Dictionary of the Extreme Universe

Did we miss a word?

Contact the CTAO Outreach and Education Coordinator,

Alba Fernández-Barral.

  • 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 orbital 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 the center of some galaxies which contain a supermassive black hole (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.
  • 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.
AGN with two relativistic jets ejected from the black hole’s vicinity and an accretion disk swirling around it. Credit: W. Steffen, UNAM
  • 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: MAGIC Collaboration

  • Cherenkov Telescope Array (CTA): The observatory for ground-based gamma-ray astronomy at high and very high energies in the range between 20 GeV and 300 TeV. It will be composed of 118 telescopes spread between two arrays: 19 telescopes in the northern hemisphere (CTA-North) and 99 in the southern hemisphere (CTA-South).
  • Circumstellar medium (CSM): The matter, such as gas and dust, which orbits a star.
  • Collection area: Area from which light can be detected for study 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 flux, meaning that if you measure the flux of a source 100 times, at least 95 times it will be below the upper limit.
  • Cosmic Microwave Background (CMB): Leftover radiation from the Big Bang that fills space, with energies in the microwave band.
  • Cosmic Rays (CRs): Energetic particles coming from 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.).
  • CTA Consortium (CTAC): Group of more than 1,500 members from 31 countries responsible for directing the science goals of the observatory and involved in the array design and supplying components.
  • CTA Council: Group of shareholders that governs the CTAO. It is composed of 11 countries, one intergovernmental organization (ESO) and two associate members (list of shareholders and representative institutes).
  • CTA Observatory (CTAO): CTAO is the entity responsible for the preparation of the implementation of the overall CTA project.
  • CTA-North: This refers to the CTA array site in the northern hemisphere, located at the Instituto de Astrofísica de Canarias’s (IAC’s) El Roque de los Muchachos Observatory (La Palma, Spain).
  • CTA-South: This refers to the CTA array site in the southern hemisphere, located near the European Southern Observatory’s Paranal Observatory (Atacama Desert, Chile).
  • Dark matter: 85% of the Universe is estimated to be made up of this unknown type of matter. We know it exists because of the way galaxies move, but we still don’t know its nature.
  • 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.
  • 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).
  • Electric field: Electric fields are created by the presence of charged particles, and create an electrical force – attraction or repulsion between matter with charge.

did you know...?

If you want to learn more, we suggest the following videos by Khan Academy about the electric and magnetic fields, as well as the electromagnetic field, respectively: https://bit.ly/2DnXRYl, https://bit.ly/2mrkGjR and https://bit.ly/2q7CSou.

  • Electromagnetic spectrum: Organization of electromagnetic radiation (light) usually 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.
  • 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 CTA 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 keV (kiloelectronvolt) = 10³ eV (electronvolt)

  • 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.
  • Extensive Air Shower (EAS): Cascade of particles originating from the interaction of a cosmic or gamma ray with the nuclei 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 that make it to Earth.

  • Fermi: Satellite that observes high-energy gamma rays from space. It has two detectors on-board: its principal instruments are 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, CTA’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 Center (GC): 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: 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). CTA is the next generation of Imaging Atmospheric Cherenkov Telescopes.

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...?

Note that 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”).

  • Gamma-ray binary: Binary system 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”.
  • 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
  • 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: 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.
  • 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 cosmic rays, scientists are not able to identify the source from which they originate. To understand cosmic-ray sources, we need to study non-charged particles from the source’s cosmic rays, such as neutrinos and, of course, gamma rays! Nevertheless, the study of cosmic rays can lead to important information, such as their abundance in our Universe.

  • 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: Beam of matter that is ejected from certain celestial objects.
  • Key Science Project: Program with special importance due to the outstanding scientific outcomes expected.
  • Large-Sized Telescope (LST): With a 23-meter diameter mirror, this is the biggest of the three classes of telescopes that will compose CTA. There will be four at the centre of each array site. The LSTs are ideal for catching gamma rays in the low edge of the CTA energy range, between 20 and 150 GeV.
  • 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).
  • Light: Photons or waves with a certain energy or frequency, and synonymous with 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. Very specific sources must be observed to test these ideas, which CTA 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.
  • 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.
  • 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 that can change the direction of their movement.
  • Magnetosphere: Region around a celestial object dominated by its magnetic field.
  • Medium-Sized Telescope (MST): With a ~10-m diameter mirror, this is the second largest telescope among the three classes of telescope that will compose CTA. A total of 15 MSTs are planned for CTA-North and 25 for CTA-South. The MSTs are ideal for catching gamma rays in the middle of CTA’s energy range, between 150 GeV and 5 TeV.
  • 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? Have a look in https://bit.ly/2SrDG6u.

  • 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 CTA), 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 “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 observations: Coordinated 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.
  • 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.
  • 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.
  • 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.
  • On-line 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.
  • Observational time: Total time a celestial object is observed by an instrument, regardless of whether all that time is useful or not for a proper analysis.
  • Particle shower or particle cascade: Cascade of particles produced by the interaction of a cosmic ray or gamma ray with nuclei in a medium, such as air or water. In the case of air, it can be used as a synonym for Extensive Air Shower.
  • 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!
  • Photon: Elementary particle that forms all electromagnetic radiation (the light).
  • Photomultiplier tube (PMT): Electronic device that transforms light into electrical pulses that can be analyzed. Used in several Cherenkov telescopes’ cameras.
  • Pion: Type of subatomic particle. There are three types according to their charge: positive (Π+,), negative (Π¯) or neutral (Πº).
  • 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 (Π+ and Π¯) give rise to neutrinos (among other particles), while neutral pions (Πº) produce gamma rays.
  • Point Spread Function (PSF): This is the radius of a circle that contains a certain percentage of light. In very high-energy gamma-ray astronomy, if you read that a certain telescope has a PSF of 0.10 degrees above some energy, it means that most of the gamma-like events from a point source (normally 68% or 95% are used) will be collected into a region of the sky with 0.10 degrees radius.

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!

  • Positron: Antiparticle of the electron, which means that it has the same characteristics but with opposite charge (in this case, positive).
  • 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 particles of the surrounding medium. This interaction gives rise to radiation, which can reach gamma-ray energies.
  • Proton: Subatomic particle with positive charge.
  • 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: Radiation of the lowest energy. Typically used for daily communications.
  • Readout (electronics): Electronics responsible for saving 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 and whose value can differ depending on the energy range.

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.

  • 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 CTA cameras.
  • Small-Sized Telescope (SST): With a ~4-m diameter mirror, this is the smallest telescope among the three classes of telescope that will compose CTA. Seventy SSTs are planned only for the CTA-South array. The SSTs are ideal for catching gamma rays at the highest energies of CTA’s energy range, between 5 and 300 TeV.
  • 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: Velocity of the light in a vacuum, which is around 300,000,000 m/s! Usually denoted with the letter c, the velocity of particles can be given with respect to it.
  • Steady (source): Refers to a celestial object with emission that does not change significantly over time.
  • Star-forming region: Regions of dense material in space containing molecular clouds where the formation of stars is likely to occur.
  • Stellar wind: Flow of matter ejected from the atmosphere of a star.
  • Supermassive Black Hole: A black hole with a mass several billion times bigger than the Sun. They are found at the centers 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.
  • 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.
  • 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 means that other observations that have been planned are removed from the schedule so that telescopes can be concentrated on the flaring object, 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 telescopes that evaluate the incoming light and decide if it should be recorded. “This is Cherenkov radiation, this is not, this is not, wait, here is some more…”.
  • Ultraviolet (UV): Radiation or light that is a bit more energetic than visible light. The Sun is our main source of UV light.
  • Variability: Refers to the property of some astrophysical objects that emit radiation that fluctuates at certain time-scales.

Did you know...?

An example of variability in astrophysical sources? In binary systems, there is the so-called orbital variability produced when the star eclipses the companion and the light cannot reach us in the same way. This is a constant variability because the flux (amount of light) increases and decreases according to the orbital movement.

  • 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.
  • 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 preceding 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: October 2019)