Astronomy glossary
A
Absolute magnitude
A star's brightness if it were placed 10 parsecs from Earth.
Absolute magnitude normalizes apparent magnitude for distance, letting astronomers compare intrinsic luminosity. The Sun's absolute magnitude is +4.83 — fairly ordinary among main-sequence stars.
Related: Apparent magnitude , Parsec (pc)
Albedo
The fraction of light a body reflects rather than absorbs.
Albedo runs from 0 (perfect absorber) to 1 (perfect mirror). Earth averages 0.30, the Moon 0.12, freshly fallen snow 0.85. Differences in albedo are why a small object can outshine a larger but darker one.
Aphelion
The point in an orbit farthest from the Sun.
Earth reaches aphelion in early July at about 152.1 million km. The opposite point — perihelion — falls in early January at 147.1 million km. The 3% difference is much smaller than the tilt that causes seasons.
Related: Perihelion
Apparent magnitude
How bright an object looks from Earth on a logarithmic scale.
Lower numbers are brighter. The Sun is −26.74, the full Moon −12.74, Sirius −1.46, the faintest naked-eye stars +6.5. A five-magnitude difference equals exactly 100× in flux.
Related: Absolute magnitude
Astronomical unit (AU)
The average Earth–Sun distance, defined as exactly 149,597,870.7 km.
Related: Light-year , Parsec (pc)
B
Black hole
A region of spacetime where gravity is strong enough that escape velocity exceeds the speed of light.
The boundary is the event horizon. Stellar-mass black holes form from massive star collapse; supermassive black holes (millions to billions of solar masses) sit at galaxy centers.
Related: Event horizon , Schwarzschild radius
C
Conjunction
When two celestial bodies appear close together in the sky.
A planet at superior conjunction is on the far side of the Sun from Earth; at inferior conjunction (only for Mercury and Venus) it is between us and the Sun. Visual conjunctions of two planets can bring them within fractions of a degree.
D
Declination
The celestial equivalent of latitude — how far north or south of the celestial equator an object lies.
Related: Right ascension
E
Eclipse
When one body passes into the shadow of another.
A solar eclipse occurs when the Moon blocks the Sun (Earth in the Moon's shadow). A lunar eclipse occurs when Earth's shadow falls on the Moon. The geometry requires the Sun, Earth and Moon to align almost exactly.
Equinox
The moment day and night are nearly equal — when the Sun crosses the celestial equator.
Spring (vernal) equinox falls around March 20, autumnal equinox around September 22. On these dates the Sun rises due east and sets due west everywhere on Earth.
Event horizon
The boundary of a black hole — the surface from which no information can escape.
Related: Black hole , Schwarzschild radius
G
Galaxy
A gravitationally bound system of stars, gas, dust and dark matter.
Galaxies range from dwarf systems with a few million stars to giants with over a trillion. They take three main shapes: spiral, elliptical and irregular. Our Milky Way is a barred spiral about 100,000 light-years across.
H
Hubble constant (H₀)
The current expansion rate of the universe.
Measured values cluster around 67-73 km/s per megaparsec depending on method. The discrepancy between local-distance ladder measurements (~73) and CMB-derived values (~67) is the "Hubble tension."
L
Light-year
The distance light travels in one Julian year — about 9.46 trillion kilometers.
Related: Astronomical unit (AU) , Parsec (pc)
M
Magnitude
See apparent magnitude or absolute magnitude.
Related: Apparent magnitude , Absolute magnitude
Meteor
A streak of light produced when a small piece of space debris (meteoroid) burns up in the atmosphere.
On the ground a recovered piece is called a meteorite. Most visible meteors come from particles the size of a grain of sand.
N
Nebula
A cloud of gas and dust in space.
Emission nebulae glow because their gas is ionized by nearby hot stars. Reflection nebulae shine by scattered starlight. Dark nebulae block background light. Planetary nebulae are shells of gas thrown off by dying low-mass stars.
O
Occultation
When one body passes in front of and hides another — a planet covering a star, for example.
Differs from a transit because the foreground body completely hides the background one. Lunar occultations are the most commonly observed.
Opposition
When an outer planet is opposite the Sun in the sky — rising at sunset, setting at sunrise.
A planet at opposition is also at its closest approach to Earth for that year and at its brightest. Mars oppositions happen about every 26 months.
P
Parallax
The apparent shift in position of a nearby object when viewed from two different vantage points.
Stellar parallax — the shift seen over six months as Earth swings to the other side of its orbit — provides the most direct measurement of distance to nearby stars. The Gaia mission has measured parallaxes for over a billion stars.
Related: Parsec (pc)
Parsec (pc)
The distance at which one astronomical unit subtends an angle of one arcsecond.
Approximately 3.262 light-years or 30.857 trillion km. The unit comes from parallax measurements and is standard in research papers.
Related: Parallax , Light-year
Perigee
The point in an Earth-orbit closest to Earth.
Lunar perigee varies between about 356,500 and 370,400 km. A full moon at perigee is a "supermoon".
Related: Apogee
Q
Quasar
An extremely luminous active galactic nucleus powered by a supermassive black hole.
The brightest quasars outshine entire galaxies, visible across most of the observable universe. The most distant quasars seen are over 13 billion light-years away.
R
Redshift
The lengthening of light wavelengths from a receding source — used to measure cosmic distance and expansion.
Cosmological redshift is caused by the expansion of space itself stretching the wavelength of light as it travels. The most distant galaxies seen (z ≈ 14) have their visible light shifted into the infrared.
Right ascension
The celestial equivalent of longitude, measured in hours, minutes and seconds along the celestial equator.
Related: Declination
S
Schwarzschild radius
The radius at which escape velocity equals the speed of light — the event horizon of a non-rotating black hole.
For a body of mass M, the radius is r = 2GM/c². The Sun would need to be compressed to 2.95 km to become a black hole.
Related: Black hole , Event horizon
Solstice
The moment the Sun reaches its northernmost or southernmost declination, producing the longest or shortest day of the year.
Related: Equinox
Supernova
The explosive death of a massive star, briefly outshining its entire host galaxy.
Type II supernovae come from core collapse of stars more than 8 solar masses. Type Ia supernovae come from white dwarfs that exceed the Chandrasekhar limit. Both types produce most of the heavy elements in the universe.
T
Transit
When a smaller body passes in front of a larger one across our line of sight — like Venus crossing the Sun.
Exoplanet detection by transit relies on the tiny dip in starlight as a planet passes in front of its star. The Kepler and TESS missions have found thousands of exoplanets this way.
W
White dwarf
The dense remnant left when a Sun-like star sheds its outer layers.
About the size of Earth but containing roughly half a solar mass. A teaspoon of white dwarf material weighs several tons on Earth.
Z
Zenithal Hourly Rate (ZHR)
The meteor rate an ideal observer would see at the peak of a shower with the radiant directly overhead and a sky dark enough to show 6.5-magnitude stars.
Real observed rates are always lower because of the radiant's altitude, light pollution and observer fatigue.