Dec. 1990)
Tell me more about BBXRT. (http://heasarc.gsfc.nasa.gov/docs/bbxrt/bbxrt.html)
Big Bang
A theory of cosmology in which the expansion of the universe is
presumed to have begun with a primeval explosion (referred to as
the "Big Bang").
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 star, producing X-rays in the process.
black dwarf
A non-radiating ball of gas resulting from a white dwarf that has radiated all its energy.
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
- First law of black hole dynamics:
For interactions between black holes and normal matter, the conservation
laws of mass-energy, electric charge, linear momentum, and angular
momentum, hold. This is analogous to the first law of thermodynamics.
- Second law of black hole dynamics:
With black-hole interactions, or interactions between black holes and
normal matter, the sum of the surface areas of all black holes involved can
never decrease. This is analogous to the second law of
thermodynamics, with the surface areas of the black holes being a
measure of the entropy of the system.
blackbody radiation
Blackbody radiation is produced by an object which is a perfect
absorber of heat. Perfect absorbers must also be perfect radiators.
For a blackbody at a temperature T, the intensity of radiation emitted
I at a particular energy E is given by Plank's law:
I(E,T) = 2
E3[h2c2(eE/kT -
1)]-1
where h is Planck's constant,
k is Boltzmann's constant,
and c is the the
speed of light.
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 motion between the object and the
observer which decreases the distance between them. See also Doppler effect.
bolometric luminosity
The total energy radiated by an object at all wavelengths, usually given
in joules per second (identical to watts).
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
observations of Mars in the last quarter of the 16th century formed
the basis for Johannes Kepler's laws of
planetary motion. Brahe lost his nose in a dual in 1566 with
Manderup Parsberg (a fellow student and nobleman) at Rostock over who
was the better mathematician. He died in 1601, not of a burst bladder
as legend suggests, but from high levels of mercury in his blood
(which he may have taken as medication after falling ill from the
infamous meal). Show
me a picture of Tycho Brahe !
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.
calibration
A process for translating the signals produced by a measuring instrument
(such as a telescope) into something that is scientifically useful. This
procedure removes most of the errors caused by environmental and instrumental
instabilities.
Cataclysmic Variable (CV)
Binary star systems with one white dwarf star and one normal star, in close orbit about each other. Material from the normal star falls onto the white dwarf, creating a burst of X-rays.
Cepheid Variable
A type of variable star which exhibits a regular pattern of changing
brightness as a function of time. The period of the pulsation pattern is
directly related to the star's intrinsic brightness. Thus, Cepheid variables
are a powerful tool for determining distances in modern astronomy.
CGRO
The Compton Gamma Ray Observatory
Tell me more about CGRO. (http://cossc.gsfc.nasa.gov/)
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.
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 likely it is to detect 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
Tellme more about the Copernicus mission. (http://heasarc.gsfc.nasa.gov/docs/copernicus/copernicus.html)
Copernicus, Nicolaus (1473 - 1543)
Polish astronomer who advanced the theory that the Earth and other
planets revolve around the Sun (the "heliocentric" theory). This was
highly controversial at the time, since the prevailing Ptolemaic model held that the Earth was the center of
the universe, and all objects, including the sun, circle it. The
Ptolemaic model had been widely accepted in Europe for 1000 years when
Copernicus proposed his model. (It should be noted, however,
that the heliocentric idea was first put forth by Aristarcus of Samos
in the 3rd century B.C., a fact known to Copernicus but long ignored
by others prior to him.). Show
me a picture of Nicholas Copernicus !
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).
COS-B
A satellite launched in August 1975 to study extraterrestrial sources
of gamma-ray emission.
Tell me more about COS-B. (http://heasarc.gsfc.nasa.gov/docs/cosb/cosb.html)
cosmic background radiation; primal glow
The background of radiation mostly in the frequency range
3 x 108 to 3 x 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 rays
Atomic nuclei (mostly protons) and electrons that are observed to strike the
Earth's atmosphere with exceedingly high energies.
cosmological constant; Lambda
A constant term (labeled Lambda) which Einstein added to his general
theory of relativity in the mistaken belief that the Universe was
neither expanding nor contracting. The cosmological constant was
found to be unnecessary once observations indicated the Universe was
expanding. Had Einstein believed what his equations were telling him,
he could have claimed the expansion of the Universe as perhaps the
greatest and most convincing prediction of general relativity; he
called this the "greatest blunder of my life".
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 dynamics
of the universe.
CXO
The Chandra X-ray Observatory. CXO was launched by the Space Shuttle
in July 1999, and named for S. Chandrasekhar.
Tell me more about CXO. (http://chandra.harvard.edu/about/axaf_mission.html)
Dark Matter
Name given to the amount of mass whose existence is deduced from
the analysis of galaxy rotation curves but which until now, has escaped all
detections. There are many theories on what dark matter could be. Not one, at
the moment is convincing enough and the question is still a mystery.
de Broglie wavelength (L. de Broglie; 1924)
The quantum mechanical "wavelength" associated with a particle, named
after the scientist who discovered it. In quantum mechanics, all
particles also have wave characteristics, where the wavelength of a
particle is inversely proportional to its momentum and the constant of
proportionality is the Planck constant.
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, and ranges from +90 degrees to -90 degrees.
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. For example, the optical analog would be to remove the
spikes and halos which often appear on images of bright stars because
of light scattered by the telescope's internal supports.
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 7.9gm/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 (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.
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.
dust tail
A stream of dust particles emitted from the nucleus of a comet. It is the most visible part of a comet.
eccentric
Non-circular; elliptical (applied to an orbit).
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 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).
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.
Show me a picture of Albert Einstein !
Einstein Observatory,
The first fully imaging x-ray telescope in space, launched by NASA in
1978. Originally named "HEAO-2" (High Energy Astrophysics Observatory
2), it was renamed for Albert Einstein upon launch. Also see
HEAO.
Tell memore about HEAO-2 (Einstein Observatory). (http://heasarc.gsfc.nasa.gov/docs/einstein/heao2.html)
ejecta
Material that is ejected. Used mostly to describe the content of a massive
star that is propelled outward in a supernova
explosion. Also used to describe the material that is blown radially
outward in a meteor impact on the surface of a planet or moon.
electromagnetic spectrum
The full range of frequencies, from radio waves to gamma rays, that
characterizes light.
electromagnetic waves (radiation)
Another term for light. Light waves are fluctuations of electric and
magnetic fields in space.
electron
A negatively charged particle commonly found in the outer layers of atoms.
The electron has only 0.0005 the mass of the proton.
electron volt
The change of potential energy experienced by an
electron moving from a place
where the potential has a value of V to a place where it has a value of
(V+1 volt). This is a convenient energy unit when dealing with the motions of
electrons and ions in electric fields; the unit is also the one used to
describe the energy of X-rays and gamma rays. A keV (or
kiloelectron volt) is equal to 1000 electron volts. An MeV is
equal to one million electron volts. A GeV is equal to one billion
(109) electron volts. A TeV is equal to a million million
(1012) electron volts.
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 Johannes Kepler 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 distance from a black hole within which nothing can escape. In
addition, 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
Tell me more about EXOSAT. (http://heasarc.gsfc.nasa.gov/docs/exosat/exosat.html)
extragalactic
Outside of, or beyond, our own galaxy.
Fast Fourier Transformation (FFT)
A Fourier Transform is the mathematical operation that takes measurements
made with a radio interferometer and transforms them into an image of the radio
sky. The Fast Fourier Transform is technique used by computer programs that
allows the Fourier Transform to be computed very quickly.
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.
flux
A measure of the amount of energy given off by an astronomical object over a fixed amount of time and area. Because the energy is measured per time and area, flux measurements make it easy for astronomers to compare the relative energy output of objects with very different sizes or ages.
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 NASA's Goddard Space Flight
Center for analyzing high-energy astronomy data.
FTP
File Transfer Protocol -- A widely available method for transferring
files over the Internet.
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.
galactic halo
A spherical region surrounding the center of a galaxy. This region may extend
beyond the luminous boundaries of the galaxy and contain a significant fraction
of the galaxy's mass. Compared to cosmological distances, objects in the halo
of our galaxy would be very nearby.
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.
Show me a picture of Galileo !
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 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. Recently, their distances were determined to be large, placing the origins of the
bursts in other galaxies.
Gamma-ray Large Area Space Telescope (GLAST)
An international mission planned for launch in 2006, GLAST will study
the universe in the energy range 10 keV - 300 Gev.
Tell me more about GLAST. (http://www-glast.sonoma.edu/)
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)
A balloon-borne instrument which uses germanium detectors for high
resolution gamma-ray spectroscopy.
Tell me more about GRIS. (http://lheawww.gsfc.nasa.gov/docs/balloon/New_GRIS_homepage/gris.html)
gas
One of the three states of matter, in which atoms, molecules, or ions move freely and are not bound to each other. In astronomy, it usually refers to hydrogen or helium.
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.
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.
Tell me more about Ginga. (http://heasarc.gsfc.nasa.gov/docs/ginga/ginga.html)
globular cluster
A spherically symmetric collection of stars which shared a common
origin. The cluster may contain up to millions of stars spanning
up to 50 parsecs.
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.)
gravitational radius
See event horizon.
gravitational waves
Ripples in space-time caused by the motion of objects in the universe. The most notable sources are orbiting neutron stars, merging black holes, and collapsing stars. Gravitational waves are also thought to eminate from the Big Bang.
gravitationally bound
Objects held in orbit about each other by their gravitational
attraction. For example, satellites in orbit around the earth are
gravitationally bound to Earth since they can't escape Earth's
gravity. By contrast, the Voyager spacecraft, which explored the
outer solar system, was launched with enough energy to escape Earth's
gravity altogether, and hence it is not gravitationally bound.
gravity
A mutual physical force attracting two bodies.
GSFC
Goddard Space Flight Center, one of the centers operated by NASA.
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 radiation (S.W. Hawking; 1973)
A theory first proposed by British physicist Stephen Hawking, that due
to a combination of properties of quantum mechanics and gravity, under
certain conditions black holes can seem to emit radiation.
Hawking temperature
The temperature inferred for a black hole based on the Hawking
radiation detected from it.
HEAO
The High Energy Astrophysical Observatory satellite series
HEASARC
High Energy Astrophysics Science Archive Research Center, located at
NASA's Goddard Space Flight Center. The HEASARC creates and maintains
archives of data from ultraviolet, x-ray and gamma-ray satellites
for use by astronomers around the world.
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 of our Sun is helium.
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, 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. Show me a picture of Edwin Hubble! (Image Credit: The Huntington Library, San Marino, California)
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. After many years in which the Hubble constant was only
known to be somewhere between 50 and 100 km/s/Mpc,
it has been determined to be 70 km/s/Mpc ± 7 km/s/Mpc by the Hubble
Space Telescope's Key Project team. (Advances in cosmology have shown
that since the Universe is self gravitating, Ho is not
truly constant. Astronomers thus seek its present value.)
Hubble's law (E.P. Hubble; 1925)
A relationship between a galaxy's distance from us and its velocity
through space. The farther away a galaxy is from us, the faster it is
receding from us. The constant of proportionality is the Hubble
constant, Ho, named after Edwin P. Hubble who discovered the relationship.
Hubble's Law is interpreted as evidence that the Universe is expanding.
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.
Show me a picture of Christian
Huygens !
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, but only a tiny fraction of the Earth.
IKI
The Space Research Institute in Russia. It is the equivalent of NASA
in the U.S.
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 Kea 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.
interstellar medium
The gas and dust between stars, which fills the plane of
the Galaxy much like air fills the world we live in. For centuries,
scientists believed that the space between the stars was empty. It wasn't
until the eighteenth century, when William
Herschel observed nebulous patches of sky through his telescope, that
serious consideration was given to the notion that interstellar space was
something to study. 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.
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, an ultraviolet
space observatory launch in 1978. Originally designed for a 3 year
mission, IUE exceeded all expectations and functioned for over 18
years, finally ceasing operation in September 1996.
Tell me more about IUE. (http://archive.stsci.edu/iue/)
jets
Beams of particles, usually coming from an active galactic
nucleus or a pulsar. Unlike a jet airplane, when
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. 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 ( = 0o C =
32o F) [ K = 273 + C = 273 + 5/9 * (F-32)]. The
Kelvin temperature scale is often used in sciences such as astronomy.
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.
Show me a picture of Johannes
Kepler !
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 line directed from the Sun to a planet sweeps out equal areas in
equal times as the planet orbits the Sun.
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)
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.
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.
L0
A representation of the luminosity of an
object in terms of Solar luminosity. The average luminosity of the
Sun is about 4x1033 erg/sec. Astronomers often express
units for other objects in terms of solar units, which makes the
resulting numbers smaller and easier to deal with.
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
solving differential equations known as variation of parameters.
Show me a picture of Joseph
Lagrange !
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, located at NASA's Goddard
Space Flight Center. The scientists, programmers and technicians
working here study the astrophysics of objects which emit cosmic ray,
x-ray and gamma-ray radiation.
light
The common term for electromagnetic radiation, usually
referring to that portion visible to the human eye. However, other
bands of the e-m spectrum are also often referred to as different
forms of light.
light curve
A graph showing how the radiation from an object varies over time.
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 x 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.
LISA (Laser Interferometer Space Antenna)
A NASA mission which will detect gravitational waves. LISA will consist of three satellites that use laser interferometry to monitor their positions relative to each other. Gravitational waves passing by the satellites cause small changes in the distances between the satellites.
luminosity
The rate at which a star or other object emits energy, usually in the
form of electromagnetic radiation.
M0
A representation of the mass of an object in terms of Solar mass.
The average mass of the Sun is about 2x1033 grams.
Astronomers often express units for other objects in terms of solar
units, since it makes the resulting numbers smaller and easier to deal with.
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.
magnetic pole
Either of two limited regions in a magnet at which the magnet's field is most
intense. The two regions have opposing polarities, which we label
"north" and "south", after the two poles on the Earth.
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
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.
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 1022 ergs = 4 x 1015 joules.
Messier, Charles (1730 - 1817)
The 18th century French astronomer who compiled a list of
approximately 100 fuzzy, diffuse looking objects which appeared at
fixed positions in the sky. Being a comet-hunter, Messier compiled
this list of objects which he knew were not comets. His list is now
well known to professional and amateur astronomers as containing
the brightest and most striking nebulae, star clusters, and galaxies
in the sky.
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.
metric system
See SI.
microquasar
Microquasars are stellar mass black holes, that
display characteristics of the supermassive black holes
found at the centers of some galaxies. For instance,
they have radio jets - something not every black hole has.
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.
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 from the explosion of a solar-like
type star.
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 with approximately the mass
of a proton, but zero charge, commonly found in the nucleus of atoms .
neutron star
The imploded core of a massive star produced by a
supernova explosion. (typical mass of 1.4 times the
mass of the Sun, radius of about 5 miles, density of a
neutron.) 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.
Show me a picture of Isaac Newton !
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 (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.
OAO 3
Orbiting Astronomical Observatory 3 - NASA ultraviolet/X-ray mission, also known as Copernicus.
Tell me more aboutOAO 3. (http://heasarc.gsfc.nasa.gov/docs/copernicus/copernicus.html)
occultation
The blockage of light by the intervention of another object; a planet can
occult (block) the light from a distant star.
opacity
A property of matter that prevents light from passing through it. 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 visible light.
orbit
The path of an object that is moving around a second object or point.
OSO 3
Orbiting Solar Observatory 3
Tell me more about OSO 3 (http://heasarc.gsfc.nasa.gov/docs/heasarc/missions/oso3.html)
OSO 8
Orbiting Solar Observatory 8
Tell me more about OSO 8 (http://heasarc.gsfc.nasa.gov/docs/oso8/oso8.html)
pair production
The physical process whereby a gamma-ray photon, usually through an interaction
with the electromagnetic field of a nucleus, produces an electron and an
anti-electron (positron). The original photon no longer
exists, its energy having gone to the two resulting particles. The inverse
process, pair annihilation, creates two gamma-ray photons from the mutual
destruction of an electron/positron pair.
parallax
The apparent motion of a relatively close object compared to a more
distant background as the location of the observer changes.
Astronomically, it is half the angle which a a star appears to move as
the earth moves from one side of the sun to the other.
parsec
The distance to an object which has a parallax
of one arc second. 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.
periapsis
The point in an orbit when two objects are closest together. Special
names are given to this point for commonly used systems: see periastron,
perihelion, and
perigee. The
opposite of apoapsis.
periastron
The point of closest approach of two stars, as in a binary star orbit.
Opposite of apastron.
perigee
The point in its orbit where an Earth satellite is closest to the
Earth. Opposite of apogee.
perihelion
The point in its orbit where a planet is closest to the Sun. Opposite
of aphelion.
photon
The smallest (quantum) unit of light/electromagnetic energy. Photons are generally regarded as particles with zero mass and no electric charge.
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:
E = h x nu
planetary nebula
A shell of gas ejected from stars like our Sun at the end of their lifetime. This gas continues to expand out from the remaining white dwarf.
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 with a positive charge commonly found in the nucleus of atoms.
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. Show me a picture of Ptolemy !
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.
PVO
Pioneer Venus Orbiter
Tell me more about PVO (http://heasarc.gsfc.nasa.gov/docs/heasarc/missions/pvo.html)
quasar
An enormously bright object at the edge of our universe which emits
massive amounts of energy. In an optical telescope, they appear
point-like, similar to stars, from which they derive their name
(quasar = quasi-stellar). Current theories hold that quasars are one
type of AGN.
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.
radial velocity
The speed at which an object is moving away or toward an observer. By
observing spectral lines, astronomers can determine how fast objects
are moving away from or toward us; however, these spectral lines cannot
be used to measure how fast the objects are moving across the sky.
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).
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
the location of objects which are near each other. 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
1.22 x lambda/d
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
Theories of motion developed by Albert
Einstein, for which he is justifiably famous. Relativity 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.
revolution
The movement of one celestial body which is in orbit around another. It is often measured as the "orbital period."
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 larger object would break up the smaller object.
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
environment around the binary system.
Röntgen, Wilhelm Conrad (1845 - 1923)
A German scientist who fortuitously discovered X-rays in 1895.
ROSAT
Röntgen Satellite
Tell me more about ROSAT (http://heasarc.gsfc.nasa.gov/docs/rosat/rosat.html)
rotation
The spin of a celestial body on its own axis. In high energy astronomy, this is often measured as the "spin period."
SAS-2
The second Small Astronomy Satellite: a NASA satellite launched November 1972
with a mission dedicated to gamma-ray astronomy.
Tell me more about SAS-2 (http://heasarc.gsfc.nasa.gov/docs/sas2/sas2.html)
SAS-3
The third Small Astronomy Satellite: a NASA satellite launched May 1975 to
determine the location of bright X-ray sources and search for X-ray
novae and other transient phenomena.
Tell me more about SAS-3 (http://heasarc.gsfc.nasa.gov/docs/sas3/sas3.html)
satellite
A body that revolves around a larger body. For example, the moon is a
satellite of the earth.
Schwarzschild black
hole
A black hole described by solutions to Einstein's equations of general relativity worked out by Karl
Schwarzschild in 1916. The solutions assume the black hole is not
rotating, and that the size of its event
horizon is determined solely by its mass.
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 half the
length of the major axis, which is the line segment passing through
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
rp =
a(1-e) and ra = a(1+e).
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
In astronomy, a term often used to refer to 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.
Mathematically, a singularity is a condition when equations do not
give a valid value, and can sometimes be avoided by using a different
coordinate system.
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, 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
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.
speed of light (in vacuum)
The speed at which electromagnetic radiation propagates in a vacuum; it is
defined as 299 792 458 m/s (186,212 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.
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).
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
Tell me more about SXG (http://hea-www.harvard.edu/SXG/sxg.shtml)
Swift
Swift is a NASA mid-sized mission whose primary goal is to study
gamma-ray bursts and address the mysteries surrounding their nature,
origin, and causes. Swift is expected to launch in 2003.
Tell me more about Swift (http://swift.sonoma.edu)
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.
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.
Tenma
The second Japanese X-ray mission, also known as Astro-B.
Tell me more about Tenma (http://heasarc.gsfc.nasa.gov/docs/tenma/tenma.html)
Thomson, William 1824 - 1907
Also known as Lord Kelvin, the British physicist who developed the Kelvin temperature scale
and who supervised the laying of a trans-Atlantic cable.
Show me a picture of Lord Kelvin!
time dilation
The increase in the time between two events as measured by an observer
who is outside of the reference frame in which the events take place.
The effect occurs in both special
and general relativity, and is quite
pronounced for speeds approaching the speed of light, and in regions
of high gravity.
Uhuru
NASA's first Small Astronomy Satellite, also known as SAS-1. Uhuru was
launched from Kenya on 12 December, 1970; The seventh anniversary of Kenya's
independence. The satellite was named "Uhuru" (Swahili for
"freedom") in honor of its launch date.
Tell me more about Uhuru (http://heasarc.gsfc.nasa.gov/docs/uhuru/uhuru.html)
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.
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 m3 / kg-sec2 (see scientific notation).
Universe
Everything that exists, including the Earth, planets, stars, galaxies, and all that they contain; the entire cosmos.
Vela 5B
US Atomic Energy Commission (now the Department of Energy) satellite with an
all-sky X-ray monitor
Tell me more about Vela 5B (http://heasarc.gsfc.nasa.gov/docs/vela5b/vela5b.html)
The Venera satellite series
The Venera satellites were a series of probes (fly-bys and landers) sent
by the Soviet Union to the planet Venus. Several Venera satellites
carried high-energy astrophysics detectors.
Tell me more about Venera 11 & 12 (http://heasarc.gsfc.nasa.gov/docs/heasarc/missions/venera1112.html)
Tell me more about Venera 13 & 14 (http://heasarc.gsfc.nasa.gov/docs/heasarc/missions/venera1314.html)
visible
Electromagnetic radiation at wavelengths which the human eye can see. We
perceive this radiation as colors ranging from red (longer wavelengths;
~ 700 nanometers) to violet (shorter wavelengths; ~400 nanometers.)
wave-particle duality
The principle of quantum mechanics which implies that light (and, indeed, all
other subatomic particles) sometimes act like a wave, and sometimes act like a
particle, depending on the experiment you are performing. For instance, low
frequency electromagnetic radiation tends to act more like a wave than a
particle; high frequency electromagnetic radiation tends to act more like a
particle than a wave.
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!
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.
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.
WWW
The World Wide Web -- a loose linkage of Internet sites which provide
data and other services from around the world.
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.
XSELECT
A software tools used by astrophysicists in conjunction with the FTOOLS software to analyze certain types of
astronomical data.
XTE
X-ray Timing Explorer, also known as the Rossi X-ray Timing Explorer (RXTE)
Tell me more about RXTE (http://heasarc.gsfc.nasa.gov/docs/xte/XTE.html)
Z
The ratio of the observed change in wavelength of light emitted by a
moving object to the rest wavelength of the emitted light. See Doppler Effect. This ratio is related to
the velocity of the object. In general, with v = velocity of the
object, c is the speed of light, lambda is the rest wavelength, and
delta-lambda is the observed change in the wavelength, z is given by
z = (delta-lambda)/lamda = (sqrt(1+v/c) / sqrt(1-v/c)) - 1.
If the velocity of the object is small compared to the speed of
light, then
z = (delta-lambda)/lamda = v/c
Objects at the furthest reaches of the known universe have values
of z = 5 or slightly greater.
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