XMM-Newton Image Gallery Glossary

 XMM-Newton Home  >  Gallery Home  >  Astronomy Glossary




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.

Absorption line: A dark line or band at a particular wavelength on a spectrum, formed when a substance between a radiating source and an observer absorbs electromagnetic radiation of that wavelength.

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.

Active Galaxy: A galaxy that is a source of excess radiation, usually radio waves, X-rays, gamma rays, or some combination.

Amplitude: the maximum variation strength of an electromagnetic wave in one wavelength.

Angstrom: A unit of length equal to one ten-billionth of a meter (about four-billionth of an inch); often used to measure the wavelength of light.

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.

Aphelion: The point in a body's orbit (e.g. Earth's) around the Sun where the orbiting body is farthest from the Sun.

Apogee: The point in a body's orbit (e.g. the Moon's) around Earth where the orbiting body is the farthest from the Earth.

Apparent magnitude: The measure of the observed brightness received from a source.

Arc minute: A measure of angular separation, - one sixtieth of a degree.

Arc second: Another measure of angular separation, - one sixtieth of an arc minute. (1/3600th of a degree.)

Astronomical unit (AU): A unit of distance equal to the average distance between the Earth and the Sun, or 149.6 million kilometers (93 million miles).


Top of page

Bandwidth: The difference between the highest and lowest frequencies to which a receiver is sensitive.

Barred Spiral Galaxy: A spiral galaxy in which the spiral arms come from the ends of a bar through the nucleus rather than from the nucleus itself

Baryon: A massive, strongly interacting elementary particle, such as a proton or a neutron.

Baryonic matter: Ordinary matter as we know it consists largely of baryons.

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.

Black dwarf: One possible final stage in the evolution of a star, in which all the energy is exhausted and it no longer emits radiation.

Black Hole: An object with such high gravity that not even light can escape. These may be formed when the most massive of stars die, and their cores collapse into a superdense mass.

Blazar or BL Lac Object: Objects that resemble quasars; thought to be the cores of highly luminous galaxies aligned so they are viewed directly down into the heart of the system.

Blue shift: Apparent shortening of the wavelength due to Doppler effect, of radiation received from a source in motion toward the observer.

Brown Dwarf: Either a supermassive planet or a failed star, a brown dwarf has insufficient mass to sustain nuclear fusion.


Top of page

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 instrument has that is capable of collecting electromagnetic radiation. Collecting area is important for a instrument's sensitivity: the more radiation it can collect (that is, the larger its collecting area), the more likely it is to detect dim objects.

Continuous Spectrum: A spectrum in which there are no absorption or emission lines.

Coronal Mass Ejection: The mass ejected from the Sun due to a solar flare.

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.

Cosmic Rays: Cosmic rays are very high energy atomic nuclei (mostly protons) traveling through space at close to the speed of light that strike the Earth's atmosphere.

Cosmology: This is the study of the large-scale structure and evolution of the Universe.

Cosmos: The Universe: the word is derived from the Greek, meaning 'everything'.

Cyclotron radiation: Electromagnetic radiation emitted when charged particles are moved within a magnetic field at non-relativistic speeds (i.e., not close to the speed of light).


Top of page

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.

Dwarf Galaxy: Small, low luminosity galaxies that are associated with larger spiral galaxies and may make up part of a galactic halo.

Dwarf star: A star, which lies on the main sequence and is too small to be classified as a giant star or a super giant star. For example, the Sun is a yellow dwarf star.


Top of page

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).

Electron: A negatively charged particle commonly found in the outer layers of atoms. The electron has only 0.0005 the mass of the proton.

Elliptical Galaxy: A galaxy that is round or elliptical in outline, typically containing little gas and dust, and no spiral arms or disk, and few hot, bright stars.

Emission Line: A bright line in a spectrum caused by the emission of photons from atoms.

Emission Nebula: A cloud of gas that is excited by the ultraviolet radiation from hot stars.

Epoch: The coordinates commonly use 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.


Top of page

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.

Flux: The rate of transfer of fluid, particles, or energy across a given surface.

Frequency: Number of wave vibrations per second; 1 Hertz is one cycle per second (e.g., 1420 MHz = 1,420,000,000 vibrations per second).

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.


Top of page

Galactic Center: The central region of a galaxy characterized by high densities of stars. The center may also contain a supermassive black hole.

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.

Globular Cluster: A spherical star cluster containing 50,000 to 1 million stars; generally old and metal-poor, globular clusters may be the left over building blocks of galaxy formation.

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.


Top of page

H I Region: A region in space, seen as cloud-like features, containing mostly neutral atomic hydrogen gas. If of sufficient density, these areas may become the birthplace of stars and planets.

H II Region: A region in space around one or more very hot, bright stars where most of the hydrogen gas is ionized.

Halo: The spherical region of a spiral galaxy composed of diffuse gas an$containing few stars and star clusters.

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.

Hertz: A measure of the number of oscillations per second. 1 Hz = 1 cycle or oscillation/second.

Hubble Constant (H): A measure of the rate of expansion of the Universe; current estimates are about 70 km/s/megaparsec.


Top of page

Irregular Galaxy: A galaxy without spiral arms that has a chaotic appearance.


Top of page

Jets: A pair of beams of particles, usually coming from an active galactic nucleus or a pulsar.


Top of page

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).

Kiloparsec: The distance of 1000 parsecs (3260 light-years).


Top of page

Light Waves: Electromagnetic radiation at light frequencies (about 1015 cycles per second), or a wavelength between 400 and 700 billionths of a meter.

Light-year: A light-year is a unit of length. The distance that light travels in one year, about six trillion miles.

Luminosity: The amount of energy a star emits in 1 second.


Top of page

Magnetar: A neutron star with a very strong magnetic field.

Magnetosphere: Region around a planet in which its magnetic field dominates the interplanetary field carried by the solar wind.

Main Sequence: The region of the H-R diagram running from upper left to lower right, which includes roughly 90 percent of all stars.

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.

Milky Way: Our Galaxy, of which the Sun is a member, seen by the naked eye as a luminous band across the sky.

Millisecond Pulsar: A pulsar with a period of one thousandth of a second.

Molecular Cloud: An interstellar gas cloud that enables the formation of molecules. Over 125 molecules have now been discovered in interstellar space through radio wavelength observations.


Top of page

Nebula: A term used by optical astronomers to denote any object that resembles a cloud, either bright or dark, and is not stellar in appearance.

Neutron Star: A small, extremely dense star made primarily of neutrons, with a radius of approximately 10 kilometers.

Non-thermal emission: Electromagnetic radiation produced by synchrotron radiation, maser line emissions from atoms and molecules, or the mechanisms not related to temperature.

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.


Top of page

Parallax: The apparent change in location of a object due to the change in position of the observer. Astronomical parallax is measured in seconds of arc.

Parsec: A unit of length approximately equal to 3.26 light-years, or nineteen trillion miles.

Peculiar Galaxy: A galaxy that has distorted features and does not fit completely into a normal galaxy category.

Phase: Angular distance between peaks or troughs of two wave forms of similar frequency.

Photon: A discrete packet of electromagnetic energy.

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.

Point Source: An idealized discrete source of radiation that subtends an infinitesimally small angle.

Polarization: The action or process of affecting radiation and especially light so that the vibrations of the wave assume a definite form.

Population I: Stars rich in atoms heavier than helium; usually relatively young stars found in the disk of a galaxy.

Population II: Stars poor in atoms heavier than helium; relatively old stars found in the halo, globular clusters, or the nuclear bulge.

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.

Protostellar Disk: A gas cloud around a forming star flattened by rotation.

Pulsar: A rapidly rotating neutron star that emits beams of radiation along misaligned magnetic axes.


Top of page

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.


Top of page

Red Dwarf: Cool, low mass star on the lower main sequence.

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 an instrument to show detail. Also known as resolving power. One common way to describe the resolution of a instrument 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.


Top of page


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.

Sidereal day: The time required for Earth to revolve 360o with respect to a celestial object outside the solar system. About 23 hours 56 minutes duration in terms of solar time.

Solar flare: A brilliant outbreak in the Sun's outer atmosphere, usually associated with active groups of sunspots.

Solar System: A star and the non-luminous objects associated with it, which may include brown dwarfs, planets, asteroids, and comets.

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 instrument 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).

Spectral Sequence: The arrangement of a star's position in the temperature classification system (O, B, A, F, G, K, M), ranging from hot to cool.

Spectrum: A plot of the intensity of light at different frequencies. Or the distribution of wavelengths and frequencies.

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.

Starburst Galaxy: A bright galaxy in which many new stars are forming.

Sun: The star associated with Earth's solar system. The Sun weighs about 2x1030 kilograms, and is about 1.4x109 meters in diameter.

Supergiant: Very luminous star 10-1000 time more massive than the Sun.

Superluminal velocity: The apparent motion of an object at greater than light speed; This appearance is caused by a "projection effect" by the object's motion toward Earth.

Supermassive Black Hole: A black hole that has a million or as much as a billion solar masses. Such huge black holes lurk at the centers of many active galaxies.

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.

Supernova Remnant: The expanding shell of gas from a supernova explosion.

Synchrotron Radiation: Radiation emitted by charged particles being accelerated in magnetic fields and moving at near the speed of light.


Top of page

T Tauri Stars: Young stars surrounded by gas and dust, believed to be contracting to main sequence stars.

Thermal emission: Radiation emitted due to an object's temperature (e.g., blackbody radiation) or by an ionized gas.


Top of page

Wavelength: The distance between two adjacent crests of a wave motion. For electromagnetic radiation, the product of frequency and wavelength is equal to the speed of light.

White Dwarf: A dying star that has collapsed to approximately the size of the Earth and is slowly cooling.