Astronomy glossary
The 51 terms used across Deep Sky Monitor, each in a sentence. On the site's pages they are underlined with dots: point at one (or tap it) for the same explanation. Explain mode can be switched off in Settings.
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90% region
The patch of sky that holds the source with 90% probability. Gravitational-wave regions can cover hundreds of square degrees.
A
Afterglow
The fading X-ray, light and radio glow after a gamma-ray burst, as its jet ploughs into surrounding gas.
Arcminute (′)
A sixtieth of a degree. The full Moon is about 30 arcminutes across.
Arcsecond (″)
A sixtieth of an arcminute: about the size of a coin 4 km away.
B
Black hole
A region where gravity is so strong that nothing, not even light, escapes. Seen through the hot gas and jets around it.
Blazar
A galaxy's central black hole firing a jet almost straight at us, so it looks very bright and flickers.
C
Characteristic age
A pulsar's age estimated from its period and spin-down (P / 2Ṗ). Rough, but usually the right order of magnitude.
Constellation
One of the 88 official regions the sky is divided into, each named after a star pattern.
D
Declination (Dec)
Position on the sky north-south, like latitude: +90° at the north celestial pole, −90° at the south.
Dispersion measure (DM)
How much a radio signal's low frequencies are delayed by free electrons on the way to us, in pc/cm³. More electrons crossed usually means farther away.
E
Equilibrium temperature
A planet's temperature from starlight alone, without any atmosphere warming it (Earth's is −18 °C).
Exoplanet
A planet orbiting a star other than the Sun. Over 6,000 are confirmed.
F
False alarm rate
How often noise alone would produce a signal at least this strong. Once in thousands of years means the event is almost certainly real.
Fast radio burst (FRB)
A radio flash lasting milliseconds from another galaxy. Some sources repeat; their cause is still being worked out.
Field of view
How much sky a telescope, camera or eyepiece shows at once, as an angle.
Fluence
The total energy received from a short burst per unit area, here in jansky-milliseconds (Jy ms).
G
Galactic latitude
Angle above or below the plane of the Milky Way; near 0° you look along the disc, full of stars and dust.
Gamma-ray burst (GRB)
The brightest explosions known: a collapsing giant star or merging neutron stars launching a jet towards us.
Gravitational wave
A ripple in space itself from two black holes or neutron stars merging, measured by LIGO, Virgo and KAGRA.
H
Habitable zone
The range of distances from a star where a rocky planet with an atmosphere could keep liquid water on its surface.
I
Infrared
Light just redder than we can see. It passes through dust, and the most distant galaxies are seen mostly in it.
J
J2000
The standard sky coordinate grid as it was on 1 January 2000. The grid slowly shifts as Earth's axis wobbles, so a date is needed.
K
Kilonova
The glow of radioactive debris from two merging neutron stars, where heavy elements such as gold are made.
Kiloparsec (kpc)
A thousand parsecs: about 3,260 light-years. Used for distances within the Milky Way.
L
Light curve
Brightness plotted against time: how a supernova rises and fades, or a variable star changes.
Light-year (ly)
The distance light travels in a year: about 9.5 trillion km.
Luminosity distance
The distance worked out from how faint something looks. For far objects it's larger than the distance light travelled.
M
Magnetar
A neutron star with a magnetic field a thousand times stronger than a normal pulsar's; its quakes give X-ray and radio outbursts.
Magnitude
Brightness on a backwards scale: smaller is brighter. The eye sees to about 6 under dark skies; each step is 2.5 times fainter.
Megaparsec (Mpc)
A million parsecs: about 3.26 million light-years. Used for distances to galaxies.
Millisecond pulsar
An old pulsar spun up to hundreds of turns a second by gas from a companion star. Among the most precise clocks known.
N
Neutrino
A nearly massless particle that passes through almost anything; high-energy ones point back to violent sources.
Neutron star
The collapsed core of a massive star: more mass than the Sun packed into a ball about 20 km across.
P
Parallax
A star's small yearly shift as Earth orbits the Sun; the smaller the shift, the farther the star.
Parsec (pc)
About 3.26 light-years: the distance at which a star shifts by one arcsecond as Earth goes round the Sun.
Proper motion
A star's slow drift across the sky as it moves through the galaxy, usually thousandths of an arcsecond a year.
Pulsar
A spinning neutron star whose beams of radio waves sweep past Earth like a lighthouse, giving regular pulses.
P–Ṗ diagram
Pulsars plotted by period against spin-down: young pulsars, millisecond pulsars and magnetars fall in different regions.
Q
Quasar
The brilliant centre of a distant galaxy, powered by gas falling into a giant black hole.
R
Redshift (z)
How much the universe has stretched light since it left the source. z = 1 means wavelengths have doubled; larger means farther and longer ago.
Right ascension (RA)
Position on the sky east-west, like longitude, measured in hours (0h to 24h) or degrees.
S
Scattering
The smearing of a radio burst as it passes through clumpy gas, so it arrives with a tail.
Signalness
IceCube's estimate of the chance that a neutrino came from space rather than from cosmic rays hitting Earth's atmosphere.
Sonification
Turning data into sound, here a pulsar's rotation as clicks at its real rate.
Spectroscopy
Splitting light into its colours to read what something is made of, how hot it is and how fast it moves.
Spin-down (Ṗ)
How fast a pulsar is slowing: the seconds its period grows each second. Tells its age and magnetic field.
Supernova
A star exploding: a massive star collapsing, or a white dwarf blowing apart. For weeks it can outshine its galaxy.
Surface brightness
Brightness spread over an object's area. A galaxy with a low surface brightness is hard to see even if its total magnitude is bright.
T
TeV, PeV
Tera- and peta-electronvolts: a TeV is a trillion times the energy of visible light's photons, a PeV a thousand times more.
V
VOTable
The Virtual Observatory's table format: data plus units and meanings, read by TOPCAT, Aladin and astropy.
X
X-rays
Very energetic light from gas at millions of degrees, around black holes, neutron stars and in galaxy clusters.
See them at work on the live sky map and the sky object pages.