Absolute Brightness (Absolute
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.
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.
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.
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.
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.
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.
A galaxy possessing an active galactic nucleus at its
Advanced Camera For Surveys (ACS)
An optical camera aboard the Hubble Space Telescope that uses CCD detectors to make images. The camera covers twice the area, has twice the sharpness, and is up to 10 times more efficient than the telescope’s Wide Field and Planetary Camera 2. The ACS wavelength range spans from ultraviolet to near-infrared light. The camera’s sharp eye and broader viewing area allow astronomers to study the life cycles of galaxies in the remotest regions of the universe. Astronauts installed the camera aboard the telescope in March 2002, but the camera experienced an electrical short in 2007 that shut down all but one data channel. During Servicing Mission 4 in 2009, astronauts replaced the failed circuit boards and added a new power supply box to restore power to the camera.
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.
A mixture of two or more metals. Brass (a mixture of copper and zinc) and bronze (a mixture of copper and tin) are common alloys.
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.
A type of telescope mounting that supports the weight of the telescope and allows it to move in two directions to locate a specific target. One axis of support is vertical (called the altitude) and allows the telescope to move up and down. The other axis is horizontal (called the azimuth) and allows the telescope to swing in a circle parallel to the ground. This makes it easy to position the telescope: swing it around in a circle and then lift it to the target. However, tracking an object as the Earth turns is more complicated. The telescope needs to be adjusted in both directions while tracking, which requires a computer to control the telescope.
To make larger or more powerful; increase. Radio signals are amplified because they are very weak.
The size of a wave from the top of a wave crest to its
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.
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.
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.
An electrical device used to send or receive electromagnetic waves. The aerial (a long piece of metal attached to the front or rear fender) on a car is the antenna for the radio.
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
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.
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.
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.
An orderly arrangement or impressive display. For radio telescopes, an array is a group of individual radio dishes that work together. The VLA (Very Large Array) has 27 telescope dishes arranged in a “Y” pattern.
Association of Universities for Research in Astronomy,
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
A small solar system object composed mostly of rock. Many of these objects orbit the Sun between Mars and Jupiter. Their sizes range anywhere from 33 feet (10 meters) in diameter to less than 620 miles (1,000 kilometers). The largest known asteroid, Ceres, has a diameter of 579 miles (926 kilometers).
A region of space between Mars and Jupiter where the great
majority of asteroids is found.
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.
Astronomy is the study of the universe and the celestial bodies that reside in it, including their composition, history, location, and motion.
The layer of gases surrounding the surface of a planet,
moon, or star.
The blurring of an image due to the layer of gases surrounding the surface of Earth. As starlight travels through the atmosphere, pockets of air act like little lenses and bend the light in unpredictable ways. This distortion causes stars to appear to twinkle.
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.
The positively charged core of an atom consisting of protons
and (except for hydrogen) neutrons, and around which electrons orbit.
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.
An imaginary line through the center of an object. The
object rotates around this line.
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.
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.
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.
Batteries provide all the electrical power to support Hubble operations during the night portion of its orbit, when the telescope is in Earth’s shadow. The telescope’s orbit is approximately 97 minutes long. Roughly 61 minutes of Hubble’s orbit are in sunlight and 36 minutes are in Earth’s shadow. During Hubble’s sunlight or daytime period, the solar arrays provide power to the onboard electrical equipment. The solar arrays also charge the spacecraft’s batteries so they can power the spacecraft during the night portion of Hubble’s orbit. Hubble has six nickel-hydrogen batteries. These batteries, which had been onboard Hubble since the telescope was launched in 1990, were replaced during Servicing Mission 4.
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.
A broadly accepted theory for the origin and evolution
of our universe. The theory says that the observable universe started roughly
13.7 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.
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.
A massive, hot star that appears blue in color. Spica
in the constellation Virgo is an example of a blue star.
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
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
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.
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.