| Term |
Definition |
| |
|
| Aberration |
failure of a mirror, refracting surface, or lens to produce exact point-to-point correspondence between
an object and its image |
| Absolute Brightness (Absolute Magnitude) |
a measure of the true brightness of an object. The absolute brightness or magnitude of an object is
the apparent brightness or magnitude it would have if it were located exactly 32.6 light-years (10 parsecs) away. For example,
the apparent brightness of our Sun is much greater than that of the star Rigel in the constellation Orion because it is so
close to us. However, if both objects were placed at the same distance from us, Rigel would appear much brighter than our
Sun because its absolute brightness is much larger |
| Absolute Zero |
the coldest possible temperature, at which all molecular motion stops. On the Kelvin temperature scale,
this temperature is the zero point (0 K), which is equivalent to –273° C and –460° F |
| Absorption |
the process by which light transfers its energy to matter. For example, a gas cloud can absorb starlight
that passes through it. After the starlight passes through the cloud, dark lines called absorption lines appear in the star’s
continuous spectrum at wavelengths corresponding to the light-absorbing elements |
| Absorption Line |
a dark line in a continuous spectrum caused by absorption of light. Each chemical element emits and
absorbs radiated energy at specific wavelengths, making it possible to identify the elements present in the atmosphere of
a star or other celestial body by analyzing which absorption lines are present |
| Accelerating Universe |
a model for the universe in which a repulsive force counteracts the attractive force of gravity, driving
all the matter in the universe apart at speeds that increase with time. Recent observations of distant supernova explosions
suggest that we may live in an accelerating universe |
| Accretion Disk |
a relatively flat, rapidly rotating disk of gas surrounding a black hole, a newborn star, or any massive
object that attracts and swallows matter. Accretion disks around stars are expected to contain dust particles and may show
evidence of active planet formation. Beta Pictoris is an example of a star known to have an accretion disk |
| Active Galactic Nucleus (AGN) |
a very bright, compact region found at the center of certain galaxies. The brightness of an active
galactic nucleus is thought to come from an accretion disk around a supermassive black hole. The black hole devours matter
from the accretion disk, and this infall of matter provides the firepower for quasars, the most luminous type of active galactic
nucleus |
| Active Galaxy |
a galaxy possessing an active galactic nucleus at its center |
| Advanced Camera For Surveys (ACS) |
an optical camera aboard the Hubble Space Telescope that covers twice the area, has twice the sharpness,
and is up to ten times more efficient than the telescope’s Wide Field and Planetary Camera 2. The camera’s wavelength
range spans from ultraviolet to near-infrared light. The camera’s sharp eye and broader viewing area enable astronomers
to study the life cycles of galaxies in the remotest regions of the cosmos. Astronauts installed the camera aboard the telescope
in March 2002 |
| Afterglow |
the fading fireball of a gamma-ray burst — a sudden burst of gamma rays from deep space —
that is observable in less energetic wavelengths, such as X-ray, optical, and radio. After an initial explosion, an expanding
gamma-ray burst slows and sweeps up surrounding material, generating the afterglow, which is visible for several weeks or
months. The afterglow is usually extremely faint, making it difficult to locate and study |
| Alpha Process |
a process by which lighter elements capture helium nuclei (alpha particles) to form heavier elements.
For example, when a carbon nucleus captures an alpha particle, a heavier oxygen nucleus is formed |
| Altaz |
Altitude and Azimuth |
| Altitude |
measurement of an object above the horizon in degrees, minutes and seconds with zero being on the
horizon, and 90 degrees being straight overhead |
| Amplitude |
the size of a wave from the top of a wave crest to its midpoint |
| Angular Momentum |
a property that an object, such as a planet revolving around the Sun, possesses by virtue of its rotation
or circular motion. An object’s angular momentum cannot change unless some force acts to speed up or slow down its circular
motion. This principle, known as conservation of angular momentum, is why an object can indefinitely maintain a circular motion
around an axis of revolution or rotation |
| Angular Resolution |
the ability of an instrument, such as a telescope, to distinguish objects that are very close to each
other. The angular resolution of an instrument is the smallest angular separation at which the instrument can observe two
neighboring objects as two separate objects. The angular resolution of the human eye is about a minute of arc. As car headlights
approach from a far-off point, they appear as a single light until the separation between the lights increases to a point
where they can be resolved as two separate lights |
| Angular Size |
the apparent size of an object as seen by an observer; expressed in units of degrees (of arc), arc
minutes, or arc seconds. The moon, as viewed from the Earth, has an angular diameter of one-half a degree |
| Antimatter |
matter made up of elementary particles whose masses are identical to their normal-matter counterparts
but whose other properties, such as electric charge, are reversed. The positron is the antimatter counterpart of an electron,
with a positive charge instead of a negative charge. When an antimatter particle collides with its normal-matter counterpart,
both particles are annihilated and energy is released |
| Apparent Brightness (Apparent Magnitude) |
a measure of the brightness of a celestial object as it appears from Earth. The Sun is the brightest
object in Earth's sky and has the greatest apparent magnitude, with the moon second. Apparent brightness does not take into
account how far away the object is from Earth |
| Arc Minute |
one arc minute is 1/60 of a degree of arc. The angular diameter of the full moon or the Sun as seen
from Earth is about 30 arc minutes |
| Arc Second |
one arc second is 1/60 of an arc minute and 1/3600 of an arc degree. The apparent size of a dime about
3.7 kilometers (2.3 miles) away would be an arc second. The angular diameter of Jupiter varies from about 30 to 50 arc seconds,
depending on its distance from Earth |
| Association of Universities for Research in Astronomy, Inc. (AURA) |
a consortium of educational and other non-profit institutions that operates world-class astronomical
observatories. Members include five international affiliates and 29 U.S. institutions, including the Space Telescope Science
Institute in Baltimore, Maryland, the science operations center for NASA’s Hubble Space Telescope |
Asterism
|
named pattern of stars that is not part of official constellations |
| Asteroid |
a small, rocky object revolving around the Sun, sometimes called a minor planet or planetoid. The
vast majority of asteroids is found in the asteroid belt, a region between the orbits of Mars and Jupiter. The largest known
asteroid, Ceres, has a diameter of 926 kilometers (579 miles) |
| Asteroid Belt |
a region of space between Mars and Jupiter where the great majority of asteroids is found |
| Astronomer |
a scientist who studies the universe and the celestial bodies residing in it, including their composition,
history, location, and motion. Many of the scientists at the Space Telescope Science Institute are astronomers. Astronomers
from all over the world use the Hubble Space Telescope |
| Astronomical Unit (AU) |
the average distance between the Earth and the Sun, which is about 150 million kilometers (93 million
miles). This unit of length is commonly used for measuring the distances between objects within the solar system |
| Atmosphere |
the layer of gases surrounding the surface of a planet, moon, or star |
| Atom |
the smallest unit of matter that possesses chemical properties. All atoms have the same basic structure:
a nucleus containing positively charged protons with an equal number of negatively charged electrons orbiting around it. In
addition to protons, most nuclei contain neutral neutrons whose mass is similar to that of protons. Each atom corresponds
to a unique chemical element determined by the number of protons in its nucleus |
| Atomic Nucleus |
the positively charged core of an atom consisting of protons and (except for hydrogen) neutrons, and
around which electrons orbit |
| Aurora |
a phenomenon produced when the solar wind (made up of energized electrons and protons) disturbs the
atoms and molecules in a planet’s upper atmosphere. Some of the energy produced by these disturbances is converted into
colorful visible light, which shimmers and dances. Auroras have been seen on several planets in our solar system. On Earth,
auroras are also known as the “Northern Lights” (aurora borealis) or “Southern Lights” (aurora australis),
depending on in which polar region they appear |
| Azimuth |
east west measurement of an object measured in degrees, minutes and seconds, with zero being North,
90 degrees being East, 180 degrees being South, 270 degrees being West, and 360 degrees being North |
| Backlash |
the apparent pause of a telescope drives in altitude or azimuth when the opposite tracking direction
of the current direction is requested |
| Barlow Lens |
an eyepiece accessory which magnifies the object being viewed by usually 2 or 3 times the power of
the eyepiece in use |
| Barred Spiral Galaxy |
a galaxy with a “bar” of stars and interstellar matter, such as dust and gas, slicing
across its center. The Milky Way is thought to be a barred spiral galaxy |
| Baseline |
The distance between two or more telescopes that are working together as a single instrument to observe
celestial objects. The wider the baseline, the greater the resolving power |
| BATSE (Burst and Transient Source Experiment) |
a high-energy astrophysics “experiment” used to investigate gamma-ray bursts (GRBs). BATSE
consisted of eight detectors that were mounted on the corners of NASA’s Earth-orbiting Compton Gamma-Ray Observatory,
whose mission ended in 2000 |
| BeppoSAX |
a space-based X-ray observatory built and operated by the Italian Space Agency and the Netherlands
Agency for Aerospace Programs. BeppoSAX has been instrumental in identifying and locating gamma-ray bursts |
| Big Bang |
a broadly accepted theory for the origin and evolution of our universe. The theory says that the observable
universe started roughly 15 billion years ago from an extremely dense and incredibly hot initial state |
| Binary Star System |
a system of two stars orbiting around a common center of mass that are bound together by their mutual
gravitational attraction |
| Black Hole |
a region of space containing a huge amount of mass compacted into an extremely small volume. A black
hole’s gravitational influence is so strong that nothing, not even light, can escape its grasp. Swirling disks of material
— called accretion disks — may surround black holes, and jets of matter may arise from their vicinity |
| Blue Star |
a massive, hot star that appears blue in color. Spica in the constellation Virgo is an example of
a blue star |
| Blueshift |
the shortening of a light wave from an object moving toward an observer. For example, when a star
is traveling toward Earth, its light appears bluer |
| Bolide |
large, brilliant meteors that enter the Earth’s atmosphere. Friction between a fast-moving meteor
and Earth’s air molecules generates tremendous heat, which causes the meteor to heat up, glow, and perhaps disintegrate.
In some cases, the meteor literally explodes, leaving a visible cloud that dissipates slowly |
| Brown Dwarf |
an object too small to be an ordinary star because it cannot produce enough energy by fusion in its
core to compensate for the radiative energy it loses from its surface. A brown dwarf has a mass less than 0.08 times that
of the Sun |
| Bulge |
the spherical structure at the center of a spiral galaxy that is made up primarily of old stars, gas,
and dust. The Milky Way’s bulge is roughly 15,000 light-years across |
| Collimation |
precise alignment of the objective lens in a refractor, or mirrors in a reflector, to centre the focal
point of the objective lens or mirror in the eyepiece |
| Coma |
the aberration of a newtonian reflector giving stars a "v"-shaped "flare" |
| Comet |
small body made of ice and dust orbiting the sun |
| Crater |
depression on surface of planet/asteroid/moon from impact of falling body, volcano eruption or collapse |
| Dark matter |
unknown substance in space that is detected by its gravitational effect on celestial bodies |
| Dec |
Declination - north south measurement in hours, minutes and seconds |
| Doppler effect |
process by which light or sound is altered in perceived frequency by motion of its source with respect
to the observer |
| Double star |
2 stars that look close to each other that may be associated or unrelated and at different distances |
| Ecliptic |
apparent annual path of the sun among the stars; the intersection of the plane of the earth's orbit
with the celestial sphere. |
| Equatorial Mount |
telescope mount that is adjusted to the celestial North Pole for tracking objects and finding objects
using R/A and Dec setting circles |
| Eyepiece |
the part of the telescope you look through which is interchangeable |
| FOV |
field of view (the section of sky in degrees that you can see with a given eyepiece/lens configuration |
| Galaxy |
huge system of many many stars sometimes with vast amounts of gas and dust |
| J2000 |
position of all objects as at 00:00 UT 1st January 2000 |
| JNow |
position of all objects Now |
| Meteor |
flash of light caused by fall of a meteoroid through Earth’s atmosphere |
| Meteorite |
meteoroid that has landed on Earth |
| Meteoroid |
rock in space made of stone and/or metal |
| Nebula |
cloud of gas and dust in space that may emit, reflect and/or absorb light |
| Neutron star |
very small star (tens of miles across) but with great mass |
| Occultation |
process by which one celestial body passes in front of another |
| Orbit |
path followed by celestial body/spacecraft |
| Penumbra |
partial shadow caused by an eclipse |
| Planet |
large, round object formed from a flattened cloud around a star, doesn't’t generate energy by
nuclear reactions |
| Planetary nebula |
glowing, expanding gas cloud expelled by dying star |
| Polar Alignment |
precise alignment of an equatorial mount's R/A axis in relation to the celestial North Pole which
allows tracking by moving just the R/A axis |
| Power |
magnification of an eyepiece calculated by dividing the telescope focal length by the eyepiece |
| Pulsar |
fast spinning, tiny, immensely dense object that emits light, radio and/or x rays |
| Quasar |
small, bright object at the centre of a distant galaxy thought to represent emission of energy from
a black hole |
| R/A |
Right Ascension - east west measurement in degrees, minutes and seconds |
| Red giant |
large, very bright star with low surface temperature, late stage of a sun like star |
| Red shift |
increase in wavelength of light or sound, often due to the Doppler effect |
| Rotation |
spinning of an object around its axis |
| Seeing |
quality of the atmospheric conditions in relation to viewing through a telescope |
| Star |
large hot mass of gas held together by its own gravity and fuelled by nuclear reactions |
| Star cluster |
group of stars held together by gravity |
| Supernova |
immense explosion that disrupts a star and may cause a neutron star or black hole |
| Terminator |
line separating illuminated/un illuminated parts of a body that shines by reflected light from the
Sun |
| Tracking |
following an object in synchronisation with the earth's rotation |
| Transit |
movement of a smaller object in front of a larger object |
| Umbra |
area of total darkness in the shadow of a total eclipse |
| Variable star |
star that changes in brightness |
| White Dwarf |
small, dense object shining from stored heat and fading away, final stage of a sun like star |
| Zenith |
point of sky directly above the observer |
