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# Glossary

absolute magnitude
  (Symbol, M.) A numerical measure of the intrinsic brightness of a star, equal to the apparent magnitude (m) the star would have if observed from a standard distance of 10 parsecs, in the absence of interstellar absorption. The absolute magnitude provides a measure of the star’s luminosity.

absorption
  The process by which a photon is absorbed by an ion, atom or molecule resulting in an increase in energy of the ion, atom or molecule.

angular separations
  How far apart two astronomical objects appear to be, calculated as the difference between the coordinates of their positions on the sky.

angular size
  How big an astronomical object appears to be, calculated as the difference between the coordinates of the extremities of the extended object.

apparent magnitude
  (Symbol, m.) A numerical measure of the apparent brightness of a body. For a star, it is a measure of the flux received.

asteroseismology
  The study of stellar interiors from observations of global oscillations of their photospheres.

Balmer series
  The series of electronic transitions in the hydrogen atom that involve a change to or from the n = 2 state, associated with spectral lines mostly in the visible.

bar
  An approximately linear extension to the central bulge seen in some spiral galaxies, from which the spiral arms emanate.

binary stars
  A situation where two stars orbit each other.

black holes
  A region of space in which matter is sufficiently dense that the action of gravity prevents both matter and electromagnetic radiation from escaping. Black holes may be classified as either ‘stellar mass’ (up to tens of solar masses) or as ‘supermassive’ (in excess of 106 solar masses) and characterised further by their Schwarzschild radius.

bulge
  The region around the centre of a spiral galaxy, where the galaxy is thicker and brighter and the concentration of matter is greater than elsewhere. Its outer parts are dominated by the light of old stars, but towards the centre it may contain material associated with the inner parts of the disc, including sites of star formation.

colour
  The difference, measured in magnitudes, of the brightness of an object in two specified wavebands (e.g. in the blue ‘B’ and red ‘R’ wavebands, in which case the difference is denoted mB – mR or simply B – R). Sometimes referred to as colour index.

colour images
  Images that incorporate information from multiple telescope filters, so that variation in colour represents the relative variation in the proportions of emitted light at different wavelengths.

computer simulations
  Numerical models within a computer used to model star or galaxy formation.

convection
  A process of energy transfer in which a fluid in a gravitational field is heated from below to the point where the hotter, less dense fluid rises upwards, displacing the cooler, denser fluid downwards.

cosmic web
  The largest-scale strucure of galaxy clusters and filaments comprising the Universe.

deep images
  Images that are sensitive to the very faintest emission that our telescopes can detect. The ‘deepest’ existing astronomical images are produced using very long exposures by space-based or very large ground-based professional observatories.

degrees
  Units of angular measure. There are 360 in a complete circle.

disc
  The flat, circular region in a spiral or lenticular galaxy in which most of the ongoing star formation is located, including spiral arms, if present. Disc-shaped structures are common in astronomy, also existing around protostars, and sometimes around stellar remnants.

Doppler effect
  The effect whereby wavelengths are shifted to longer or shorter values as a result of relative movement between the source and the observer.

dust
  Small solid particles, around 10-7 or 10-6 m across, found mixed with interstellar gas. Dust grains are predominantly composed of carbonaceous material and silicates, but may be surrounded by an icy mantle. Dust is very effective at absorbing and scattering ultraviolet and visible light.

electromagnetic radiation
  A form of radiation in which the transfer of energy from place to place may be attributed to the passage of wave-like disturbances in the electric and magnetic fields between those places (i.e. electromagnetic waves), or to the passage of photons from one place to the other. The wavelength or frequency of the waves (or the energy of the photons) may be used to assign particular kinds of electromagnetic radiation to specific parts of the electromagnetic spectrum, resulting in their classification as light, radio waves, X-rays, etc.

emission line
  A narrow wavelength or frequency range in a spectrum where the spectral flux density is greater than at adjacent wavelengths (or frequencies).

extended
  An astronomical source of light whose angular size is larger than the point-spread function of a particular telescope.

far-infrared
  The longest wavelength part of the infrared region of the electromagnetic spectrum. Far-infrared emission traces dusty regions, including planet-forming discs and distant protogalaxies.

filaments
  In the context of star formation, a filament is a linear region of denser material. They result from the partial collapse of a molecular cloud. Material streams along them and stars typically form at their intersections. In the context of galaxy structures, filaments are thread-like structures comprising hundreds or thousands of gravitationally bound galaxies as part of the cosmic web.

flux
  (Symbol, F.) A quantity describing the rate at which energy transferred by radiation is received from a source, per unit area facing the source. The SI unit of flux is the watt per square metre (W m−2).

Galactic coordinates
  A celestial coordinate system, whose two elements are Galactic longitude l, and Galactic latitude b, resembling longitude and latitude on the Earth. The orientation of the coordinate system is defined to make it useful for describing the locations of objects in the Galaxy from the viewpoint of the Sun. The Galactic equator is chosen to coincide with the Galactic plane, the direction (l, b) = (0°, 0°) is in the direction of the Galactic centre.

galaxy
  A collection of luminous stars, non-luminous dark matter, and in the case of spiral and irregular galaxies some amount of gas and dust that are gravitationally bound to one another, and are separated from other similar structures usually by distances of tens of kiloparsecs or more. Various categories of galaxies may be defined based on their appearance, such as spiral galaxies (barred or normal), elliptical galaxies, lenticular galaxies and irregular galaxies.

galaxy clusters
  A collection of hundreds or thousands of galaxies that are bound together by gravity.

gamma ray
  The highest-energy form of electromagnetic radiation, produced in energetic processes like nuclear fusion.

globular cluster
  Clusters of 105 to 107 very old stars, tightly bound by gravity into a spherical region of space less than about 50 pc in diameter. The 150 or so globular clusters associated with the Milky Way are found in a spherical distribution about the centre of our Galaxy. Similar distributions are seen in other galaxies.

Hubble–Lemaître law
  A relationship between the apparent speed at which a galaxy is receding from us and its distance away.

inclinations
  The angle between the plane of an orbital system and the plane of the sky.

jets
  A long narrow linear feature emanating from an active galactic nucleus, seen in many radio galaxies and quasars, most often via its radio synchrotron emission but sometimes also at optical and X-ray wavelengths. Jets are also present in a small number of X-ray binaries and believed to be present in gamma ray bursts.

light curves
  A diagram showing the variation of brightness (e.g. magnitude, flux or luminosity) with time, for a celestial object.

luminosity
  (Symbol, L.) A quantity describing the rate at which energy is carried away from a luminous object by electromagnetic radiation. The SI unit of luminosity is the watt (W), where 1 W = 1 J s−1.

magnetic fields
  The quantity, specified throughout some region of space, that determines the magnetic force that would act on a particle of given electric charge moving with given velocity through any point within that region. At each point in the region, the magnetic field possesses a strength and a direction. The magnetic field in a region may be produced by magnets or by moving charged particles (e.g. electric currents), but is deemed to exist irrespective of whether there are any other moving charged particles or magnets present to ‘feel’ its effect.

multiwavelength astronomy
  Astronomy that makes use of images, spectra and light-curves made in multiple regions of the electromagnetic spectrum.

narrow-band image
  An image made using a narrow-band telescope filter through which only light within a small range of wavelength can pass.

near-infrared
  The shortest wavelength part of the infrared region of the electromagnetic spectrum.

nebulae
  Extended astronomical objects having a fuzzy or cloud-like appearance in small telescopes.

neutral
  With reference to an atom or molecule: not ionised; having no net electric charge.

open cluster
  Clusters of up to a few thousand stars, loosely bound by gravity, that were formed from the same giant molecular cloud and so have roughly the same age. There are more than 1000 open clusters in the Milky Way, confined to the disc of the galaxy.

parallax
  The phenomenon whereby the apparent position of an object changes when seen from different viewpoints.

parsecs
  The distance to a celestial body that has an annual parallax of one arcsecond: 1 pc = 3.09 × 1013 km, or just over 3 light-years.

photometric apertures
  A region (often circular) superimposed on an astronomical image within which the brightness values of individual pixels are added together in order to determine the total brightness of a particular astronomical object.

photometry
  The science of the measurement of the brightness (magnitude, flux or luminosity) of astronomical objects.

photons
  The particle of electromagnetic radiation in the photon model of light. The photon energy, E, is proportional to the frequency, f, of the associated radiation: E = hf, where h is Planck’s constant.

pixel scale
  The conversion factor between image pixels and angular separation for images from a particular telescope or instrument set-up.

planetary nebulae
  The gaseous remains of the outer envelope of a low-mass star, which is illuminated by a luminous central white dwarf.

point-like
  An astronomical source of light whose angular size is smaller than the point-spread function of a particular telescope.

point-spread function (PSF)
  A mathematical description of the blurring effect caused by the telescope optics and atmospheric effects, so that light from a point-like source such as a star is spread over multiple pixels of an image.

projection
  The shape that a 3-dimensional object produces when viewed in a 2-dimensional image, which may depend on its orientation.

protostar
  A molecular cloud that has begun the process of collapsing into a star and has become opaque to radiation but which has not yet started nuclear fusion.

radians
  Unit of angular measure. There are radians in a complete circle.

radio astronomy
  Astronomy carried out using images and spectra in the radio part of the electromagnetic spectrum, consisting of radiation with the lowest frequencies/longest wavelengths.

red giant
  A large star with a photospheric temperature less than about 6000 K. Main sequence stars with masses of less than about 11 M⊙ evolve to become red giants.

redshift
  The increase in the observed wavelength of radiation relative to the wavelength at which the radiation was emitted (the frequency is correspondingly decreased). This can be due to either the Doppler effect arising from motion through space or the expansion of the Universe.

resolution
  A measure of the smallest angular separation for which a particular telescope can distinguish two separate sources of light without them blurring together.

resolved
  Indicating that a particular telescope can distinguish two separate sources of light without them blurring together.

right ascension
  In the equatorial coordinate system, one of the coordinates used to define the positions of objects on the celestial sphere, analogous to terrestrial longitude. Often abbreviated to RA. It is measured in hours, minutes and seconds such that 1 hour of RA is equal to 15 degrees along the celestial equator.

seeing
  The effect of atmospheric blurring which often characterises the resolution limit of ground-based astronomical telescopes.

semimajor axis
  A distance equal to half the longest axis of an ellipse or elliptical orbit.

sky maps
  Either an image or plot of catalogued positions that covers a large area of the celestial sphere.

spatial resolution
  A measure of the ability of a telescope to distinguish between closely spaced features in an image.

spectroscopy
  The range of electromagnetic radiation that is emitted, absorbed or reflected by an object, dispersed according to its constituent wavelengths, frequencies or energies is called a spectrum (plural spectra). The study of such spectra is referred to as spectroscopy.

spiral galaxy
  A member of the Hubble class of galaxies that is characterised by having a disc and spiral arms. Membership of this class is indicated by the letter S, or SB in the case of a barred spiral galaxy.

stars
  A ball of matter which, at some stage in its life, is self-luminous as a result of energy released by nuclear reactions in its core.

star cluster
  A group of stars with more than a few members in a relatively small volume of space.

star-forming regions
  A region of gas within the interstellar medium of a galaxy in which stars are in the process of forming.

supergiant
  A star that lies along the top of the Hertzsprung–Russell diagram, i.e. the stars with the greatest luminosity. Main sequence stars with masses greater than about 11 M⊙ evolve to become supergiants. Later, such stars become Type II supernovae.

supernova remnant
  The gaseous remains of a stellar explosion, which may be caused by collapse of a massive star or thermonuclear explosion of a white dwarf. The explosion sends a shock wave into the interstellar medium, causing it to radiate in multiple parts of the electromagnetic spectrum. Often abbreviated to SNR.

surface brightness
  A quantity that describes the brightness at a particular position on an extended object, such as a galaxy. The surface brightness at any chosen point is the amount of radiant flux that would reach 1 m2 at Earth from a small, uniformly bright, square region, of angular area 1 arcsec2, surrounding the chosen point. A common SI unit of surface brightness is W m−2 arcsec−2.

surface brightness profiles
  Plots of the surface brightness of extended astronomical objects as a function of radial distance from its centre.

time-domain surveys
  An astronomical survey that records images or other measurements at the same locations at multiple points in time, so as to record information about how positions and brightness of objects change.

unresolved
  In relation to an astronomical image, an object whose light is contained within a region smaller than the size of details that may be distinguished.

voids
  Under-dense regions of the cosmic web between the filaments and galaxy clusters.

wavelength
  The distance over which a periodic wave repeats itself, e.g. the distance from one peak of the wave to the next.

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