Dictionary of the Extreme Universe

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  • Accretion disk: This refers to a flow of matter that is attracted to a celestial object by its gravity and is forced to move around it in an orbit due to the object’s rotational movement – as it does so, it forms a disk, called an accretion disk. This flow of matter can be detected at different wavelengths and plays a key role in important sources for gamma-ray astronomy, such as microquasars or Active Galactic Nuclei (AGNs).
  • Active Galactic Nucleus (AGN): This refers to galaxies with a supermassive black hole at their centre (some tens of billions of times more massive than our Sun!) that is eating up all the surrounding material, forms an accretion disk and becomes very luminous. Some AGN show two beams of particles (relativistic jets) moving almost at the speed of light.
  • Apastron: In a binary system, it refers to the point along the elliptical orbit of greatest separation between the two celestial objects.

Scheme of the most important phases of a binary system, in this case composed of a massive star (blue) and a black hole (black dot). Credit: CTAO.

  • Axion: Hypothetical particle that is thought to be the main component of dark matter.
  • Azimuth: Coordinate used to define the position of an object in the sky as seen by a person on Earth, and is the angular distance in the horizontal plane from North to the object’s position in the sky, measured clockwise in degrees. It is part of the Horizontal Coordinate System, along with the Elevation. Unlike the Equatorial Coordinate System, an object’s position in the Horizontal Coordinate system changes with time, as the object rises, crosses the sky, and sets.


Animation of an AGN where one of the relativistic jets – emitted from the supermassive black hole’s vicinity – is shown, as well as accretion disk around the black hole. Credit: CTAO, animation “CTAO Science: Emission to Discovery.”

  • Binary system: Association of two celestial objects attracted to each other by gravity, which orbit a common point. It can be formed by a star and a compact object (like a black hole or a neutron star), two compact objects, or by two stars.
  • Black hole (BH): The densest objects in the Universe, black holes are formed after the death of a very massive star (between 25 and 40 times more massive than our Sun) in a supernova explosion. Nothing can escape from these objects, not matter or light… that is why they are called “black.” If their mass is around thousands to billions of times larger than the Sun, they are formed differently, and are called supermassive black holes (SMBH).
  • Blazar: This is a type of AGN with its relativistic jets pointing almost directly towards Earth.
  • Bremsstrahlung: Physical process in which a particle with a charge (like an electron) emits light when being accelerated within an electric field.
  • Cataclysmic Variable: Type of binary system composed of a white dwarf and a star that undergoes sudden increases in its luminosity.
  • Charge: Physical property of matter, expressed through the force (attraction or repulsion) that it experiences when located in an electromagnetic field. It can be positive, like the charge in protons, or negative like in electrons. While particles with the same charge will be repulsed from one another, particles with opposite charges will be attracted to each other.
  • Cherenkov light: Bluish light produced when particles move faster than light can travel in a particular medium, such as the air in our atmosphere, glass or water. As nothing can travel faster than light in a vacuum, Cherenkov radiation cannot be produced in a vacuum.

Extended Air Shower created by a gamma ray. When the particles of the shower move faster than the speed of light in air, they produce the Cherenkov light that will eventually be detected by the telescopes on Earth’s surface. Credit: CTAO, animation “CTAO Science: Emission to Discovery.”

  • Cherenkov Telescope Array Consortium (CTAC): Group of hundreds of scientists from different countries that support the scientific programme of the Observatory. The CTAC works in close cooperation with the CTAO Project Scientist in the definition and progress of the Key Science Projects (KSPs). The KSPs, which include surveys of the sky and deep observations of key targets, are designed to efficiently address the broad-ranging science questions of the CTAO’s study themes.
  • Cherenkov Telescope Array Observatory (CTAO): The first open ground-based gamma-ray observatory and the largest and most sensitive instrument for gamma-ray astronomy at high and very high energies in the range between 20 GeV and 300 TeV. It will be composed of 64 telescopes spread between two arrays: one in the northern hemisphere (CTAO-North) and one in the southern hemisphere (CTAO-South). Additionally, the CTAO has the Headquarters in Italy and the Science Data Management Centre in Germany. The CTAO Headquarters in Bologna is the central office responsible for the overall administration of Observatory operations. The Science Data Management Centre will coordinate science operations and make the CTAO’s science products available to the worldwide community. More information in “About.” and “Locations.”
  • Circumstellar medium (CSM): The matter, such as gas and dust, which orbits a star.
  • Collection area: Area from which light can be detected by a particular telescope on the ground or in space.
  • Colliding-Wind Binary: Type of binary system formed by two massive stars (more massive than our Sun) that emit strong stellar winds. These winds interact and produce light at different energies, including gamma rays.
  • Compact object: Collective name for white dwarfs, neutron stars and black holes, given because they are the densest objects in the Universe. They are all outcomes of the death of stars.
  • Confidence Level (C.L.): This refers to the probability that a particular parameter falls inside a certain range – in other words, how confident we are about a given result. In astronomy, a 95% C.L. is typically used for example in upper limits of the gamma-ray flux (have a look to the definition of Upper limit).
  • Cosmic Rays (CRs): Energetic particles coming from space, outside our atmosphere. Despite their name, they are not rays, but particles: their composition is 99% protons and helium nuclei, while the remaining 1% are electrons, heavier nuclei, etc.
  • Cosmic Voids: Most of the matter in space is clustered, such as in galaxies or galaxy clusters. On the largest scales of the Universe, these form filaments like the strands of a spider’s web or a soap-bubble foam. The cosmic voids refer to the vast space between those filaments.
Image of the Cosmic Web

Image of the dark matter distribution from the Millennium Simulation. The filamentary structure of the Universe at large-scale is clearly visible, where every bright point represents a galaxy cluster and the space between the filaments corresponds to the cosmic voids. Credit: V. Springel, Max Planck Institute für Astrophysik.

  • Cosmos: Synonym of Universe that is typically used by astrophysicists to refer to the Universe as an orderly entity.
  • Crab Units (C.U.): This unit is related to the Crab Nebula, which is a very luminous well-known gamma-ray source with the steadiest emission. Astrophysicists measure the flux of other sources in the same energy range in terms of the Crab Nebula’s flux (the Crab Nebula’s flux corresponds to 1 C.U.).
  • CTAO ERIC: The CTAO ERIC (European Research Infrastructure Consortium) will be the final legal entity of the Observatory, responsible for the construction and operation of the CTAO.
  • CTAO gGmbH:The Cherenkov Telescope Array Observatory gGmbH (CTAO gGmbH) is the interim legal entity in the preparation of the design and implementation of the CTAO until the final legal entity, a European Research Infrastructure Consortium (ERIC), is achieved.
  • CTAO-North: The CTAO’s northern hemisphere array or CTAO-North is located on the island of La Palma in the Canary Islands (Spain) at the Instituto de Astrofísica de Canarias’ (IAC’s) Observatorio del Roque de los Muchachos in Villa de Garafía . At 2,200 metres in altitude and nestled on the slope below the rim of an extinct volcanic crater, the site currently hosts a prototype of one of the CTAO’s Large-Sized Telescopes, the LST-1. In addition to the telescope array, the CTAO will have an operations building and calibration devices on site, as well as an office at sea level. The sum of these facilities is known as the “Northern Station.” More information in “Locations.”
  • CTAO-South: CTAO’s southern hemisphere array or CTAO-South is located less than 10 km southeast of the European Southern Observatory’s (ESO’s) existing Paranal Observatory in the Atacama Desert in Chile. The telescope array along with the support facilities located in Santiago, Chile, is known as the “Southern Station.” More information in “Locations.”
  • CTAO Council: Group of shareholders that governs the CTAO gGmbH. It is composed of 11 countries, one intergovernmental organization (ESO) and two associate members (list of shareholders and representative institutes).

The CTAO will host two sites to observe the entire sky: one in the northern hemisphere (CTAO-North, top rendering) in the island of La Palma, Spain, and one in the southern hemisphere (CTAO-South, bottom rendering) in the Atacama Desert, Chile. Credit: Gabriel Pérez Diaz (IAC)/Marc-André Besel (CTAO). More images on our Flickr channel.

  • Dark matter: 85% of the Universe is estimated to be made up of this unknown type of matter. We know it exists because of its gravity effects on its surroundings (for example, the way galaxies move in the galaxies – the movement would be different without dark matter), but we still don’t know its nature. There are different theories to explain its nature, such as the WIMPs.
  • Declination (Dec): Similar to latitude on Earth, this is one of the coordinates used to define the position of an object in the sky. The declination is the angle from the celestial equator plane to the object in the sky: it is positive to the north and negative to the south, and is measured in degrees. It is part of the Equatorial Coordinate System, along with Right Ascension.
  • Differential flux: The amount of light that arrives at Earth per unit time, energy and area.
  • Dwarf Spheroidal Galaxy (dSph): This is a type of small-sized galaxy with low luminosity. These galaxies are thought to contain more dark matter than ‘ordinary’ matter.
  • Effective time: Total time that a source is observed and that leads to useful data (for example, the total time after removing data taken under poor weather conditions). Not to be confused with Observational Time.
  • Electric field: Electric fields are created by the presence of charged particles, and create an electrical force – attraction or repulsion between matter with charge.
  • Electromagnetic field: When they were first discovered, it was thought that electrical and magnetic fields were distinct, but in fact we now know that electric and magnetic fields are tangled, so we refer to an electromagnetic field. It propagates as a wave, called an electromagnetic wave or electromagnetic radiation (which we can see as light).

Did you know...?

You can find more information about the electric field, the magnetic field and the electromagnetic field, in these videos by Khan Academy.

  • Electromagnetic spectrum: Classification of electromagnetic radiation (light) according to its energy, frequency or wavelength. From less to more energetic, the spectrum spans from radio waves, microwaves, infrared and visible light, then passing through ultraviolet, X-rays and finally gamma rays.


You can find this image of the electromagnetic spectrum and more multimedia material on our website.

  • Electronvolt (eV): Unit of energy commonly used in astroparticle physics. Visible light has an energy of around 1 eV, but gamma rays are much more energetic. The gamma rays that the CTAO will observe have energies from billions (gigaelectronvolts, GeV) to trillions (hundreds of teraelectronvolts, TeV) the energy of the visible light!

Prefixes used in different physics units, for example with energy: 1 GeV (Gigaelectronvolt) = 109 eV (electronvolts).

  • Elevation: Coordinate used to define the position of an object in the sky as seen by a person on Earth, and is the vertical angular distance from the horizon to the object’s position in the sky, measured in degrees. It is part of the Horizontal Coordinate System, along with the Azimuth. Unlike the Equatorial Coordinate System, an object’s position in the Horizontal Coordinate system changes with time, as the object rises, crosses the sky, and sets.
  • Energy: Property that all objects in the Universe have, and it can be understood as the capability to perform an activity. Depending on the activity itself, the energy can be divided into different types, for example: kinetic energy (property of moving objects), rotational energy (property of rotating objects) or thermal energy (property of objects with temperatures above absolute zero). Energy is transferred from one object to another, sometimes changing type, because energy cannot be created or destroyed, only interchanged or transferred. One example of this is electrical energy being transformed into light and heat when a lightbulb is switched on.
  • ESCAPE: Alliance formed by several research infrastructures and organisations across astronomy, astrophysics, particle and nuclear physics, including the CTAO, to work together on common challenges to find solutions in the areas of large-scale data management, data processing and analysis in support for open data and research. ESCAPE is one of the five science clusters that exist as part of the European Open Science Cloud (EOSC).
  • Extensive Air Shower (EAS): Cascade of particles originating from the interaction of a cosmic or gamma ray with the molecules in the Earth’s atmosphere. When the shower is created by a gamma ray, it is called an electromagnetic cascade.
  • Extragalactic Background Light (EBL): Accumulated light in our Universe coming from different objects that spans almost all energies. This is important to gamma-ray astronomy because the EBL can interact with gamma rays (a process known as “pair production”), reducing the amount of gamma rays  – especially those at the highest energies – that make it to Earth.

Did you know...?

The light’s energy, frequency and wavelength are related one to another. The energy and frequency are directly proportional, which means that the higher the energy, the higher the frequency too. On the contrary, the wavelength is inversely proportional to the other two features: the electromagnetic radiation (light) with longer wavelength, will be less energetic and will have lower frequency. Gamma rays have very high energy and so, high frequency and short wavelength. Since they are all correlated… The electromagnetic spectrum can be shown using any of these three light’s properties!

  • Fermi: Satellite that observes high-energy gamma rays from space. It has two detectors on-board: the Large Area Telescope (LAT, which observes from 20 MeV to 300 GeV) and the Gamma-ray Burst Monitor (GBM, between ~10 keV to 25 MeV), which hunts Gamma-Ray Bursts.
  • Field of View (FoV): This refers to the area of sky visible to a particular telescope. For example, CTAO’s Small-Sized Telescopes will have a field of view of around 9 degrees, meaning they will observe a circular area of sky with a diameter of 9 degrees. For comparison, the full Moon has a diameter of half a degree as seen from Earth.
  • Flare: Sudden increase in the emission of a source – often very short in duration (seconds or days).
  • Flux: The amount of light that we observe from a source per time and surface unit – often given per square centimeter per second.
  • Frequency: Number of repetitions of a wave, like light or sound, per time unit. Normally, its units are Hertz (Hz), with one Hertz corresponding to one repetition per second. The shorter the wavelength of a wave, the higher its frequency and the higher the energy, so gamma rays have extremely high frequencies.
  • Galactic Centre (GC): It refers to the centre of our Galaxy, the Milky Way, which hosts a supermassive black hole named Sagittarius A* (Sgr A*).
  • Galactic Latitude (b): Coordinate used to define the position of an object in our Galaxy. It is measured in degrees and is the angle from the Galactic Centre to the object’s position in the sky, taken in an easterly direction along the Galactic Plane.  It is part of the Galactic Coordinate System, along with the Galactic Longitude.
  • Galactic Longitude (l): Coordinate used to define the position of an object in our Galaxy. It is measured in degrees and is the angle from the Galactic Plane to the object’s position in the sky: it is positive to the north and negative to the south. It is part of the Galactic Coordinate System, along with the Galactic Latitude.
  • Galactic Plane: This is the region in which most of the matter lies in our Galaxy, which is formed of a spherical central region surrounded by a disk of material containing spiral “arms.”

Our Galaxy, the Milky Way, from different perspectives. Credit: ESO, S. Brunier (picture) / NASA (right top artistic illustration)

  • Galaxy: A group of billions of stars, planets, dust and different objects bound together by the force of gravity. Our Galaxy is called the Milky Way.
  • Galaxy cluster: A group of hundreds of galaxies held together by gravity.
  • Gamma-ray binary: Binary system composed of a star and a compact object that emits most of its light in the gamma-ray energy range.
  • Gamma-Ray Burst (GRB): Extremely energetic explosions and the brightest events in our Universe! According to their duration, GRBs can be classified as “short” (if this initial prompt phase lasts less than a couple of seconds) or “long” (if it lasts more, up to hundreds or even thousands of seconds). The short bursts are produced by colliding neutron stars, and the most of the long bursts are thought to be created by particularly energetic supernova explosions, sometimes called “hypernovae.”

Animation of a neutron star merger, with the consequently production of gravitational waves (pale ripples) and a gamma-ray burst (GRB). The GRB creates two jets with particles moving almost at the speed of light, giving rise to a sudden emission of very high-energy gamma rays (magenta). Credit: NASA’s Goddard Space Flight Center.

  • Gamma ray: The most energetic light in the Universe! Gamma rays are the product of the most extreme events, such as exploding stars, due to the acceleration of cosmic particles or their interaction with the surrounding medium. Gamma rays cover a vast energy range, from approximately ~1 MeV and without a well-defined upper limit. They can be classified according to their energy range (see table below). So far, only gamma rays from the first three energy ranges have been detected, but scientists expect to see gamma rays with energies greater than hundreds TeV, considering the cosmic particle energies. As gamma rays cover a huge energy range – greater than that between microwaves and X-rays for example – we need to use different techniques to observe different energies. Three types of instruments can be used to cover high (HE) and very high (VHE) regimes: satellites (for MeV and GeV energies), Imaging Atmospheric Cherenkov Telescopes (for GeV and TeV energies) and Water Cherenkov detectors (above hundreds GeV). The Earth’s atmosphere prevents gamma rays from reaching the ground and hence, the latter two types of instruments perform indirect observations of gamma rays. In the case of IACTs, the information about gamma rays is obtained through the detection of Cherenkov light produced in the Extensive Air Shower (EAS). It is worth mentioning that the CTAO, thanks to its three types of telescopes (LST, MST and SST), will be able to observe uncharted energies (up to 300 TeV) with unprecedented precision, opening an entirely new window to the most extreme Universe.

Classification of the energy domain in gamma-ray astronomy. Energy ranges are approximate and might slightly differ from one publication to another.

Did you know...?

The noun and adjective of gamma rays are spelled differently. The noun is spelled as gamma rays (for example, “gamma rays are very energetic”), while the adjective is written as gamma-ray (for example, “gamma-ray astronomy is a young field”).


(Crédito animación: ESA/Hubble, M. Kornmesser)

  • Globular cluster: Densely-packed, spherical-shaped group of stars that orbits a galaxy.
  • Gravitational Waves: Disturbances or “ripples” in space-time due to extremely powerful events, such as the merger of two black holes. Gravitational waves propagate through space-time at the speed of light like the waves caused by a drop of water into a pool of water.
  • Gravity: Fundamental force that keeps matter together on large scales.
  • Hadrons: Subatomic particles (such as protons and pions) that are made up of elementary particles and are subject to the strong nuclear force. In gamma-ray astronomy, when scientists talk about “hadronic gamma-ray emission,” they refer to emissions produced by the interaction of this type of particles (like the interaction of protons or the decay of pions).
  • High Altitude Water Cherenkov Observatory (HAWC Observatory): Observatory with 300 water tanks for the observation of the very high-energy gamma-ray sky (between 100 GeV and 100 TeV), located in Mexico and inaugurated in 2015.
  • High Energy Stereoscopic System (H.E.S.S.): Array of five Cherenkov telescopes located in Namibia for the study of very high-energy gamma rays (between tens of GeV and tens of TeV) and inaugurated in 2002.
  • IceCube Neutrino Observatory: Observatory for neutrinos located in the Antarctic and formed by 5000 detectors situated under ground level inside 1 km² of ice.
  • Imaging Atmospheric Cherenkov Telescope (IACT): Instrument used on Earth to observe gamma rays indirectly. The telescope detects Cherenkov light, produced by the interaction of a gamma ray with Earth’s atmosphere. This light is then used to gain information about the gamma ray that produced it, such as its point of origin or its energy.
  • In-Kind Contributor (IKC): An In-Kind Contribution (IKC) teams is that who provide goods and services for the software and hardware development and construction of the CTAO. Of the many IKC teams involved in the Observatory, the three telescope collaborations are the most substantial: LST Collaboration, MST Collaboration and SST Collaboration.
  • Inferior conjunction: A binary system is in conjunction when the celestial objects that composed it are aligned as seen from Earth. The inferior conjunction means that one of the objects (typically the compact object, like a black hole) is in front of the companion according to our line of sight.
  • Infrared light or infrared radiation (IR): A type of light included in the electromagnetic spectrum that is less energetic than visible light. Most of the radiation emitted by bodies at room temperature (~27 °C) falls into the infrared range, including human bodies! Sub-classifications in the range include: Near-Infrared (NIR) and Far-Infrared (FIR).
  • Instrument Response Function (IRF): Group of characteristics that define the behavior of an instrument, such a Cherenkov telescope, and which must be considered during data analysis (one of them is, for example, the collection area of the instrument).
  • Integral flux: The amount of light per area and time integrated over a certain energy range.
  • Intergalactic magnetic field (IGMF): Magnetic field that spreads between galaxies.
  • Interstellar magnetic field (ISMF): Magnetic field that spreads between stars in a galaxy.

Did you know...?

Given that cosmic rays are charged particles (such as electrons and protons), they get randomly deflected in their travel through space by the IGMF and ISMF. That is why by observing directly cosmic rays, scientists are not able to identify the origin source from which they originate. To understand cosmic-ray sources, we need to study non-charged messengers produced by cosmic-ray interactions within the source, such as neutrinos and, of course, gamma rays! Nevertheless, the study of cosmic rays can lead to important information, such as their abundance in the Universe.


(Image credit: IceCube)

  • Interstellar Medium (ISM): All the matter and light that lie between the stars in a galaxy.
  • Inverse Compton (IC): Physical process in which electrons moving close to the speed of light transfer most of their energy to low-energy photons, turning them into gamma rays.
  • Jet: Collimated beam of relativistic matter (particles moving close to the speed of light) and electromagnetic radiation, typically associated with the accretion process and the creation of an accretion disk around the celestial object that is emitting the jet.
  • Key Science Project: Observational program with special importance due to the outstanding scientific outcomes expected.
  • Large-Sized Telescope (LST): The LST is the biggest of the three classes of telescopes that will compose the CTAO. It has a 23-m diameter dish, corresponding to a 400 square metres reflective area, composed of almost 200 segmented mirrors. Although it stands 45 metres tall and weighs around 100 tonnes, it is extremely nimble, with the ability to re-position within 20 seconds. Four LSTs will be built at the centre of the northern hemisphere array. The LSTs are ideal for catching gamma rays at the low edge of the CTAO energy range, and are responsible for CTAO sensitivity between 20 and 150 GeV.

The following video shows the construction of the LST prototype, the LST-1, on the CTAO-North site, located at the Roque de los Muchachos Observatory in La Palma (Canary Islands, Spain). Subscribe to our YouTube channel to follow our series of outreach and educational videos:

  • Large Magellanic Cloud (LMC): Small galaxy that orbits the Milky Way and that hosts important objects for gamma-ray astronomy as well as dense regions with molecular clouds. Due to its proximity and content, it is a very interesting target to search for very high-energy gamma-ray emitters.
  • Laser Interferometer Gravitational-wave Observatory (LIGO): Instrument dedicated to the observation of gravitational waves. There are two observatories in the United States: LIGO Livingston Observatory (in Louisiana) and LIGO Hanford Observatory (Washington).
  • Leptons: Elementary particles (such as electrons) that are not affected by the strong nuclear force. In gamma-ray astronomy, when scientists talk about “leptonic gamma-ray emission,” they refer to emissions produced by the interaction of this type of particles.
  • Light: Photons or waves with a certain energy or frequency, and synonymous with electromagnetic radiation. Sometimes this term is used to describe only “visible light” to distinguish it from other less or more energetic electromagnetic radiation.
  • Light curve: This is a plot that represents the flux of a source as a function of time. Very useful in order to study different behaviors over time, such as flares.
  • Light year: This is a unit of astronomical distance and corresponds to the distance traveled by light in space in one year. One light year is equivalent to ~9.5 trillion kilometers!
  • Line of sight: This is the straight line between the observer (for example, someone on Earth) and what they are looking at (for example, the Crab Nebula in the sky).
  • Lorentz Invariance Violation (LIV): The so-called Lorentz invariance tells us that the laws of the physics are invariant under transformation between two coordinate frames moving both at constant velocity. In other words, two people in different circumstances, moving at the same velocity one from another, would observe the laws of physics behaving equally. This concept is related to Einstein’s theory of Special Relativity and is one of the principles of modern physics… but could this principle be violated? Some theories predict that photons could be dispersed in space in different ways depending on their energy, which would imply a LIV. In order to test these ideas, very specific sources must be observed, which CTAO is planning to do!
  • Luminosity: Intrinsic characteristic of a celestial object that represents the amount of energy per time unit (for example, eV/second) that the object emits.
  • Magnetic field: Distribution of the magnetic force – attraction or repulsion of objects with the property of magnetism – in space-time. A magnetic field is created by the flow of charged particles.
  • Magnetism: Intrinsic property of some objects expressed through their interaction with magnetic fields that can change the direction of their movement. This property makes objects act as “magnets.”
  • Magnetosphere: Region around a celestial object dominated by its magnetic field.
  • Major Atmospheric Gamma Imaging Cherenkov (MAGIC): Two Cherenkov telescopes located in La Palma, Spain, for the study of very high-energy gamma rays (from ~30 GeV up to 100 TeV) and inaugurated in 2003.
  • Medium-Sized Telescope (MST): The MST is the second largest telescope among the three classes of telescope that will compose the CTAO. It has a ~12-m diameter dish and weights around 80 tons. One of its main features is its large field of view (~ 8 degrees), which will allow MSTs to perform fast surveys of the night sky. Nine MSTs are planned for the CTAO-North and 14 for CTAO-South. The MSTs are ideal for catching gamma rays in the middle of the CTAO’s energy range, and they are responsible for the CTAO sensitivity between 150 GeV and 5 TeV. There is an alternative prototype proposed with two mirror structure, named Schwarzschild-Couder Telescope (SCT).
Photo of MST prototype in Berlin

MST prototype installed in Berlin (Germany). More images on our Flickr channel.

  • Memorandum of Understanding (MoU): Agreement between different parties that includes the common line of action. It can also include the rights, requirements or responsibilities of each party.
  • Microquasar: System composed of a star and a black hole or neutron star. The latter gobbles up the matter of the star and can produce, on occasion, jets of radiation and particles moving almost at the speed of light!

Did you know...?

One of the most famous microquasars is Cygnus X-1, which caused Stephen Hawking to lose a bet with Kip Thorne. Do you know the story?


(Image credit: NASA/CXC/M.Weiss)

  • Microwave: Low-energy radiation of the electromagnetic spectrum (just above radio waves). This is extensively used in modern technology, including to warm up your food!
  • Monte Carlo (MC): Simulation of gamma or cosmic rays that is extremely important in gamma-ray astronomy. Monte Carlo simulations are needed to analyze data taken by telescopes (for example, with Cherenkov telescopes like those used by CTAO), which describe the simulated development of particle showers initiated by gamma or cosmic rays in our atmosphere, as well as the response of the telescope to those showers. They are named after the Monte Carlo casino in Monaco, since performing Monte Carlo simulations is like rolling the dice many, many times in a game of chance.
  • Multi-messenger astronomy: This is a sub-field in astronomy based on the observation of the Universe with instruments that detect different cosmic “messengers.” There are four possible messengers that are used at present: electromagnetic radiation (at its different energies, such as gamma rays), cosmic rays, neutrinos and gravitational waves. Each messenger can provide different information from the same source.
  • Multi-wavelength astronomy: This is a sub-field in astronomy based on the observations among different instruments that can detect different energy ranges of electromagnetic radiation. For example, observing a microquasar at the same time with radio observatories, infrared telescopes, X-ray satellites and gamma-ray Cherenkov telescopes can provide valuable information on the state of the source.
  • Neutrino: Elementary particle without charge that interacts extremely weakly with matter, making it hard (but not impossible!) to detect.
  • Neutron star (NS): Extremely small and dense star, formed after the death of a massive star (between ~10 and 25 times more massive than our Sun) in a supernova explosion.
  • Night Sky Background (NSB): Light coming from stars, airglow, polar and zodiacal light and artificial light that permeates the sky at night and which can affect astronomical observations.
  • Nova: Abrupt increase (over a few days) of the luminosity produced within a binary system composed of a white dwarf and a normal star. This type of increase can be recurrent, unlike the supernova explosions that only happen once and lead to the death of the star.
  • Observational time: Total time a celestial body is observed by an instrument, before applying any cut to the data during the analysis (which could reduce this time) during the analysis.
  • On-line analysis or Real-time analysis: Analysis performed by an automatic program simultaneously with the data collection of an instrument. This is very useful to check if a source’s activity has increased suddenly, which could lead to prolonging observations or even to issuing an alert to the astronomy community.
  • Pair production: Physical process in which a high-energy photon (such as a gamma ray) interacts with another low-energy photon (for example, an infrared photon) giving rise to the creation of an electron and a positron. In very high-energy gamma-ray astronomy, we say that this process is responsible for “gamma-ray absorption,” which basically ends up decreasing the number of gamma rays that reach Earth.
  • Parsec (pc): Unit of length typically used in astronomy to describe huge distances. One parsec is equivalent to 3.26 light-years or 30 trillion km!
  • Particle cascade or particle shower: Continue production of particles initiated by the interaction of a cosmic ray or a gamma ray with the molecule of a medium, such as air or water, and which ends when the particles reach a critical energy from which they cannot generate other particles. In the case of air, it is used as a synonym for extensive air cascade or EAS.
  • Periastron: In a binary system, it refers to the point along the elliptical orbit of closest approach between the two celestial objects.
  • PeVatron: It refers to a cosmic source capable of accelerating particles up to PeV (Petaelectronvolts, 1015 eV) energies. Gamma-ray emission from this type of sources, at energies of 100 TeV and above, is expected from the interaction of such high-energy particles.
  • Photomultiplier tube (PMT): Electronic device that transforms light into electrical pulses that can be analyzed. Used in several Cherenkov telescopes’ cameras.
  • Photon: Elementary particle that forms all electromagnetic radiation (the light).
  • Pion: Type of subatomic particle. There are three types according to their charge: positive pions (π +), negative pions (π ) and neutral pions (π 0).
  • Pion Decay: Over time, the pions can use their energy in creating new particles, which means they get transformed (a process named particle decay). This is an important physical process in astronomy, because charged pions give rise to neutrinos (among other particles), while neutral pions produce gamma rays.
  • Point Spread Function (PSF): This is the radius of a circle that contains a certain percentage of light emitted by a point source observed by an instrument. In very high-energy gamma-ray astronomy, if you read that a certain Cherenkov telescope has, for example, a PSF of 0.10 degrees above some energy, it means that most of the gamma rays (normally referred to 68% or 95%) from a point source at those energies, will be collected into a region of the sky with 0.10 degrees radius. Thus, the PSF is used to describe the angular resolution of the telescope: a point source does not have to be seen strictly as a point, but is seen as a “blur” dot of radius equal to the PSF (no object with smaller extension than the PSF will be resolved by the instrument).

Did you know...?

The smaller the PSF of an instrument the better. This is because with a smaller PSF, an instrument can disentangle smaller and finer details in the sky… so more information!


(Image credit: CTAO)

  • Positron: Antiparticle of the electron, which means that it has the same characteristics but with opposite charge (in this case, positive).
  • Proton: Subatomic particle with positive charge.
  • Pulsar: Extremely rapidly rotating and magnetized neutron star. It emits two beams of radiation, which can be detected from the radio up to even the gamma-ray band. However, it is like a cosmic lighthouse – it is only visible when passing through our line of sight!

Artistic animation of a pulsar, where its beams of light stand out. Credit: NASA

  • Pulsar Wind: Flow of particles emitted by pulsars into space from all round the pulsar (not to be mixed up with their light beams!).
  • Pulsar Wind Nebula (PWN): Astrophysical object produced when particles from the pulsar wind interact with the surrounding medium. This interaction gives rise to radiation, which can reach gamma-ray energies.
  • Quasar: Extremely luminous type of AGN, but where the jets are not pointing directly towards us.
  • Radiation or electromagnetic radiation: This is the emission of energy as waves or as particles (photons), which can propagate through space or interact with matter. Depending on the energy or frequency of the radiation, it will be called radio, microwave, infrared, visible, ultraviolet, X-rays or gamma rays. In daily life, radiation is just called light – although sometimes this term is reserved to talk about only visible light.
  • Radio waves: Radiation of the lowest energy. Typically used for daily communications.
  • Readout (electronics): Electronics responsible for saving Cherenkov telescope data (after the trigger said so!).
  • Region of Interest (RoI): In astronomy, this is a region of the sky or in an image on which the scientist wants to focus because of its physics importance.
  • Relativistic: Refers to something that moves close to the speed of light.
  • Right Ascension (RA): Similar to longitude on Earth, this is one of the coordinates used to define the position of an object in the sky. The Right Ascension is measured eastwards along the celestial equator plane and is the angle between the object in the sky and the point at which the Sun crosses the equator at the vernal equinox (around March 20th). It is measured in hours and minutes or sometimes degrees, with one hour being equal to 15 degrees. It is part of the Equatorial Coordinate System, along with Declination.
  • Satellite: Instrument placed in orbit for different purposes, such as the observation of Earth or outer space, communication and navigation goals, etc. Satellites for the observation of outer space are also called space satellites or space observatories.
  • Sensitivity: Minimum amount of light that an instrument can reliably detect. Its value can differ depending on the energy range and so, instruments can show different sensitivity values for each energy range, which is represented in a “sensitivity curve.”

Did you know...?

The lower the sensitivity’s value, the better the instrument! For example, if we have two instruments, one with a sensitivity of 0.6 C.U. (have a look to the definition of Crab Units) and another with 0.4 C.U. for a certain energy range, it means that the first instrument will be able to detect sources whose flux is above 0.6 C.U., while the second will do so for sources with flux already above 0.4 C.U. So the second can detect even weaker sources, which is definitely better.


(More information in “Performance”)

The CTAO-North and CTAO-South sensitivities compared to currently operative instruments. The plot shows, from one side, the different sensitivity vales for each energy range (which leads to a sensitivity curve) and, from the other side, that the CTAO will have much better sensitivity than any other instrument with same or even less observation time.

  • Silicon photomultiplier (SiPM): Device that detects light and converts it into an electrical signal to analyze it. An alternative to the photomultiplier tubes used in some CTAO cameras.
  • SKAO: The Square Kilometre Array Observatory (SKAO) is a next-generation radio astronomy-driven Big Data facility that will revolutionise the understanding of the Universe at the radio wavelengths.
  • Small-Sized Telescope (SST): The SST is the smallest telescope among the three classes of telescope that will compose the CTAO, with a ~4-m diameter primary dish. It is a dual-mirror telescope, a segmented primary mirror and a monolithic secondary one. A total of 37 SSTs are planned only for the CTAO-South array. The SSTs are ideal for catching gamma rays at the highest energies of CTAO’s energy range, and they are in charge of the CTAO sensitivity between 5 and 300 TeV.

SST prototype installed in Mount Etna (Catania, Italy). More images on our Flickr channel.

  • Space: Existing expanse between the objects in the Universe. Although almost entirely empty, it is not completely so! Between bodies in the Cosmos there are cosmic rays, electromagnetic radiation, magnetic fields, etc.
  • Spectral energy distribution (SED): Relative contribution of each wavelength to the total energy released by the source.
  • Speed of light (c): It typically refers to the velocity of the light in a vacuum, which is the highest possible speed in the Universe: 300,000 km/s! Nothing can travel faster than the speed of light in a vacuum, but in other mediums (like the air, glass or water), the light slows down and high-energy particles can exceed the velocity of light in those mediums – this is the principle of Cherenkov light.  Speed of light in a vacuum is usually denoted with the letter c, and the velocity of particles can be given with respect to it (for example, 0.95c corresponds to 95% of the speed of light).
  • Star-forming region: Regions of dense material in space containing molecular clouds where the formation of stars is likely to occur.
  • Steady (source): Refers to a celestial object with emission that does not change significantly over time.
  • Stellar wind: Flow of matter ejected from the atmosphere of a star.
  • Superior conjunction: A binary system is in conjunction when the celestial objects that composed it are aligned as seen from Earth. The superior conjunction means that one of the objects (typically the compact object, like a black hole) is behind the companion according to our line of sight.
  • Supermassive Black Hole: A black hole with a mass several billion times bigger than the Sun. They are found at the centres of galaxies, including the Milky Way. Astronomers are not sure exactly how they are formed.
  • Supernova (SN): Powerful explosion that happens upon the death of a massive star. One of the most energetic events in the Universe – in one moment, it can release more energy than the energy released by our Sun over its entire life!
  • Supernova remnant (SNR): Astrophysical object produced by the interaction of the matter expelled in a supernova explosion with the matter of the surrounding medium. This interaction gives rise to light, which can reach gamma-ray energies. They are very important sources in very high-energy astrophysics, because SNRs are expected to accelerate particles up to PeV energies, it means they are thought to be PeVatrons. More information in our page “Science.”
  • Synchrotron: Physical process in which a charged particle (like an electron) moving at relativistic speeds emits light due to the presence of a magnetic field.
  • Synchrotron Self Compton (SSC): This is Inverse Compton radiation in which the low-energy photon that hits the electron originates from a Synchrotron process from the same electron population.

Animation of a supernova explosion, the death of a massive star. The material expelled in the explosion interacts with the surrounding medium, giving rise to a new astrophysical source, known as Supernova Remnant (SNR). Credit: NASA.

  • Target of Opportunity (ToO): When a celestial object becomes extremely interesting, perhaps due to a major flare, astronomers will declare it a “Target of Opportunity.” This usually affects the observation program: other planned observations could be removed or postponed from the program so that the telescopes can focus on this objective, as its state does not usually last long.
  • Transient: Event in the Cosmos with a limited duration (seconds, days, weeks, etc.).
  • Trigger (electronics): Electronics inside the Cherenkov telescopes that evaluate the incoming light (typically to distinguish between an interesting event we want to keep, such as Cherenkov light produced by a gamma ray and events we want to discard, such as those produced by cosmic rays, NSB…) and decide if it should be recorded.
  • Ultraviolet (UV): Radiation or light that is a bit more energetic than visible light. The Sun is our main source of UV light.
  • Upper limit (U.L.): In high-energy astronomy, it typically refers to a maximum value in the gamma-ray flux  that is calculated when a source is not detected. We know that this source cannot emit an amount of gamma rays above the computed upper limit, otherwise our instrument would have detected it. It is usually indicated at a 95% C.L. (Confidence Level), which means that in 95% of the cases an hypothetical gamma-ray emission from the source will be below the U.L.
  • Variability: Refers to the property of some astrophysical objects whose emission fluctuates at certain time-scales. The variability can have a predictable modulation (if we can foresee it because it is produced constantly over time) or unpredictable (if we cannot foresee the increase or decrease of emission). We can therefore say that an object has a periodic variability or not, respectively.

Did you know...?

Binary systems can show different types of variability, such as the orbital variability: a predictable modulation in the gamma-ray emission produced by the change of the objects’ positions along the orbit. For example, given our line of sight, the star can eclipse the compact object (from whose vicinity gamma rays are emitted), reducing this way the gamma rays that reach Earth. In other cases, the collision of both objects’ emissions intensifies at their closest orbital point, leading to an increased gamma-ray flux.


(Animation: Binary system composed of a neutron star (magenta) and a massive star (blue). The neutron star accelerates high-energy electrons, and when they collide with star’s light  as it passes the periastron, there is an increase of gamma-ray luminosity. Credit: NASA’s Goddard Space Flight Center)

  • Very Energetic Radiation Imaging Telescope Array System (VERITAS): Group of four Cherenkov telescopes located in Arizona (US) for the study of very high-energy gamma rays (between 100 GeV and 10 TeV) and inaugurated in 2005.
  • VIRGO: Instrument to detect gravitational waves, located in Pisa (Italy) and inaugurated in 2003.
  • Visible light: Refers to the light that we can see with our naked eyes.
  • Water Cherenkov Detector (WCD): Technique used to detect gamma and cosmic rays, which detects the Cherenkov light produced as a consequence of the very high-energy particles from particle showers hitting a tank of water equipped with detector devices (such as photomultiplier tubes).
  • Wavelength: Distance between the peaks of the light wave – the larger the distance, the longer the wavelength and the less energetic the light (for example, gamma rays have a very short wavelength).
  • Weakly Interacting Massive Particle (WIMP): Hypothetical particle that is thought to be the main component of dark matter. The WIMP model has undoubtedly been the preferred and most studied model by a large part of the astronomical community dedicated to dark matter, since its properties manage to “naturally” explain all the content of dark matter in the Universe. This is known as the “WIMP miracle.” The WIMPs have the peculiarity that they are their own antiparticles, so when two WIMPs meet they annihilate themselves, giving rise to known particles that in turn are annihilated into gamma rays. This allows searching for WIMPs with Cherenkov telescopes like the CTAO!
  • White Dwarf: Final evolutionary state of a star, which was not massive enough to die in a supernova explosion. Our Sun will likely end its life as a white dwarf.
  • X-ray binary: Binary system composed of a star and a compact object that emits most of its light in the X-ray energy range. According to the mass of the star, the system can be classified as Low-Mass X-ray Binary (LMXB) or High-Mass X-ray Binary (HMXB).
  • X-rays: Radiation that, according to its energy, preceeds gamma rays in the electromagnetic spectrum. In daily life, X-rays are used in medical radiography or airport security scanners.
  • Zenith: Imaginary point in the sky vertically above a certain location, such as above our telescopes.
  • Zenith angle: Refers to the separation in degrees from the zenith to a particular object’s position in the sky.

(Last Updated: July 2023)