Absolute Visual Magnitude
The apparent visual magnitude of a star, as would be obtained at a standard distance of 10 parsecs from the star, in the absence of interstellar matter.
Accretion
Or Aggregation. Where a body increases in mass due to collisions with smaller bodies that stick to it. Sufficiently large bodies can speed up the accretion process due to their gravitational attraction.
Accretion Disc
A disc shaped region of of in falling material surrounding a binary star or black hole.
Active Galaxy / AGN (Active Galactic Nucleus)
The Andromeda Galaxy
Anisotropy
This term is frequently used to describe the temperature fluctuations in the Cosmic Microwave Background. The word means not uniform -
Aphelion
The point in the orbit of a planet, comet, or other body in solar orbit that that is farthest from the Sun. See perihelion.
Axion
A hypothetical particle which is a candidate for dark matter. Axions have zero charge and low mass.
Balmer series
The Balmer series is one of a set of six different named series describing the spectral line emissions of the hydrogen atom. The Balmer series is characterized by the electron transitioning from n ≥ 3 to n = 2, where n refers to the principal quantum number of the electron. Balmer lines appear in the spectrum of many stellar objects due to the abundance of hydrogen in the universe. The Balmer lines can be used to help determine the age of stars because younger stars are composed almost entirely of hydrogen, while older stars have greater quantities of heavier elements.
Big Bang Theory
Big Crunch
The Balmer series is one of a set of six different named series describing the spectral line emissions of the hydrogen atom. The Balmer series is characterized by the electron transitioning from n ≥ 3 to n = 2, where n refers to the principal quantum number of the electron. Balmer lines appear in the spectrum of many stellar objects due to the abundance of hydrogen in the universe. The Balmer lines can be used to help determine the age of stars because younger stars are composed almost entirely of hydrogen, while older stars have greater quantities of heavier elements.
Big Bang Theory
Big Crunch
The collapse of a closed universe to a singularity.
Big Smash
A final singularity in which the Universe is destroyed in a finite proper time by excessive expansion.
Black Hole
Blazar
A final singularity in which the Universe is destroyed in a finite proper time by excessive expansion.
Black Hole
Blazar
Blazars are AGNs with a relativistic jet that is pointing in the general direction of the Earth. Blazars have a compact and highly variable energy source associated with a supermassive black hole at the centre of a host galaxy. The observed emission from a Blazar is greatly enhanced by relativistic effects in the jet, a process termed relativistic beaming.
Bolometer Array
For low spectral resolution experiments, bolometers are more sensitive than radio detectors. Bolometers work by measuring the change in temperature of an absorber when illuminated by electromagnetic radiation. Space based missions require longer design periods which often means the detector technology is superseded by the time the mission is launched. This is a particular problem with missions that utilise bolometers.
The Buran Orbiter
Brahe, Tycho
Cardassian models
Cardassian expansion is a modification to the Friedmann equation that allows the universe to be flat, matter dominated, and accelerating, without a vacuum component.
The Cassini Mission to Saturn
For low spectral resolution experiments, bolometers are more sensitive than radio detectors. Bolometers work by measuring the change in temperature of an absorber when illuminated by electromagnetic radiation. Space based missions require longer design periods which often means the detector technology is superseded by the time the mission is launched. This is a particular problem with missions that utilise bolometers.
The Buran Orbiter
Brahe, Tycho
Cardassian models
Cardassian expansion is a modification to the Friedmann equation that allows the universe to be flat, matter dominated, and accelerating, without a vacuum component.
The Cassini Mission to Saturn
Cepheid Variables
A type of pulsating variable star. They are very luminous and have periods of around one to 200 days. There is a relation between the period of the pulsation and the average brightness of the star. This enables their use as standard candles.
Cherenkov Radiation
electromagnetic radiation emitted when a charged particle passes through an insulator at a speed greater than the speed of light in that medium. The characteristic "blue glow" of nuclear reactors is due to Cherenkov radiation. It is named after Soviet scientist Pavel Alekseyevich Cherenkov.
Copernican Principle
The philosophical assertion that the Earth does not occupy a preferred place in the universe.
According to this principle there are no special or preferred places, or observers, anywhere in the universe. This, together with the cosmological principle, is absolutely fundamental to modern ideas about the universe.
Copernicus, Nicolas
Correlation Radiometer Array
Copernicus, Nicolas
Correlation Radiometer Array
A type of polarimeter and interferometer. The correlation radiometer is the complement of the spatial interferometer. It images two points in the focal plane instead of two patches in the aperture plane, and extracts the corresponding uncorrelated signal instead of the correlated signal. A Fourier transform of the cross-correlation function yields the power density cross-spectrum. If you are interested see this technical paper -
Spectroscopy with Multi-
CMB - Cosmic Microwave Background
This is the radiation "leftover" after the decoupling of photons from matter about 300,000 years after the Big Bang. We study the perturbations, or disturbances, in the radiation in the hope of one day discovering the ultimate fate of our universe.
The study of the origin and development of the Universe as a whole.
Cosmological Principle
The assumption that the Universe is, on average, homogeneous and isotropic. In other words, on sufficiently large scales, the universe is smooth. Therefore, we do not occupy a special position in the universe (see Copernican principle); indeed there is no special position anywhere in the universe. An observer anywhere in the universe will observe a similar distribution of galaxies, on a large enough scale.
Critical Density
The minimum density needed for gravity to overcome the expansion of the universe and cause it to eventually re-collapse.
Dark Matter
The minimum density needed for gravity to overcome the expansion of the universe and cause it to eventually re-
Dark Matter
Decoupling
Term that describes the event 300,000 years after the Big Bang where the universe had finally cooled off enough so that ordinary matter could form and photons could travel in a free path. During decoupling the universe goes from opaque to transparent.
Eddington Limit
The upper limit to the luminosity of an object. The limit arises because the radiation pressure in the emitting region cannot exceed the gravitational forces holding the object together.
Effective Temperature of a Star
The effective temperature is the best measure of the actual of the gas in a star’s outer layers. More precisely, the effective temperature of a star is the temperature of a black body with the same luminosity per surface area as the star and is defined according to the Stefan-Boltzmann law. Even more rigorously, the effective temperature corresponds to the temperature at the radius that is defined by the Rosselan optical depth.
Ethane
Ethane is a chemical compound which consists of two carbons and six hydrogen atoms. It is the only two-carbon alkane, that is, an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colourless, odourless gas.
The chemical structure of ethane
Etherodyne Correlation Receiver
Etherodyne Correlation Receiver
SIS (Superconductor - Insulator - Superconductor) junctions are used to build low noise mixers for etherodyne receivers at millimetre and sub-millimetre wavelengths.
SIS mixers use tunnel junctions, which employ a quantum effect in which an electron "tunnels" through an insulator. This kind of device is able to detect very weak signals with very little noise. Receivers built with SIS mixers are the most sensitive kind of receiver that current technology can build, and are nearly at the theoretical performance limits.
European Space Agency (ESA),
ESA was established in 1975. It is an inter-governmental organization dedicated to the exploration of space, currently with 17 member states.
Evershed Effect
The Evershed effect is the radial flow of gas across the photospheric surface of the penumbra of sunspots from the inner border with the umbra towards the outer edge. The true nature of these flows remains enigmatic, and this presently prevents a full understanding of penumbral physics. The effect was discovered in 1909 by the British astronomer John Evershed.
Galaxy
A galaxy is an accumulation of dark matter, stars, dust and gas that is held together by gravity. The gravitational pull within a galaxy is stronger than the force of the Hubble Expansion, so the elements of a galaxy do not expand away from each other. Galaxies can be spiral-shaped, elliptical, lenticular or irregular and are found in many different sizes. There are about 10^11 galaxies in the observable universe. Our home galaxy is called the Milky Way.
Galaxy Cluster
A collection of galaxies that is drawn together by gravity. Galaxy clusters are the largest objects in the universe that have a gravitational pull strong enough to overcome the Hubble Expansion, and thus galaxies in a cluster do not expand away from each other.
A galaxy is an accumulation of dark matter, stars, dust and gas that is held together by gravity. The gravitational pull within a galaxy is stronger than the force of the Hubble Expansion, so the elements of a galaxy do not expand away from each other. Galaxies can be spiral-
Galaxy Cluster
A collection of galaxies that is drawn together by gravity. Galaxy clusters are the largest objects in the universe that have a gravitational pull strong enough to overcome the Hubble Expansion, and thus galaxies in a cluster do not expand away from each other.
Gunn-Peterson Effect
Quasar spectrums may be affected by the Gunn-Peterson effect. This is a trough in the spectrum which is due to the absorption of light by neutral intergalactic hydrogen. It's a trough rather than the usual line because the quasar is so far away from the Earth that its spectrum is shifted by the expansion of the universe.
High Electron Mobility Transistors
The High Electron Mobility Transistor or HEMT is a form of field effect transistor. It offers a combination of low noise combined with the ability to operate at high levels of performance at microwave frequencies. The key element within a HEMT is the specialised pn junction, known as a hetero-
HEMT polarimeters and correlation receivers are much more common in current CMB polarization experiments than bolometers because of their higher sensitivity below observing frequencies of 100 GHz. They also display lower susceptibility to systematic effects.
Homogeneous Universe
A universe that has the same density of matter/energy everywhere, on average. A homogenous universe is by implication isotropic.
Horizon
Term relating to how far we can see out into the universe. Because the Universe is about 14 billion years old, we cannot observe any part of the universe that is farther away than about 14 billion light years. Actually, the horizon is slightly larger than we might expect based purely on the distance light has travelled, because of the expansion of the universe. Our horizon will continue to get bigger with time.
Hubble Expansion
Or Hubble flow. Used to describe the expansion of the universe based on the redshifted light of distant galaxies. The relationship between recession velocity and distance is described by Hubble's Law.
Or Hubble flow. Used to describe the expansion of the universe based on the redshifted light of distant galaxies. The relationship between recession velocity and distance is described by Hubble's Law.
Hubble time
The Hubble time is one divided by the Hubble constant, which gives a number from 10 to 20 billion years. For a flat universe with no cosmological constant, the age of the universe is two-thirds of the Hubble time. If there is no cosmological constant, the universe is younger than the Hubble time.
Infinite
When we talk about the possibility of the universe being infinite this relates to both space and time. In terms of space, infinitely big means literally without limits. Secondly, the universe may continue until infinitely old, into the infinite future.
Inflation
A period of extremely rapid expansion thought to have occurred in the early universe.
Interferometer
Usually a ground based detection technique. A device that combines signals radiating from a common source but received at different locations, to produce fringes which result from the interference between the received signals. From analysis of the fringe visibility the angular extent of the source may be inferred. A map of the source may be produced by using a range of different baselines.
Usually a ground based detection technique. A device that combines signals radiating from a common source but received at different locations, to produce fringes which result from the interference between the received signals. From analysis of the fringe visibility the angular extent of the source may be inferred. A map of the source may be produced by using a range of different baselines.
Isotropic Universe
An isotropic universe has no preferred direction for any observer. It looks and acts the same in every direction. For example, the redshift of distant galaxy clusters looks the same from our location as it does from another distant cluster in the universe.
Kelvin Scale
The Kelvin scale is a thermodynamic temperature scale where absolute zero is defined as zero kelvin. The unit increment of the Kelvin scale is the kelvin (symbol: K), which is the SI unit of temperature. Note the unit is the singular kelvin not “degrees kelvin”. The Kelvin scale is named after British physicist and engineer William Thomson, 1st Baron Kelvin.
L2 point
Lagrangian point L2. Named after the Italian-French mathematician Joseph Louis Lagrange (1736-1813). In a system of two large bodies (e.g. Sun-Earth or Earth-Moon), there are five points where a small third body will keep a fixed position relative to the other two. L2 lies on the night side, away from the Sun. A satellite can be placed into a Lissajous orbit around the L2 point. There are many advantages to the L2 orbit. The thermal environment is very stable. There is little radiation from the earth or moon to interfere with observations and there are no radiation belts at this location.
Light Echoes
Light echoes are produced by radiation scattered by dust near or along the line of sight to an astrophysical explosion.
Light echoes are produced by radiation scattered by dust near or along the line of sight to an astrophysical explosion.
Light year
A light year is a unit of distance, NOT a length of time! It is the distance that light can travel (in a vacuum) in one year. One light year is about 6 trillion miles, or 10 trillion kilometers.
Lissajous orbits
Jules Antoine Lissajous (1822-1880) was a French physicist who was interested in waves.
Electronics engineers use Lissajous patterns to measure radio signal frequencies. They do this by analysing the type of pattern an unknown signal makes when it is combined with a signal of known frequency.
Quasi-periodic, bounded but not precisely repeating, Lissajous orbits exist in an n-body system (e.g. A system involving the Sun, the Earth and a spacecraft). Although such orbits are not perfectly stable, by employing station-keeping methods a spacecraft can stay in a desired Lissajous orbit for an extended period of time.
Lyman Break Galaxies
A population of high-redshift galaxies that are caught in the act of forming large numbers of new stars. Lyman-Break Galaxies are selected by performing deep imaging in three broad band filters.
Mars Shop
Maser
Microwave (or molecular) amplification by stimulated emission of radiation. A maser is an intensely concentrated beam of coherent microwaves. An astrophysical maser is formed through the interaction between high-energy starlight and a nearby region rich in molecules.
Such emission is observed from water (H2O), hydroxyl radicals (OH), methanol (CH3OH), formaldehyde (CH2O), and silicon monoxide (SiO).
Some galaxies possess central black holes into which a disk of molecular material is falling. Excitations of these molecules in the disk or in a jet can result in megamasers with large luminosities.
Malmquist Bias
Obviously, we can always see more luminous objects at greater distances than we can see less luminous objects. This means that the relative numbers of intrinsically bright and faint objects that we see may be nothing like the relative numbers per unit volume of space; instead, bright objects are over-represented; and the average luminosity of the objects we see inevitably increases with distance. This is the Malmquist bias.
Megaparsec
One million parsecs. The main unit of distance used on cosmological scales.
New General Catalogue of Nebulae and Star Clusters
The NGC is a catalogue of nearly 8,000 nebulae, star clusters and galaxies. It was originally published in 1888, and later expanded by the publication of the IC or Index Catalogue.
Observable Universe
The sum total of all the matter, energy and space we are capable of experiencing. There may exist objects that are potentially unknowable as they lay beyond the horizon and so outside the observable Universe. The study of the large scale structure of the Universe is called cosmology.
Orbit
The path followed by an object moving in a gravitational field. For a single object this is usually an ellipse. See eccentricity.
Parsec
Short for parallax second. The distance at which a star would have an annual parallax of one arc second. Equivalent to 3.2616 light-years.
Perihelion
The point in the orbit of a planet, comet or other body in solar orbit that is nearest the Sun. See Aphelion.
Phantom Cosmology
Cosmology involving phantom energy, which is a hypothetical form of dark energy with an equation of state of w less than -1. If it exists, it could cause the expansion of the universe to accelerate so quickly that the Big Rip would occur. If w is less than -1, the energy density increases as the universe expands. The growing energy density increases the negative pressure, driving the acceleration of the expansion harder, leading to more volume and more energy, etc. This produces a runaway feedback loop that makes the universe expand explosively.
Planet
Unfortunately, there is no straightforward definition of what constitutes a planet as this is a somewhat controversial subject. For our purposes, a planet is an object orbiting a star that does not produce energy by fusion reactions. The Earth is a planet.
Photon
One of the elementary particles. All electromagnetic radiation is quantised as photons - they are the smallest amount of electromagnetic radiation that can exist. Photons have spin 1 and are therefore classified as bosons.
Photons range in energy from low energy radio-wave photons, through microwave, infra-red, visible, ultraviolet, x-ray and gamma-ray photons.
Polaris
The brightest star located within one degree of the north celestial pole. It can be used as a reference to establish where north is located. Sometimes referred to as the North Pole star. Polaris is located in the constellation Ursa Minor -
Primordial Soup
The name for the time in the evolution of the universe when the universe was too hot to form ordinary matter. At this stage the universe was opaque and glowed due to photons that were being continuously scattered by electrons.
A rapidly rotating neutron star which emits a radio beam at frequent intervals. Pulsars were first discovered in 1967 by graduate student Jocelyn Bell Burnell. The combination of the powerful magnetic field and the rapid rotation of the neutron star produces extremely strong electric fields. Electrons are accelerated to high velocities by the electric fields. These high-energy electrons produce the radiation we observe in ways that are not fully understood.
A shortened form of Quasi-Stellar Object (QSO), from a contraction of QUASi-stellAR radio sources, the term quasar is still disliked by many scientists. Currently, the best explanation for quasars is that they are powered by supermassive black holes.
Red-giant Star
A red giant is a type of star about a thousand times the volume of the Sun which is approaching the end of its evolution. The supply of hydrogen in its core has been used up and the star has switched to fusing hydrogen in a shell outside the core. The dominant nuclear process in the core is now the 3-alpha process.
Redshift
When a radiating object moves away from us, we observe a redshift in its light. The light waves it emits are getting longer (shifting to the red part of the spectrum) due to the Doppler effect. This should not be confused with the redshift arising from the expansion of the Universe.
Scalar field
A scalar field associates a scalar to every point in space. A scalar is a physical quantity which assumes a single value which is independent of the coordinate system being used to describe the physical system.
A vector field associates a vector to every point in space. Some examples of vector fields include the electromagnetic field and the Newtonian gravitational field. A vector is a quantity characterized by a magnitude and a direction.
A tensor field associates a tensor to every point in space. In general relativity, gravity is associated with a tensor field.
Singularity
A singularity is a region of extremely high - perhaps infinite - density into which matter and light are attracted. Singularities are thought to exist within Black Holes. The laws of normal physics break down at a singularity.
Standard Siren
The gravitational wave analog to a standard candle.
Star
A celestial body composed of hot plasma and gas that radiates energy derived from thermonuclear reactions inside the star. Stellar masses vary between 0.01 and 100 times the mass of the sun.
Sunyaev-Zeldovich effect
This effect has been predicted by Sunyaev & Zeldovich. They showed that the passage of the cosmic microwave background radiation through a hot electron cloud would distort the spectrum of the radiation. The photons gain energy due to the Compton interaction between the photons and the electrons.
Ultra-Luminous X-ray source (ULX)
An astronomical source of X-rays that is not in the nucleus of a galaxy. At present, it is not known what powers ULXs. Proposed models include beamed emission of stellar mass objects, accreting intermediate-mass black holes, and super-Eddington emission.
Uniform
When we talk about a homogeneous universe, we are making the assumption that the universe is uniform, or has the same makeup throughout. So the amount of galaxies, stars, gas and dust in a given volume is pretty much the same anywhere in the universe.
Triple Alpha Process
See 3-alpha process
Visible Spectrum
This is the part of the Electromagnetic Spectrum that contains the light we can see. The colours in the visible Spectrum from longest wavelength to shortest are: red, orange, yellow, green, blue, and violet. These are the same colours we see in a rainbow.
WIMPs
Weakly Interacting Massive Particles. Such particles are a dark matter candidate.
X-ray Bursters
X-ray bursters are a class of binary stars which have periodic outbursts luminous in X-rays. Neutron stars in x-ray binaries accrete material from a companion star and flare to life with a burst of x-rays.
Zenith
A reference point in the sky that is directly overhead.
| Eccentricity |
| Electromagnetic Spectrum |
| Maxwell's Equations |
| Metallic Hydrogen |
| Newtons Laws of Motion |
| Orbit |
| Standard Candle |