NRAO Radio Astronomy GlossaryA B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Absolute magnitude: The apparent magnitude that a star would possess it if were placed at a distance of 10 parsecs from Earth. In this way, absolute magnitude provides a direct comparison of the brightness of stars.
Accretion Disk: A disk of gas that accumulates around a center of gravitational attraction, such as a white dwarf, a neutron star, or a black hole. As the gas spirals in, it becomes hot and emits energy at a variety of wavelengths, including X-ray and radio waves.
Active Galactic Nuclei (AGN): A galaxy with an unusually bright central region thought to contain a supermassive black hole actively pulling in tremendous amounts of matter from a swirling disk of gas, stars, and dust.
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. Thus planets in elliptical orbits travel faster when they are closest to the Sun, and more slowly when farthest from the Sun. A spinning body also possesses spin angular momentum.
Balmer lines: Emission or absorption lines in the spectrum of hydrogen that arise from transitions between the second and higher energy states of the hydrogen atom. They were discovered by Swiss physicist J. J. Balmer.
Big Bang Theory: The theory that the Universe began with all matter and energy concentrated to very high density and temperature some 15 billion years ago. The present universe expanded from that epoch and is still expanding.
Binary star: A system of two stars orbiting around a common center of mass due to their mutual gravity. Binary stars are twins in the sense that they formed together out of the same interstellar cloud.
Cepheid Variable Stars: A luminous giant star whose brightness varies periodically: growing very bright quickly, and then dimming slowly. The period of variation is related to luminosity. This property makes Cepheids useful for estimating astronomical distances.
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.
Correlator: A specialized supercomputer that multiplies the data from two antennas and averages the result over time, thus selecting only the signals that are seen by both antennas and dropping unwanted noise.
Cosmic Microwave Background Radiation: Radiation left over from the Big Bang. Because of the expansion of the Universe, the radiation is detected in the microwave portion of the spectrum, and has a temperature of only 2.7 K.
Cosmos: The Universe: the word is derived from the Greek, meaning 'everything'.
Dark Matter: Any nonluminous astronomical object or particle that is detected only by its gravitational influence. Examples include planets, black holes, white dwarfs (because they are low luminosity) and more exotic things like weakly interacting particles (WIMPs).
Declination: One of two coordinates for the celestial sphere, which are analogous to latitude and longitude for the Earth's surface. The declination of an object is how many degrees it is north or south of the celestial equator. The other coordinate is called right ascension, and it is measured eastward from a somewhat arbitrary "prime meridian" on the sky. The "prime meridian" passes through the position of the Sun at the time of the vernal equinox. Thus its position changes slowly over the years, due to the precession of the equinoxes. The position of the celestial poles also changes with precession. Thus, to locate an object from its right ascension and declination, you must also know the date for which those coordinates are valid; that date is called the epoch of the coordinates.
Doppler Shift: The change in length of a wave (light, sound, etc.) due to the relative motion of source and receiver. Things moving toward you have their wavelengths shortened. Things moving away have their emitted wavelengths lengthened.
Dust: Tiny grains of material (e.g., carbon and silicate grains) that are about 0.1-1.0 micron in size. Dust in interstellar space blocks and scatters visible light. The longer wavelengths of radio waves, however, are able to pass through dust in space, allowing astronomers to image previously hidden objects, such as the center of the Milky Way Galaxy.
Electromagnetic Radiation: A collective term for radiation consisting of oscillating electric and magnetic fields. It includes from shortest to longest wavelengths (gamma rays, X-rays, ultraviolet, visible light, infrared, microwave, and radio waves).
Epoch: The coordinates commonly used for the celestial sphere, which are analogous to latitude and longitude for the Earth's surface, are called right ascension and declination. The "prime meridian" of this system passes through the position of the Sun at the time of the vernal equinox. Thus its position changes slowly over the years, due to the precession of the equinoxes. The position of the celestial poles also changes with precession. Thus, to locate an object from its right ascension and declination, you must also know the date for which those coordinates are valid; that date is called the epoch of the coordinates.
False color: A technique scientists use to help them see details in images of objects. The colors the scientist picks for his or her image may have nothing to do with the color of the object. Sometimes the chosen colors represent the density, temperature, or other properties of parts of the object. Thus, the viewer of a false color image must always be told by the creator of the image what the colors mean.
Fast Fourier Transform (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.
Fusion: A process where nuclei collide so fast they combine, overcoming the natural repulsion of the positively charged protons. In the center of most stars, hydrogen fuses together to form helium. Fusion is so powerful it supports the star's enormous mass from collapsing in on itself, and heats the star so high it glows as the bright object we see today.
Galaxy: A large body of gas, dust, stars, and their companions held together by their mutual gravitational attraction. They are grouped into three main categories: spiral galaxies; elliptical galaxies, containing mostly older stars, which range in shape from spherical to "football" shaped; and irregular galaxies, which, as their name implies, are irregularly-shaped and generally smaller in size. Another class of galaxies is peculiar galaxies, which are thought to be distorted normal galaxies.
Gamma-ray Burst: Among the most powerful events in the Universe, a gamma-ray burst is believed to be the extremely powerful energy discharge of a black hole forming from the death of a giant star. Though the gamma-ray emission lasts only a few minutes to hours, the radio wave afterglow from the burst can last more than a year, making long-term observations of these enigmatic objects possible.
General relativity: The theory of gravitation developed by Albert Einstein. The theory has consequences for the bending of light by massive objects, the nature of black holes, and the fabric of space and time.
Gravitational Lens: The effect when light from a distant object, such as a galaxy, is bent by the gravity of a massive object, such as another galaxy, before it reaches the Earth. If the two objects are perfectly aligned, the light from the distant object appears as a ring from Earth. This then is called an Einstein Ring, since its existence was predicted by Einstein in his theory of General Relativity.
H-R Diagram: The H-R (Hertzsprung-Russell) diagram is used by astronomers to classify stars according to their luminosity, spectral type, color, temperature and evolutionary stage. Most stars fall into the Main Sequence, which runs from upper left to lower right.
Interference: For radio telescopes, this typically means unwanted signals, noise, or static. It also describes the result of combining the signals that two telescopes receive when observing the same source, which results in a pattern of oscillating values or "fringes" that depends on the separation of the two telescopes.
Interferometer: A radio telescope consisting of two or more antennas at some distance from one another. It uses the phenomenon of interference in order to increase the effective resolving power of the antennas.
Kelvin (after Lord Kelvin, 1824-1907): A scale that measures an object's temperature over absolute zero, the theoretical coolest temperature where all molecular and atomic motion ceases. On the Kelvin scale, the freezing point of water is 273 (273 K = 0o C = 32o F).
Maser: "Microwave-amplified stimulated emission of radiation". An amplifier of radio waves (similar to a laser, which amplifies visible light). This may be a natural feature, such as water molecules in space, or can be created by using special properties of certain crystals, such as ruby, at temperatures near absolute zero and in strong magnetic fields. Water molecules in space can form masers that help astronomers study radio emission from objects that would normally be too faint to detect.
Nova: A star that abruptly increases in brightness by a factor of a million. A nova is caused in a binary star system where hydrogen-rich material is transferred to the surface of a white dwarf until sufficient material and temperatures exist to kindle explosive nuclear fusion.
OH Line: Dark absorption lines on an electromagnetic spectrum or the emission of photos by hydroxyl (OH) molecules (one atom of oxygen and one atom of hydrogen). At presents, four principal lines are known in the radio domain at frequencies of 1612, 1665, 1667, and 1720 MHz, or wavelengths of approximately 18 centimeters.
Planetary Nebula: The gaseous outer layers of a star that have been ejected into space as the star collapses into a white dwarf. The ultraviolet radiation from the white dwarf causes the gas to fluoresce.
Proper motion: Apparent angular motion of a star on the celestial sphere, usually measured in seconds of arc per year. A star's transverse velocity, i.e., its motion across the line of sight to the star (as opposed to its radial velocity, or line-of-sight velocity), is calculated in kilometers per second.
Quasar: Objects of small angular size and immense power output. Some quasars (quasi-stellar objects, or QSOs) are strong radio sources. Radio-emitting quasars were the first to be discovered. These are some of the most distant objects in the Universe, and are believed to be fueled by supermassive black holes residing in ancient galaxies.
Radio Galaxy: A galaxy that emits radio waves from its central core. The energy to produce these emissions is generated by a supermassive black hole, which sends out massive jets of radio energy many millions of light-years into interstellar space.
Radio Telescope: A large and precise instrument that basically has three important parts: an antenna to pick up the extraterrestrial radiation, a receiving system to amplify and measure the signal, and a computer.
Redshift: The shift of all the spectral lines toward longer wavelengths due to the object's recession as seen from the Earth, this recession, at great distances, is due to the overall expansion of the Universe.
Resolution: The ability of a telescope to show detail. Also known as resolving power. One common way to describe the resolution of a telescope is to state the minimum angular separation at which a double star, whose two components are fairly bright and have very nearly the same brightness, can be distinguished as two separate stars.
Right ascension: The equatorial coordinate specifying the angle (usually specified in hours, minutes and seconds), measured eastward along the celestial equator from the vernal equinox to the hour circle passing through an object in the sky.
Seyfert Galaxy: A galaxy that appears to be a normal spiral galaxy, but whose core fluctuates in brightness. It is believed that these fluctuations are caused by powerful eruptions in the core of the galaxy.
Special Relativity: The specific set of rules relating observations from one frame of reference to the observations of the same phenomenon in another frame of reference. It states that the speed of light is the same for all observers. It also equates matter and energy through the equation E = mc2.
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).
Speed of light (c): The speed at which electromagnetic radiation propagates in a vacuum; it is defined as c = 299,792,458 m/s (186,000 miles/second). Einstein's Theory of Relativity implies that nothing can travel faster than the speed of light.
Spiral Galaxy: A galaxy consisting of a flattened rotating disk of young stars, a central bulge of generally older stars, and a surrounding halo of older stars and dense clusters of old stars called globular clusters. The disk is prominent due to the presence of young, hot stars in a spiral pattern.
Supernova: An extremely violent explosion of a star many times more massive than our Sun. During this explosion, the star may become as bright as all the other stars in a galaxy combined, and in which a great deal of matter is thrown off into space at high velocity and high energy. The remnant of these massive stars collapse into either a neutron star or a black hole.
21-cm Hydrogen Line: Radio emission of a very specific radio frequency of 1420 MHz by a neutral hydrogen atom when its single electron flips, emitting a single photon of energy with a wavelength of 21 centimeters.