Imagine the Universe! Dictionary
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(Note - Greek letters are written out by name - alpha, beta etc.)
Accumulation of dust and gas onto larger bodies such as stars, planets and
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.
A unit of length equal to 0.00000001 centimeters. This may also be
written as 1 x 10-8 cm (see
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.
The point of greatest
separation between two stars which are in orbit around each other.
See binary stars. Opposite of
The point in its orbit where
a planet is farthest from the Sun. Opposite of
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
The point in its orbit where
an Earth satellite is farthest from the Earth. Opposite of
An angular measurement equal to 1/60th of a degree.
An angular measurement equal to 1/60th of an arc minute or 1/360th of a degree.
A UK X-ray mission, also known as UK-5
The Japanese Asuka spacecraft (formerly Astro-D), an X-ray mission
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
astronomical unit (AU)
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.
149,597,870 km; the average distance from the Earth to the Sun.
The scientific study of matter in outer space, especially the positions,
dimensions, distribution, motion, composition, energy, and evolution of
celestial bodies and phenomena.
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.
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.
Balmer lines (J. Balmer)
The Advanced X-ray Astrophysics Facility. AXAF was renamed Chandra
X-ray Observatory, CXO, and launched in July 1999.
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.
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 (Burst and Transient Source Experiment) was an instrument aboard the
Compton Gamma Ray Observatory that detected and
located gamma-ray bursts in the sky.
The Broad Band X-Ray Telescope, which was flown on the Astro-1 space
shuttle flight (Dec. 1990)
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 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.
A non-radiating ball of gas resulting from a white dwarf that has radiated all its energy.
black-hole dynamic laws; laws of black-hole
An object whose gravity is so strong that not even light can escape
- 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 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
where h is Planck's constant,
k is Boltzmann's constant,
and c is the the
speed of light.
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
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.
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
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 !
"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.
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
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.
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.
The Compton Gamma Ray Observatory
Chandra X-ray Observatory (CXO)
One of NASA's Great Observatories in Earth orbit, launched in July
1999, and named after S. Chandrasekhar.
It was previously named the Advanced X-ray Astrophysics
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.
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.
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.
NASA ultraviolet/X-ray mission, also known as OAO-3
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
A satellite launched in August 1975 to study extraterrestrial sources
of gamma-ray emission.
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.
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".
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.
An effect where light emitted from a distant source appears redshifted
because of the expansion of spacetime itself. Compare
The astrophysical study of the history, structure, and dynamics
of the universe.
The Chandra X-ray Observatory. CXO was launched by the Space Shuttle
in July 1999, and named for S. Chandrasekhar.
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.
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.
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.
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
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.
(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
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
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.
A stream of dust particles emitted from the nucleus of a comet. It is the most visible part of a comet.
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