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Absolute
magnitude is the brightness unique to a celestial
body, which is not affected by
distance. It is an apparent magnitude when you place a celestial body at the
distance of 10 parsec. Usually, two celestial bodies with same brightness seem
to be darker if the distance becomes far. So this absolute magnitude is used
when describing the real brightness of the celestial body.
The temperature at which substances possess no thermal energy, equal to -273.15°C, or -459.67°F.
A nebula that is seen as a dark cloud or silhouette as it absorbs light from behind.
Absorption
Line Spectrum
Accumulation of dust and gas into larger bodies.
Accretion
Disk
Active
Galactic Nuclei (AGN)
A "hyperactive" galaxy that emits more energy than sum of the individual stars within it. The following three galaxy types are all know as "active galaxies" because of the powerful, concentrated, non-thermal (not starlight) energy sources at their cores. These galaxies emit energy at wavelengths across the electromagnetic spectrum, from radio to X-ray. However, some are unusually bright at radio, infrared or X-ray wavelengths compared with our galaxy.
Active
Region
Reflectivity of an object; ratio of reflected light to incident light.
The ratio of the amount of light reflected by an object and the amount of incident light; a measure of the reflectivity or intrinsic brightness of an object (a white, perfectly reflecting surface would have an albedo of 1.0; a black perfectly absorbing surface would have an albedo of 0.0).
1: Material that is formed or introduced from somewhere other than the place it is presently found. 2: Fragmented rock thrown out of the crater during its formation that either falls back to partly fill the crater or blankets its outer flanks after the impact event.
The closest bright star to our solar system.
Alpha
Particle
Amplitude
A unit of length used by astronomers and physicists to measure the wavelength of light. One angstrom equals 0.0000000001 meter, about the size of an atom.
Angular
Diameter
Angular
Momentum
The destruction of a subatomic particle and its antimatter opposite when they meet.
An eclipse of the Sun in which the Moon does not completely mask the Sun's light, resulting in a ring of light around the Moon at the peak of the eclipse.
Antapex
(solar)
A gravitational field that repels, rather than attracts, matter and light rays.
The exact opposite of matter; when matter meets the tiny amount of antimatter in the universe, the two annihilate each other.
An atomic particle that has exactly the opposite properties of its counter-part.
The point that is directly on the opposite side of the planet.
A roman numeral indicates the quality of seeing according to the following scale: I – Perfect seeing, without a quiver. II – Slight undulations, with some moments of calm lasting several seconds. III – Moderate seeing, with larger air tremors. IV – Poor seeing, with constant troublesome undulations. V – Very poor seeing, scarcely allowing the making of a rough sketch.
Apastron
The diameter of an opening through which light passes in an optical instrument.
Apex
(solar)
The point in its orbit where a planet is farthest from the Sun.
Apoapsis
The point in orbit farthest from the Earth.
When observing a celestial body, the object seems to be darker than actual brightness if the distance between observation point and the object increases. Because of this, the luminosity observed from Earth is called apparent magnitude. But it is the brightness not considering absorption by atmosphere, so when observed from Earth, the absorption is adjusted.
Those points in an orbit at which a body is at either the greatest or the least distance from its center of attraction.
A unit used by astronomers to measure the size or separation of objects in the sky. One arc second is equal to 1/3600 degrees.
Having the form of a bow; curved; arc-shaped.
To place or set in order: arranged.
Array
(interferometer)
66.5 degrees north latitude, 23.5 degrees latitude south of the Earth's north pole. Above this latitude, the summer sun never sets, and the winter sun never rises.
Association
A small group of stars.
A small rocky object orbiting the sun, less than 1,000 kilometers in diameter.
Asteroid
Belt
Asteroids are assigned a serial number when they are discovered. It has no particular meaning except that asteroid N+1 was discovered after asteroid N.
An ancient astronomical instrument used to measure the position and movement of objects in the sky.
One who is skilled in astronomy or who makes observations of celestial phenomena.
A book of astronomical facts and figures that astronomers use to locate stars, planets, and asteroids, plus forecast eclipses, and lunar phases.
Earth's average revolution radius, other words, distance between Sun and Earth is 1 astronomic unit. Light takes 500 seconds and converted into km is about 149,597,870 million.
Astronomy
A branch of astronomy dealing with the physical and chemical constitution of the celestial bodies.
1: A gaseous mass enveloping a heavenly body (as a planet). 2: The whole mass of air surrounding the earth.
One of the basic units, consisting a matter. It is the smallest in the matter that reflects unique characteristic of that matter. Thus if you divide this even smaller, it loses its characteristic. It has a structure which electrons fly around atomic nucleus. Its size is one hundred millionth of one cm.
If you make something smaller and smaller, it becomes a molecule. An atom forms molecules and if you make an atom even smaller, it becomes electrons and atomic nucleus. The size compared to an atom is far smaller, but majority of the mass of atom is concentrated there. Because it consists of protons with plus charge electricity and neutrons with electrically neutral condition, overall electric charge is plus.
A glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun.
The Northern Lights caused by the interaction between the solar wind, the Earth's magnetic field and the upper atmosphere; a similar effect happens in the southern hemisphere where it is known as the aurora australis.
Autumnal
Equinox
1. The imaginary line through the center of a planet, star, or galaxy around which it rotates; also, a similar line through a telescope mounting. 2. A straight line about which a line, curve, plane figure, body, or any geometric figure rotates or may be supposed to rotate.
Azimuth
A faint radio signal emitted by the entire sky - the remnant of radiation from the Big Bang.
Balmer
Lines A distinct pattern of hydrogen
emission or absorption spectral lines that appear at a specific set of
wavelengths. This is one of hydrogen's spectral signatures. Since stars are
mostly composed of hydrogen, the Balmer series of spectral lines is one of the
most prominent features in the visible spectrum of a star. Some familiar Balmer
lines are:
These are the first four Balmer lines of the Balmer series.
Equals
0.987 atmosphere = 1.02 kg/cm2 = 100 kilopascal = 14.5 lbs/inch2.
Another name for dark nebulae which
obscure the light from background
stars. This name is used to honor an American
astronomer who discovered them in about 1900.
Barred
Spiral Galaxy
The local name for Mars in Edgar
Rice Burroughs' SF books.
Barycenter
A particle with spin 1/2 (or 3/2)
which consists of three elementary particles known as
quarks. Baryons are the
nucleons (protons and neutrons) which form atomic nuclei and also very short-lived
(<10-10 second) hyperons such as sigma particles.
A form of degenerate matter in which
electrons have been forced into the confines of the atomic nucleus by the
weight
of overlying material. Once inside the nucleus the electrons merge with the
protons to form neutrons. It is the neutrons, obeying Pauli's Exclusion
Principle, which provide the degeneracy pressure to halt further collapse.
The first star to be discovered
using infrared detection methods. This star is deeply embedded within the Orion
star forming nebula. It is invisible at optical
wavelengths because the light is
completely scattered or absorbed due to the high density of dusty material in
the Orion nebula. Investigation has shown that the Becklin-Neugebauer object has
a spectral classification of type B and is currently on the main sequence of
stellar evolution.
One type
of radiation decay of nucleus. In other words, the nucleus falls apart by
emitting radiation. This radiation is a 'beta line' which consists of fast
velocity electron and positron, and neutrino.
A high speed electron emitted in a
form of radioactive disintegration from an atomic nucleus which has a surfeit of
electric charge and energy. The mechanism involves the conversion of one type of
nucleon into another and the creation of an electron ( no
electrons can exist in the nucleus (the so-called Klein paradox )).
The beta particle is emitted at high speed at any one energy up to a maximum
which is unique to the radio-nuclide. Likewise the half life is characteristic.
Energy is conserved by the simultaneous emission of a neutrino with very little
(or no mass) that travels close to (or at the
speed of light). An example of the
equation of 'nuclear chemistry' that illustrates negative beta decay is the decay
of tritium with a 13 year half-life
1H3 -> 2He3 + -e +
antineutrino If the electron emitted is the
antimatter counterpart or positron then there must first be an excess of
internal energy in the nucleus greater than the rest mass energy of two
electrons (i.e. 2 m c2). An 'ordinary' matter neutrino shares energy
with the positive electron. For example: 8O15
+ (2 m c2) -> 7N15 + +e +
neutrino
A star which infrared observations showed to have a dusty disc of material around it. Although originally thought to be an accretion disc, from which planets form, the dimensions of the disc are far too large for that to be the answer. With a diameter of ten times that of Pluto's orbit, current ideas indicate that the disc may be a Kuiper belt.
A star in the constellation of Orion
which is a red giant. It has a spectral classification of M2. It is a
variable
star that changes its brightness between apparent magnitudes +0.1 and +1.2, that
is, about 8000 times the sun.
The Star of Bethlehem, has
significance in the Christian religion as auguring the birth of Jesus its
founder. The 'star' may have been a supernova explosion, with a characteristic
half life of about 60 days, as reported by Chinese astronomers. Or it could have
been the conjunction, over several months, of the
planets Saturn and Jupiter.
A theory which states that the
universe came into being in an instantaneous event between 15 and 20
billion
years ago. Everything contained within the universe was created in that initial
event and as time has gone by, the universe has expanded and the contents have
evolved into the stars and galaxies of today. The theory uses the known laws of
physics to describe everything that occurred in the universe following the
Planck time, which ended when the universe was 10-43 seconds old.
Following this space and time became distinct from matter and energy. Matter and
energy continued to be completely interchangeable until the universe was fifteen
seconds old. At this time, the elementary particles which make up atoms became
stable. Atoms could still not form, however, because collisions between photons
and sub-atomic particles prevented them binding together. The forces of
nature evolved into gravity, electromagnetism, the weak nuclear force and the
strong nuclear force. As the temperature fell, because the universe expanded,
atoms formed. Now that the sub-atomic particles were in orderly structures, the
obstructions they had caused to the photons were removed. This allowed the
photons to travel large distances through space without interacting with other
particles. In effect, space became transparent to them and caused the releases
of energy which are observed today as the cosmic background
radiation. This also
removed the radiation pressure from the matter and allowed it to be influenced
by the force of gravity. This, in turn, caused galaxies to form as matter
congregated together.
The theorized demise of a closed
universe. If the universe contains enough mass to halt the Hubble flow and
reverse its expansion, all the matter contained within it will be drawn together
by the force of its mutual gravitational attraction. This collapse will cause
the matter to be concentrated in an ever decreasing volume with densities and
temperatures reaching those which were reached in the big
bang. Some ambitious
ideas suggest that, since a big crunch would match the conditions of the big
bang, perhaps it would cause another phase of expansion and the universe would
begin again. This concept is known as the oscillating universe.
The North American name for the asterism, the Plough.
A very large number: 1,000,000,000.
A stellar system which is composed of two stars orbiting one another about their common center of mass. The two stars are held together by the force of their mutual gravity. Binary stars are twins in the sense that they formed together out of the same interstellar cloud. Since they may have formed with different masses to one another, however, they will evolve at different rates. As stars grow to become red giants, once nuclear fusion of helium begins in their cores, they can fill their Roche lobes. This means that matter can pass through the inner Lagrangian point and form an accretion disc around the companion star. This increases the mass of the companion and, thus, causes it to evolve faster. Roughly one half of the stars in the sky are binaries or multiple systems. In some cases, the stars are far enough apart so that they can be clearly seen as having two components. These are known as visual binaries. Other binary stars are too close together for the separation between them to be detected directly. A star, which displays a periodic 'wobble' in its passage through space, is being acted upon by a force of gravity. That gravity is caused by the mass of a companion star. So, although the star cannot be seen its presence can be inferred by the visible star's motion. These are known as astrometric binaries. If the motion caused by the companion star is too small to be detected by astrometry, spectroscopy can reveal its presence. The spectra of stars sometimes reveal the presence of two stars, either by containing incompatible spectral lines or displaying movement of the lines caused by the Doppler effect as the stars orbit one another. These are known as spectroscopic binaries (see Figure). Some binary stars appear to be variable stars. These are the eclipsing binaries and have an orbital plane which is inclined to the Earth so that the stars pass in front and behind of one another, causing eclipses which dim the light output. Another, more violent, variable, known as a nova, is also thought to be caused within binary star systems in which one member is a white dwarf.
The energy equivalent of the mass
defect of an atomic nucleus. The energy required to separate entities (a relatively
great distance). The energy to pull an outer electron from an atom is of the
order of a few electron volts (eV) and that needed to eject a proton or neutron
from most atomic nuclei is around 7 million electron volts (MeV). If the reverse
takes place, that is, a particle is captured the binding energy is released. The
energy in all cases, is either comes from, or turns into, mass according
to the energy-mass equivalence equation of Einstein.
A stellar wind which eminates, by
preference, the star's two polar regions rather than all other
latitudes.
Bipolar outflows often represent significant periods of mass loss in a star's
life. They tend to occur during the protostar and pre-main sequence
phase and,
again, during the red giant phase just before the production of a planetary
nebula. It is uncertain whether the bipolar flow is caused by a lack of material
being ejected at other stellar latitudes or whether something, such as an
accretion disc, blocks the material in the equatorial regions and only allows
that which is ejected at the poles to escape. It has also been suggested that
magnetic fields may constrain the out flowing material. The outflows carve out
cavities in the surrounding interstellar medium and cause
reflection nebulae to
be produced.
A perfect absorber and radiator of
energy. Its spectrum is totally dependent on the
temperature of the black body;
cold black bodies appear red, hot black bodies appear blue, or white. Stars
actually mimic blackbodies closely...cool stars are red while hot stars are blue
or white.
The radiation emitted after a perfect absorber reaches a temperature which is higher than that of its surroundings. Black (or full) body radiation covers the entire electromagnetic spectrum with the peak emission taking place at a wavelength which is dependent only on the emitting body's temperature. A perfect absorber does not reflect any electromagnetic radiation which falls upon it. All radiation is absorbed and converted into internal energy, or heat, as it is commonly known. It then re-radiates that heat energy as black body radiation. Perfect absorbers are often referred to as black bodies since anything black will tend to absorb radiation rather than reflect it. A close approximation is a very small hole into a constant temperature cavity, such as the human eye. The radiation emitted from stars approximates to that of a black body. This is why the color of a star, which corresponds to its peak emission, can be used to determine its effective temperature.
Blackbody Temperature
Black Dwarf
Stellar core remnants which are so dense that not even photons of electromagnetic radiation can escape from them. The overwhelming property of a black hole is its mass. During the big bang, so called primordial black holes are theorized to have been formed. These contain very little mass, perhaps only a few kilograms. At the other end of the scale, active galactic nuclei (AGN) are thought to be powered by supermassive black holes. These have masses which can be thousands or even millions of times greater than that of the Sun. A black hole is a region of space which outside observers can never know anything about, since information cannot escape its gravitational field. The spherical boundary at which information is lost forever is known as the event horizon. At the center of this region is the singularity. This is the point at which matter is compressed to an infinitely dense state and possibly lost from our universe altogether. In a rotating black hole, a region surrounding the event horizon is known as the ergosphere. In this region, the space-time continuum, and everything on it, is dragged around by the rotation of the black hole. The boundary of the ergosphere is called the stationary limit. As matter spirals down towards a black hole, it is theorized to form an accretion disc. The gases in these discs are so hot that they emit electromagnetic radiation in the form of X rays. It is these x rays which allow astronomers to detect black holes. The primary formation scenario for black holes of stellar mass is in supernovae explosions. If a collapsar contains more mass than the Oppenheimer-Volkoff limit, it will overcome the baryon degeneracy pressure and become a black hole.
A type of active galaxy that is angled in such a way to us that we look almost directly at its accretion disk and jet.
The first discovered blazar. BL
Lacertae was originally classified as a variable star in 1941 but was eventually
found to be much more complex when, in 1968, it was discovered to be a strong
radio source. A class of active galaxy is now named after this object, they are
known as the BL Lacertae objects. They are thought to be the superbright nucleus
of an elliptical
galaxy. These objects contain no spectral lines in their
spectra. This is theorized to indicate that there are no gas clouds surrounding
the AGN in BL Lacertae objects. Thus, no atomic absorption or atomic emission
can take place and so no spectral lines are visible. The electromagnetic
radiation from the visible part of the spectrum is highly polarized and so is
thought to be synchrotron emission from
electrons spiraling around magnetic
field lines.
A displacement of absorption-line patterns toward the blue end of the spectrum. Such a shift caused by the speed of an object relative to an observer is an example of the Doppler effect. As a star travels toward an observer, the wavelength of the star light decreases. The observer sees the star as "bluer" than the same star at rest.
Any star in the upper left section of the Hertzsprung-Russell diagram. Blue stars have spectral classifications of O and B. Supergiant stars are many tens of times the mass of the Sun. They typically have absolute magnitudes of between -5 and -10.
Spherical dark nebulae which are
believed to be regions of interstellar clouds, undergoing gravitational
collapse. They appear as round clouds which are silhouetted by background stars
or emission
nebulae. They consist predominantly of gas, although they do have a
dust component. The temperature of these globules is usually about 10 - 15 Kelvin, which is sufficiently low for the kinetic energy of the gas
particles not to resist the collapse. As the globule collapses, it breaks up and
then the fragments, in turn, collapse into protostars. Bok globules vary in the
mass they contain from a fraction of a solar mass to several hundred solar
masses. They are named after Bart Bok, a Dutch astronomer who first
observed the less massive variety earlier this century.
An exploding meteorite. Bolometric
Luminosity Boltzmann
Constant (k) A very few light short lived
nuclei,
such as 6He, exhibit a halo of two neutrons
'orbiting' at around 1
fermi beyond the densely packed nucleus. Mathematically this is a
quantum
mechanical three-body Borromean system.
Particles which do not obey the Pauli exclusion principle, for example, photons. Thus there is no limit to the number that can occupy the same region at the same time yet have the same quantum numbers; so, for example, there is no limit to the brightness of light or the loudness of sound. They have integer (intrinsic) spin quantum number. Mesons have spin 0; and photons 1; these are termed carrier bosons because they are associated with the interaction between nucleons and charged particles, respectively. These bosons (here called virtual) can, naively, be looked upon as jumping forward and back binding the particles. If there is a carrier boson (a graviton) associated with the gravitational force between masses it is predicted to have spin 2.
The outermost part of a planetary magnetosphere; the place where the supersonic flow of the solar wind is slowed to subsonic speed by the planetary magnetic field.
Bremsstrahlung (or Braking) Radiation When a light fast moving charged
particle (for example a positive or negative beta
particle), passes within the
strong electric field of an atomic nucleus or other heavy charged
particle it will be decelerated. It can thus be the origin of electromagnetic
radiation in the form of a photon that is created and whose
quantum of energy is
equal that lost by the particle. The particle continues with less energy and on
a deflected path.
Brightness is a measure of the amount of energy that reaches Earth from a source, such as a star, galaxy, or planet. Brightness also depends on the distance between Earth and the source. For instance, at the orbit of Pluto, the Sun appears as an extremely bright star in an otherwise dark sky. At Alpha Centauri, a nearby star system 4 light years away, the Sun appears about as bright as the star Vega in our night sky.
Brown
Dwarf An instrument which can record the passage of high energy particles because of the ionization they cause to the atoms or molecules of, for example, liquid hydrogen or helium, contained in the bubble chamber. As the high energy particles ionize the atoms they strike, energy is released which causes local boiling of the superheated liquid. This produces bubbles which show the path that the particle has taken. The tracks of bubbles are photographed and analyzed. Their characteristics can reveal which particles created them, often together with their mass, momentum and charge. |