Gaia in the UK

Taking the Galactic Census

Alerts Glossary

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2MASS: 2MASS is the Two Micron All-Sky Survey. 2MASS imaged the whole sky in three filters at infrared wavelengths. The survey produced two catalogues, one for extended sources (such as galaxies), and one for point sources (such as stars or quasars).


Accretion disc: a disc-shaped structure of material formed around a central body (usually a star), where the material is being pulled inwards by the gravity of the central object. The material does not plunge directly onto the object because it has angular momentum which causes it to orbit around the central object getting closer and closer until eventually being deposited on the surface of the central object.

AGN: AGN stands for Active Galactic Nucleus which is the unusually bright compact region right in the centre of some galaxies. Their unusual brightness is thought to come from material that is heated to high temperatures, causing it to glow, as it falls at high speeds into a supermassive black hole in the centre of the galaxy. The brightness of AGN often varies quite a bit in visible light.

AKARI: Akari was a Japanese infrared satellite which imaged almost the whole sky. Akari means light in Japanese.

AM CVn: AM Canum Venaticorum star: This is a rare type of cataclysmic variable star. It is a binary star system where one star is taking material, which does not contain hydrogen, from the other star. Gaia has discovered one AM CVn star so far, called Gaia14aae.

APMUKS(BJ): APMUKS was a galaxy survey made using photographic plates on the UK Schmidt Telescope in Australia. The term (BJ) refers to the filter used on the telescope and the emulsion that was used in the photographic plates.

Arcmin: An arcminute is a measure of angle used by astronomers. There are 60 arcminutes in one degree. The Moon is about 31 arcminutes in diameter as seen from the Earth (or about half of one degree). A 22cm diameter football subtends one arcminute when about 756 metres away.

Arc second: An arcsecond is a small angular distance used by astronomers - often to measure seperations between pairs of celestial objects. There are 60 arcseconds in one arcminute and 3600 arcseconds in one degree. An arcsecond is also the diameter of a 22cm diameter football from 45.4 kilometres (28.2 miles) away.

ASAS-SN: ASAS-SN is the All Sky Automated Survey for SuperNovae which is a project operating robotic telescopes in both the northern (Hawaii) and southern (Chile) hemispheres. ASAS-SN uses telephoto lenses and finds many bright supernovae, but not the fainter ones we find with Gaia.

ATEL: ATEL stands for the Astronomer's Telegram. It is a website and e-mail list used by astronomers to quickly share information about celestial events as they happen.

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Balmer Absorption: Absorption lines (dips) on a spectrum which show where photons have been absorbed due to hydrogen atoms present in the astronomical source.

Binary System: A binary system contains two stars bound together by gravity, and in orbit around their common centre of mass. Many stars are found in binaries, or in even larger groupings, called multiple star systems.

Black Hole: A black hole is thought to be formed when enough mass gets compressed into a small enough volume to deform spacetime so strongly, that nothing (not even light) can escape. Black holes can be formed by the collapse of a massive star (in a supernova). Supermassive black holes (weighing millions of times the mass of our Sun) are thought to live in the centres of every large galaxy (including our own).

BP/RP: Blue and red photometry as recorded by the Gaia satellite in the form of low-resolution dispersion spectra.

BP/RP Spectrograph: Also known as the BP/RP Photometer, this instrument generates spectra for every object that Gaia observes, and can be used to study the temperature and composition of the star or alerting source. Read more about Red and Blue Photometer.

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Chandrasekhar Mass: The maximum mass of a stable white dwarf star.  White dwarfs stars with masses higher than Chandrasekhar mass may become neutron stars or black holes. See:

CCD: Charge-Coupled Device

CRTS: CRTS is the Catalina Real-Time Transient Survey which searches for transient events in images from the Catalina Sky Survey CSS and publishes them to the world.

CSS: CSS is the Catalina Sky Survey which is searching for moving objects in the sky (Solar System Objects such as comets and asteroids).

Cataclysmic variable star: A Cataclysmic Variable (or CV) is a binary system containing two stars, one of which is an ultra dense white dwarf star which is devouring its companion. CVs can show bright outbursts, appearing then disappearing some time later. Read more about CVs.

CCSN see core-collapse supernova.

Core-Collapse Supernova: Very massive stars will undergo a collapse in their cores when they run out of fuel to sustain nuclear fusion. This collapse can result in the violent ejection of the outer layers of the star which we see as a supernova. The core may collapse to form a neutron star or a black hole. Sometimes this is abbreviated to CCSN.

CV: CV is an abbreviation for Cataclysmic Variable.

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Dec: Declination: The angular position of a source as measured with respect to the celestial equator.

Delta Magnitude: Change in magnitude of a source compared to one or more earlier measurements.

Doppler Effect: An increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move towards (or away from) each other. The effect causes the change in pitch noticeable in a passing siren, as well as the redshift seen by astronomers. Electromagnetic radiation emitted by a star moving away from the Earth appears to be shifted towards the red part of the spectrum (a red shift) and towards the blue, for a star moving towards the Earth (a blue shift).

DSS2: DSS2 is the second generation of the Digitized Sky Survey. It was produced by digitally scanning photographic images of the sky taken mostly at Palomar Observatory (US) and the Anglo-Australian Observatory.

Dwarf Nova: A Dwarf Nova is a type of Cataclysmic Variable which displays semi-regular changes in brightness of up to several magnitudes.

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Electromagnetic Spectrum: The electromagnetic spectrum is the collective term for all possible frequencies of electromagnetic radiation. Different parts of the spectrum are given different names. Starting from the low-frequency (long-wavelength) to the high-frequency (short-wavelength) end, these parts of the spectrum are called radio, microwave, far infrared, near infrared, visible light, ultraviolet light, X-rays, Gamma-rays.

EPSL: An Ecliptic Poles Scanning Law (EPSL) in which the spin axis of the spacecraft always lies in the ecliptic plane (no precession), such that the field-of-view directions pass the northern and southern ecliptic poles on each six-hour spin. This scanning law has been in place since the start of nominal operations of the spacecraft on 25 July 2014 until 22 August 2014.

ESO: ESO is the European Southern Observatory and is a collaboration between 16 nations for ground-based astronomy. ESO is responsible for building and operating some of the largest telescopes in the world, such as the Very Large Telescope (VLT), and the Atacama Large Milimeter Array (ALMA). ESO observatories are sited in northern Chile.

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Field of view: The area of sky which is observable to the telescope or satellite at a specific moment (also referred to as FOV).

Filters: Filters are devices that selectively transmit light of different wavelengths, hence observations in multiple filters can give us an idea of the colour of an object.

Flux: The amount of light observed from an astronomical object, per unit area per unit time.

Frequency: The frequency of a wave is the number of peaks in that wave seen to pass the observer each second. Electromagnetic waves can be distinguished by their frequency; higher frequency waves correspond to blue light (or x-rays and ultraviolet), while lower frequency waves are seen as red light (or radio waves).

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G-Band Fluxes: Unfiltered (white) light observed by the Gaia satellite, from about 350 to 1000 nanometres (10-9 metres).

Galactic Plane: In our own Galaxy, the Milky Way, most of the stars live in a disk - or plane - around the galactic centre. The Sun lies about 26,000 Light Years from the centre of the Milky Way (or about half way between middle and the edge of the disk).

Galaxy: A galaxy is a collection of stars, gas, dust and dark matter all held together by gravity. Our own galaxy is the Milky Way, which can be seen as a stripe of fuzzy or unresolved stars across the sky from a dark enough place. Galaxies are usually classified according to their shape. For example, we live in a spiral galaxy, a little like the Andromeda Galaxy (one of nearest neighbours). Other galaxies can be irregular in shape (for example the Large and Small Magellanic Clouds), or even elliptical. Many galaxies are thought to have massive black holes at their centres.

Galaxy names: Galaxies (and stars) are usually named after the survey(s) which catalogued them. Galaxies can have many different names. Some of the names you will see in our Gaia Alerts stream include 2MASS (also 2MASX), SDSS, GALEX. Sometimes we can’t be sure if an object is a galaxy or a star, and sometimes we get the classification wrong.

GALEX: GALEX refers to the Galaxy Evolution Explorer satellite which mapped the sky at ultraviolet wavelengths between 2003 and 2012. Just over half the sky was imaged. Typically ultraviolet sources are very hot, for example Cataclysmic Variables or galaxies full of massive stars.

GALEXASC GALEXASC stands for the GALEX (GALaxy Evolution eXplorer) All-Sky Catalogue.

Gamma Ray: Gamma rays are radiation at the high-frequency (short wavelength) end of the electromagnetic spectrum. They contain a lot of energy and hence would be hazardous if you were to encounter them on Earth, but they are blocked by the Earth's atmosphere.

G-band: Unfiltered (white) light observed by the Gaia satellite, with wavelengths between about 350 and 1000 nanometres. There are 1 million nanometres in 1 millimetre.

GSA: Gaia Science Alerts

GS-TEC: (the Gaia spectrophotometry transient events classifier) software, developed by Nadia Blagorodnova as part of her PhD project, for initial Alerts classification based on their prism spectra. More information about GS-TEC can be found in "GS-TEC: the Gaia Spectrophotometry Transient Events Classifier" (Blagorodnova, N. et al (2014), "GS-TEC: the Gaia spectrophotometry transient events classifier", Monthly Notices of the Royal Astronomical Society, Volume 442, Issue 1, p.327-342. DOI: 10.1093/mnras/stu837. [Online] Also available from

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Hostless: A hostless transient is one in which we see no obvious underlying host galaxy or star. For example, some supernovae don't seem to be connected with a galaxy, but it could just be that the underlying galaxy is incredibly faint, and hasn't been discovered yet.

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IC: IC is the Index Catalogue of Nebulae and Clusters of Stars dating back to the end of the 19th Century and is a supplement to the NGC catalogue.

ICRS: International Celestial Reference System.

Infrared: Infrared (or IR) light is too long in wavelength to be seen by the human eye. The Sun emits more than half its light at infrared wavelengths. Infrared light can be used to see objects which are hidden in dusty regions of space (for example where stars are being formed), or to study objects at high redshift.

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L2: Second Lagrange point of the Sun and Earth-Moon system.

Light Curve: A lightcurve is a graph of brightness against time for an object of interest such as a star or a galaxy. The shape of the lightcurve can help us tell different kinds of transients apart. For example, supernovae get much brighter than cataclysmic variables, but take longer to do so (often staying bright for months instead of days or weeks). Stellar Flares are some of the quickest transient events and can be over and done within a few hours.

LCRS: LCRS stands for Las Campanas Redshift Survey. This was the first attempt to map a large area of the universe out to a redshift of z = 0.2.

Logarithmic Scale: A logarithmic (log, for short) scale is a nonlinear scale used when a quantity can have a very large range of possible values. It is used for easier representation of the data (e.g. when creating a lightcurve). Common uses include the strength of earthquakes, sound loudness, light intensity (magnitude), and pH of solutions.

LBVs: (Luminous Blue Variables) are very massive (typically more than 50 times the mass of the Sun), hot and bright stars, which sometimes undergo outbursts where they eject material into their surroundings. Eta Carina is the best known example of such a star in our own galaxy.

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Magnitude: Magnitude is a measure of the brightness of an object. It is inverted, so a smaller number means a brighter object. The brightest objects will have negative values. For example, the Sun has a magnitude of -26.7, while the moon has a magnitude of -12.7. The next brightest object in the sky is usually Venus (which varies between -3.8 and -4.9), while the brightest star in the sky is Sirius at a magnitude of -1.4 in the visible. Many of the Gaia Alerts will be much fainter than this, and need telescopes to be observed. The faintest transients could be magnitude 19, which is 145 million times fainter than Sirius. Magnitude is measured on a logarithmic scale such that 5 steps in magnitude corresponds to a factor of 100 in brightness. (e.g. a magnitude 14 star is 100 times brighter than a magnitude 19 star).

MCG: MCG stands for Morphological Catalogue of Galaxies. It is a Russian catalogue of 30,642 galaxies.

Microlensing Event: Gravitational microlensing can occur when a foreground object passes directly in front of a background object. The light from the background star, or galaxy, or QSO is bent by the gravitational field of the foreground object, and can make it appear to be much brighter. We can use this effect to sudy otherwise invisible or very faint objects in the foreground, such as planets or black holes.

Milky Way: The Milky Way is our own Galaxy which contains about 100 billion stars. The Gaia spacecraft is measuring the positions, distances and motions of 1 billion stars in our Galaxy. The Milky Way is so named because from a dark site we can see a dim glowing band stretching across the night sky, where the individual stars cannot be separated by the naked eye.

MRSS: MRSS stands for Muenster Red Sky Survey.

M Star: Relatively cool star, with a surface temperature of less than 4000K.

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NGC: NGC stands for the New General Catalogue of Nebulae and Clusters of Stars and is a well known catalogue of (mostly) galaxies which was compiled during the 1880s.

NSL: A Nominal Scanning Law (NSL) with a precession rate of 5.8 revolutions per year. The transition from the EPSL to this scanning law took place on 22 August 2014.

NVSS: NVSS stands for the NRAO (National Radio Astronomy Observatory) VLA (Very Large Array) Sky Survey. The VLA is a collection of radio telescopes at the NRAO which is in New Mexico in the USA. This survey covers all but the most Southern part of the sky in the radio part of the electromagnetic spectrum.

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OGLE: OGLE stands for the Optical Gravitational Lensing Experiment which is a Polish astronomical survey using a telescope located in Chile and run by the University of Warsaw. OGLE finds thousands of Variable Stars, Microlensing Events and Supernovae. 

Outburst: When a star becomes much brighter, for example due to a flare, astronomers will refer to it as being "in outburst".

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Photometry: Photometry means measuring the brightness (or intensity) of an astronomical object. Often we measure photometry in a specific filter, e.g. B (Blue), V (Visible) or R (Red). We can compare the differences between the blue and red photometry to measure a colour. Photometry is usually expressed in an ancient system using units called magnitudes, which is a logarithmic measure.

Photometric Error: The uncertainty assigned to the measurement of light from an astronomical object.

Principal Investigator (PI): A principal investigator is the holder of an independent grant for the grant project.

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Quasar: A Quasar or quasi-stellar object (QSO) is a compact region in the centre of a massive galaxy surrounding a central supermassive black hole. A quasar is a type of AGN. The energy emitted by a quasar derives from mass falling into the black hole. Quasar have high redshifts (which means they are very distant) and are some of the most luminous objects in the universe.

QSO Quasi-stellar object - see Quasar.

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RA: Right Ascension: The angular position of an object as measured along the celestial equator, measured with respect to the Vernal Equinox.

RCB: RCB is an abbreviation for an R Coronae Borealis star.

R Coronae Borealis: An R Coronae Borealis star (often abbreviated as RCrB or RCB) is a star that occasionally puffs out a huge amount of Carbon soot which hides it from view until the soot is blown away by the stellar wind. The image shows a lightcurve of an RCB star generated from OGLE data.

RCrB: RCrB is an abbreviation for an R Coronae Borealis star.

Redshift A redshift occurs when an object travels away from the observer (the Doppler Effect). However, the redshift we see for distant galaxies is caused by Cosmological Redshift. The spacetime through which the photons of light emitted by the galaxies are travelling is stretched, which increases their wavelength, and this is observed as a redshift.

RV: Radial Velocity

RVS: Radial Velocity Spectrometer

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SDSS: SDSS stands for the Sloan Digital Sky Survey,, which imaged about one-third of the whole sky in 5 optical filters, and took spectra for more than 3 million objects. Many of the faint galaxies which play host to supernovae have been found by SDSS and are named after the survey.

Seyfert Galaxy: Seyfert galaxies are a class of active galaxies which have QSO-like nuclei, but otherwise look like normal spiral galaxies.

Simbad: Astronomical database,

SN: see supernova

SN Ia: SN Ia is a type of supernova (also known as a thermonuclear supernova) is thought to arise from a binary system containing at least one white dwarf, orbiting together with a second star. If the white dwarf is close enough to its companion, it can pull material slowly of it, becoming heavier in the process. If it gains enough material, it can reach a point where it explodes as a supernova. Alternatively, a SN Ia can arise from two white dwarf stars which merge or collide. Thermonuclear supernovae can be used as "standard candles" to measure distances in the Universe.

SN Ib: SN Ib is a class of supernova thought to arise from the core collapse of a massive star which has lost its hydrogen-rich outer layers.

SN Ic SN Ic is a class of supernova thought to arise from the core collapse of a massive star which has lost both its outer hydrogen and helium layers.

SN IIP: SN IIP is a class of supernova thought to arise from the core collapse of a hydrogen-rich massive star where the lightcurve shows a distinctive phase of constant brightness for about 100 days.

SN IIL: SN IIL is a class of supernova thought to arise from the core collapse of a hydrogen-rich massive star where the lightcurve shows a steady decline over time.

SN IIn: SN IIn is a class of supernova thought to arise from the core collapse of a massive star which has exploded inside a dense cocoon of gas and dust.

SN IIb: SN IIb is a class of supernova which is has properties intermediate between a SN IIP and a SN Ib. The star which exploded has lost most (but not all) of its outer hydrogen-rich layers.

Solar System Object: A Solar System Object (or SSO) could refer to anything in our own Solar System. In Gaia Alerts we usually use SSO to refer to asteroids (we also call them rocks). Solar System Objects can usually be identified by their motion in the sky as they orbit the Sun. We are not trying to find SSOs, there is another team dedicated to doing this (see, but sometimes they creep into our Alert stream as an object that suddenly appeared when there was nothing there before.

Spectra: When the light from a star (or a galaxy, or even a streetlight) is split into many colours (for example by a prism), we call it a spectrum. Spectra allow us to study the temperature and composition of the object. In Gaia Alerts we use the BP/RP spectrograph to help understand the transients. See Why we take spectroscopy for more.

SSO: see Solar System Object

Starburst Galaxy: While many galaxies are continually forming new stars, a starburst galaxy is doing so at an exceptionally high rate. This high star formation rate leads to many more supernova explosions in these galaxies.

Stellar Flare: Our own Sun is variable - we have seen outbursts and eruptions called solar flares. But the strongest solar flares pale in comparison to the extreme energetics of the flares seen in other stars. The source of energy for these flares lies in the magnetic field of the star. Magnetic fields trace out arcs on the surface of the star, similar to the pattern you can see in iron filings when you hold the pole of a magnet nearby, or the magnetic field lines around the Earth. If the magnetic fields on the surface of a star become tangled and twisted, they can build up a store of energy that is released suddenly in a flare. This process is called "magnetic reconnection".

Supernova: A Supernova (often abbreviated as SN) is the violent, explosive event that signifies the end of a star's life. The plural of supernova is "supernovae". Read more about supernovae.

Super pixel: Part of a digital image with the same qualities as a pixel such as colour and brightness, but however is larger in size.

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TDE: TDE is an abbreviation for Tidal Disruption Event.

Tidal Disruption Event: A Tidal Disruption Event (abbreviated as TDE) occurs when a star gets to close to a black hole and is torn apart by the incredibly strong gravity of the black hole. It emits a huge flare in optical light (but also X-rays and the radio) as it spirals into the black hole. Read more about TDEs.

Transient: A transient is anything in the sky which appears, disappears or changes. Some of these transients will be stars exploding as supernovae, or black holes swallowing stars. The Gaia Alerts project is working to find such events in the data from Gaia, and announce them to the world in real time.

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UGC: UGC stands for Uppsala General Catalogue of Galaxies. It is a catalogue of 12,921 galaxies visible from the northern hemisphere.

ULENS: ULENS is an abbreviation for a Microlensing Event. The U is supposed to look a bit like the Greek letter 'mu' which is the symbol for micro in the metric system.

Ultraviolet: Ultraviolet (or UV) light is light that is too short in wavelength to be seen by the human eye. The sun emits UV light which can give rise to suntan, freckling or sunburn. UV radiation is at longer wavelengths than X-ray and Gamma-ray radiation.

UTC: The Coordinated Universal Time, see

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Vernal Equinox: Position on the celestial equator where it crosses the ecliptic plane, and corresponding to the direction of the Sun at the moment when in spring the lengths of day and night are identical.

Viscosity: Viscosity describes a fluid's resistance to flow. Read more:

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Wavelength: The wavelength of a wave is the distance between a point on one wave and the same point on the next wave. For electromagnetic waves, long wavelength light is red, and short wavelength light is blue.

WD: White Dwarf (star): A white dwarf is a very dense star. A low to medium mass (mass of up to 8 times the mass of the Sun) star becomes a white dwarf after exhausting its nuclear fuel.

WFAU: (Edinburgh) Wide Field Astronomy Unit

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X-ray: X-rays are the same type of radiation used to produce X-rays of bones in hospital. X-rays are not quite as energetic as gamma rays, but they are also at the high-frequency (short-wavelength) end of the electromagnetic spectrum.

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YSO: Young stellar object (YSO) refers to a star in the early stages of its life.

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