Astronomy glossary – terms explained simply

Glossary

From magnitude to circumpolar star: the key terms about stars, constellations and star catalogues – short and easy to understand.

Absolute magnitude: Absolute magnitude is the brightness a star would have if it were exactly 10 parsecs (about 32.6 light years) away. It lets astronomers compare stars fairly, regardless of their distance. Our Sun dazzles in the daytime sky, yet from that distance it would shine at only about 4.8 mag – a rather modest point of light. Many faint-looking stars you might name are actually far more luminous than the Sun.; See also: Apparent magnitude, Parsec (pc), Light year
Apparent magnitude: Apparent magnitude describes how bright a star looks from Earth, measured on the magnitude scale (mag). The scale runs backwards: the smaller the number, the brighter the star, and five magnitudes correspond to a brightness factor of 100. Sirius, the brightest star in the night sky, is about −1.4 mag. On sternschenker.de you can choose stars down to magnitude 6.5 – bright enough to be seen with the naked eye under a dark sky.; See also: Absolute magnitude, Naked-eye star, Light pollution
Asterism: An asterism is a recognisable pattern of stars that is not an official constellation. It is often part of a constellation or links stars from several. The best-known example is the Plough, or Big Dipper, which belongs to the constellation Ursa Major. The Summer Triangle of Vega, Deneb and Altair even spans three constellations. Asterisms are ideal signposts for finding a particular star in the sky.; See also: Constellation, Polaris (Pole Star), Planetarium app
Bayer designation: A Bayer designation names a star with a Greek letter plus the Latin genitive of its constellation, for example Alpha Lyrae for Vega. Johann Bayer introduced it in 1603 in his star atlas “Uranometria”. The brightest star usually received Alpha, though there are many exceptions. The designation tells you straight away which constellation your star belongs to.; See also: Flamsteed designation, Proper name, Constellation
Bortle scale: The Bortle scale rates how dark the night sky is at a location on nine levels, from 1 (excellent, truly dark sky) to 9 (inner-city sky). The amateur astronomer John Bortle published it in 2001. The lower the level, the fainter the stars you can see. From around level 4 downwards the Milky Way becomes clearly visible. Light-pollution maps online show you where to find a dark sky.; See also: Light pollution, Naked-eye star, Milky Way
Celestial equator: The celestial equator is the projection of Earth’s equator onto the sky – an imaginary great circle dividing the sky into northern and southern halves. It is the zero line of declination. Stars lying on the celestial equator rise due east and set due west from anywhere on Earth. The three belt stars of Orion lie very close to it.; See also: Declination (Dec), Ecliptic, Right ascension (RA)
Circumpolar star: A circumpolar star is a star that never sets from your location but circles the celestial pole all year round. At 50° north this applies to all stars with a declination north of +40°, such as the stars of the Plough or Cassiopeia. A circumpolar star makes a lovely gift, because you can find it in the sky on every clear night.; See also: Polaris (Pole Star), Declination (Dec), Horizon and visibility
Colour index (B−V): The B−V colour index is the difference between a star’s brightness in blue (B) and yellow-green (V) light, and therefore a measure of its colour. Small or negative values mean bluish-white, hot stars; large values mean orange-red, cooler stars. Vega is close to 0, the Sun at about 0.65. Star maps and apps often use this value to display stars in their natural colour.; See also: Spectral class, Apparent magnitude, Red giant
Constellation: A constellation is a region of the sky with precise boundaries defined by the International Astronomical Union (IAU); there are 88 of them, covering the entire sky without gaps. Every star therefore belongs to exactly one constellation. Many date back to ancient figures such as Orion or Ursa Major. On sternschenker.de you can preselect a constellation – your zodiac constellation, for example – and choose your star within it.; See also: Asterism, International Astronomical Union (IAU), Zodiac
Culmination: Culmination is the moment a star reaches its highest point in the sky as it crosses the meridian – the north–south line through the zenith. It then stands highest above the horizon, and its light passes through the least air. If you want to observe your star, the time around its culmination is therefore usually best. Planetarium apps show you when that is.; See also: Horizon and visibility, Planetarium app, Circumpolar star
Declination (Dec): Declination is a star’s “celestial latitude”: its angular distance north (+) or south (−) of the celestial equator, from +90° at the north celestial pole to −90° at the south pole. It helps decide whether you can see a star from where you live. At 50° north – roughly Central Europe – a star with a declination south of −40° never rises above the horizon.; See also: Right ascension (RA), Circumpolar star, Horizon and visibility
Double star: A double star consists of two stars that appear very close together in the sky. If they actually orbit each other, it is a physical (binary) double; if they merely lie in the same line of sight by chance, it is an optical double. A famous pair is Mizar and Alcor in the Plough, which good eyes can separate without binoculars. Many stars you can name are in fact double or multiple systems.; See also: Variable star, Asterism, Star cluster
Ecliptic: The ecliptic is the apparent path the Sun traces against the background stars over the course of a year. It is tilted by about 23.4° to the celestial equator because Earth’s axis is tilted. The Moon and planets always stay close to it too. The constellations the ecliptic passes through form the zodiac – which is why you almost always find planets in zodiac constellations such as Leo or Virgo.; See also: Zodiac, Celestial equator, Zodiac sign vs constellation
Flamsteed designation: A Flamsteed designation names a star with a number plus the genitive of its constellation, for example 61 Cygni. The numbers go back to the star catalogue of the English astronomer John Flamsteed and increase from west to east within each constellation, that is, with right ascension. This gave many stars without a Greek letter a unique designation – including some you can choose on sternschenker.de.; See also: Bayer designation, Right ascension (RA), Hipparcos catalogue (HIP)
Gaia mission: Gaia is an ESA space telescope launched in 2013 that measured the positions, distances and motions of more than a billion stars in the Milky Way. It succeeded Hipparcos and is many times more precise; its observations ended in 2025. Gaia data provide the most precise distances yet for many stars – including many you can choose on sternschenker.de.; See also: Hipparcos catalogue (HIP), Parsec (pc), Milky Way
Galaxy: A galaxy is a vast, gravitationally bound system of stars, gas, dust and dark matter. Our home galaxy is the Milky Way. The Andromeda Galaxy is about 2.5 million light years away and can be seen with the naked eye under a dark sky as a faint smudge. By contrast, every individual star you see in the sky and can choose on sternschenker.de belongs to the Milky Way.; See also: Milky Way, Nebula, Light year
Henry Draper catalogue (HD): The Henry Draper catalogue is a star catalogue compiled at Harvard College Observatory that gives the spectral class of about 225,000 stars; it was published between 1918 and 1924. It is named after the astronomer Henry Draper, whose widow funded the work. The classification was carried out mainly by Annie Jump Cannon and her colleagues. HD numbers are still used today – your star often has one too.; See also: Spectral class, Hipparcos catalogue (HIP), Gaia mission
Hipparcos catalogue (HIP): The Hipparcos catalogue is a star catalogue of about 118,000 stars, compiled from measurements by ESA’s Hipparcos satellite (1989–1993) and published in 1997. It contains precise positions, brightnesses and distances. Every star has a unique HIP number – Vega, for example, is HIP 91262. Sternschenker uses exactly this catalogue: your star is uniquely identified by its HIP number and can be found in any astronomy software.; See also: Gaia mission, Henry Draper catalogue (HD), Parsec (pc)
Horizon and visibility: Whether a star is visible depends on whether it is currently above the horizon – which is determined by your latitude, the season and the time of night. Stars low on the horizon look fainter because of haze and air turbulence. From Central Europe, stars far south of the celestial equator never rise. So before giving a star, check that it can be seen from where the recipient lives.; See also: Declination (Dec), Culmination, Circumpolar star
International Astronomical Union (IAU): The IAU, founded in 1919, is the worldwide organisation of professional astronomers and the only recognised authority for official names of celestial objects. It defined the 88 constellations and approves proper names of stars. The IAU does not sell star names or assign them to private individuals. That is why every private star naming is symbolic – including those on sternschenker.de, which we say openly.; See also: WGSN (IAU Working Group on Star Names), Star registry, Constellation
Light pollution: Light pollution is the brightening of the night sky by artificial light from street lamps, buildings and advertising. It drowns out faint stars: in a big city you often see only the brightest stars, while in the countryside you can see thousands and the Milky Way. If you give a faint star, a trip to a dark place is worthwhile – or choose a brighter star that is visible from town as well.; See also: Bortle scale, Apparent magnitude, Naked-eye star
Light year: A light year is the distance light travels in one year – about 9.46 trillion kilometres. It is a unit of distance, not of time. The nearest star after the Sun, Proxima Centauri, is about 4.2 light years away. A lovely detail: when you look at your star, you see light that left it years ago – for a star 30 light years away, that light set off around the time someone turning 30 was born.; See also: Parsec (pc), Absolute magnitude, Gaia mission
Main sequence: The main sequence is the stage of life in which a star fuses hydrogen into helium in its core – the longest and calmest phase of its life. In a diagram of temperature versus luminosity these stars form a distinct band. Most stars, including our Sun, are on the main sequence. Vega and Sirius are main-sequence stars too, though hotter and more luminous than the Sun.; See also: Spectral class, Red giant, White dwarf
Milky Way: The Milky Way is the barred spiral galaxy our Sun belongs to; estimates put its number of stars at a few hundred billion. From inside, we see its disc as a shimmering band of light across the sky, especially impressive on summer nights. To see it you need a truly dark place free of light pollution. Every star you can name is part of it.; See also: Galaxy, Light pollution, Gaia mission
Naked-eye star: A naked-eye star is a star that can be seen without binoculars or a telescope. Under a very dark sky the limit is around magnitude 6 to 6.5; in a city it is often only magnitude 3 to 4. That is why Sternschenker offers only stars down to magnitude 6.5. This means you can, in principle, look for your star with your own eyes – most easily if you choose a brighter one.; See also: Apparent magnitude, Light pollution, Bortle scale
Nebula: A nebula is a cloud of interstellar gas and dust. Some glow because young hot stars excite them, others reflect starlight or, as dark clouds, block what lies behind them. New stars are born in many nebulae. The Orion Nebula in Orion’s “sword” can be seen as a fuzzy patch with the naked eye under a dark sky. You cannot name nebulae themselves on sternschenker.de – only stars.; See also: Star cluster, Galaxy, Milky Way
Parsec (pc): A parsec is the distance at which the radius of Earth’s orbit appears at an angle of one arcsecond; it equals about 3.26 light years. The name comes from “parallax second”: astronomers measure the tiny apparent shift of a star as Earth orbits the Sun and calculate its distance from it. Satellites such as Hipparcos and Gaia measured the distances of many of the stars you can find on sternschenker.de this way.; See also: Light year, Hipparcos catalogue (HIP), Gaia mission
Planetarium app: A planetarium app is a program that calculates and displays the night sky for any place and time. Many apps show the sky in whatever direction you point your smartphone; the free software Stellarium is also available for computers. With the HIP number or the coordinates on your certificate, you can quickly find your star in such an app and see when it is best placed for observing.; See also: Hipparcos catalogue (HIP), Right ascension (RA), Culmination
Polaris (Pole Star): Polaris (Alpha Ursae Minoris), the Pole Star, is the brightest star in Ursa Minor and lies less than one degree from the north celestial pole. That is why it seems to stand almost still while the rest of the sky turns around it, reliably marking north. It is not the brightest star in the sky, but it is easy to find: extend the two end stars of the Plough’s bowl about five times their distance.; See also: Circumpolar star, Asterism, Bayer designation
Proper name: A proper name is a star’s official name, such as Sirius, Vega or Aldebaran, as opposed to a catalogue number. Many proper names come from Arabic, Greek or Latin; since 2016 the IAU’s WGSN has formally approved them. Only a small fraction of stars have a proper name. The vast majority carry only catalogue designations – the name you give on sternschenker.de is a personal, symbolic one.; See also: WGSN (IAU Working Group on Star Names), Bayer designation, Hipparcos catalogue (HIP)
Red giant: A red giant is an ageing star that has used up the hydrogen in its core and, as a result, has swollen enormously and cooled at its surface. It shines orange to reddish and is often very bright. Well-known examples are Arcturus in Boötes and Aldebaran in Taurus. Our Sun will also become a red giant in the distant future. Bright reddish stars are easy to spot – a bonus when you want to find yours again.; See also: Main sequence, White dwarf, Spectral class
Right ascension (RA): Right ascension is a star’s “celestial longitude”: its angular distance from the vernal equinox point along the celestial equator, given in hours, minutes and seconds from 0 h to 24 h. Together with declination it forms a star’s celestial coordinates, much like longitude and latitude on Earth. With these two values you can find your star precisely in any planetarium app or star chart.; See also: Declination (Dec), Celestial equator, Planetarium app
Spectral class: Spectral class sorts stars by their surface temperature, which shows up in their light spectrum. The main classes are O, B, A, F, G, K and M – from very hot, bluish stars to cooler, reddish ones. Our Sun is a yellowish G star, Rigel in Orion a bluish B star and Betelgeuse a reddish M star. So the spectral class tells you roughly what colour your star is.; See also: Colour index (B−V), Henry Draper catalogue (HD), Main sequence
Star cluster: A star cluster is a group of stars that formed together from the same gas cloud and are held together by gravity. Open clusters are young and loose, like the Pleiades in Taurus; globular clusters are ancient and densely packed, like M13 in Hercules. The Pleiades are easily visible to the naked eye – their individual stars make a particularly atmospheric choice for naming.; See also: Nebula, Double star, Milky Way
Star registry: A star registry is a private list in which a provider stores the star names given by its customers. Such registries are not recognised by the IAU and have no astronomical meaning: no observatory or star catalogue adopts these names. That is why sternschenker.de deliberately does without a database – your name lives only in your link and your certificate. This is honest, protects your data and makes free star naming possible.; See also: International Astronomical Union (IAU), WGSN (IAU Working Group on Star Names), Proper name
Variable star: A variable star is a star whose brightness changes measurably over time. This can be caused by pulsations, outbursts or two stars eclipsing each other. Algol in Perseus, for example, dims noticeably roughly every three days because a companion partly covers it. So if your star is a variable, its listed magnitude is only a typical value.; See also: Double star, Apparent magnitude, Red giant
WGSN (IAU Working Group on Star Names): The WGSN (Working Group on Star Names) is an IAU working group founded in 2016 that catalogues and formally approves proper names of stars. It standardised traditional names such as Betelgeuse and Vega and has also adopted names from many of the world’s sky cultures. The proper names you see for stars on sternschenker.de come from this official list – the personal name you give is separate from it and symbolic.; See also: International Astronomical Union (IAU), Proper name, Star registry
White dwarf: A white dwarf is the burnt-out, extremely dense core of a star like the Sun – roughly the size of Earth but with about the mass of a star. It no longer produces energy by nuclear fusion and slowly cools down. White dwarfs are faint and invisible to the naked eye; the best known is Sirius B, the inconspicuous companion of bright Sirius.; See also: Red giant, Main sequence, Double star
Zodiac: The zodiac is the band of sky along the ecliptic in which the Sun, Moon and planets move. Traditionally it is divided into twelve sections of 30° each, the zodiac signs from Aries to Pisces. In reality, however, the ecliptic passes through 13 constellations – including Ophiuchus, the Serpent Bearer. When giving a star as a birthday present, a star in the matching zodiac constellation is a popular, personal choice.; See also: Ecliptic, Zodiac sign vs constellation, Constellation
Zodiac sign vs constellation: A zodiac sign is an astrological 30° section of the ecliptic, whereas a constellation is an astronomically defined sky region with boundaries of unequal size. They often share a name but do not match: because of the precession of Earth’s axis, the sky has shifted by almost a whole sign since antiquity. For someone born a “Leo”, the Sun on their birthday today usually stands in the constellation Cancer. For your star gift you can still simply choose the constellation of the same name.; See also: Zodiac, Constellation, Ecliptic

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