Circumpolar star

A circumpolar star is a star, as viewed from a given latitude on Earth, that never sets below the horizon due to its apparent proximity to one of the celestial poles. Circumpolar stars are therefore visible from said location toward the nearest pole for the entire night on every night of the year (and would be continuously visible throughout the day too, were they not overwhelmed by the Sun's glare).

All circumpolar stars lie within a relative circumpolar circle, whose radius equals the observer's latitude. This was in fact the original meaning of "Arctic Circle", before the current geographical meaning, meaning "Circle of the Bears" (Ursa Major, the Great Bear; and Ursa Minor, the Little Bear), from Greek αρκτικός (arktikos), "near the Bear", from the word άρκτος (arktos) bear.

Zirkumpolar ani
Northern circumpolar stars appearing to revolve around the north celestial pole. Note that Polaris, the bright star near the center, remains almost stationary in the sky. The north pole star is constantly above the horizon throughout the year, viewed from the Northern Hemisphere. (The graphic shows how the apparent positions of the stars move over a 24-hour period, but in practice, they are invisible in daylight, in which sunlight outshines them.)
Circumpolar AZ81
Circumpolar star trails in a long-exposure photo of several hours. Note that the stars near the celestial pole leave shorter trails with the long exposure.


As Earth rotates daily on its axis, the stars appear to move in circular paths around one of the celestial poles (the north celestial pole for observers in the Northern Hemisphere, or the south celestial pole for observers in the Southern Hemisphere). Stars far from a celestial pole appear to rotate in large circles; stars located very close to a celestial pole rotate in small circles and hence hardly seem to engage in any diurnal motion at all. Depending on the observer's latitude on Earth, some stars – the circumpolar ones – are close enough to the celestial pole to remain continuously above the horizon, while other stars dip below the horizon for some portion of their daily circular path (and others remain permanently below the horizon).

The circumpolar stars appear to lie within a circle that is centered at the celestial pole and tangential to the horizon. At the Earth's North Pole, the north celestial pole is directly overhead, and all stars that are visible at all (that is, all stars in the Northern Celestial Hemisphere) are circumpolar. As one travels south, the north celestial pole moves towards the northern horizon. More and more stars that are at a distance from it begin to disappear below the horizon for some portion of their daily "orbit", and the circle containing the remaining circumpolar stars becomes increasingly small. At the Equator, this circle vanishes to a single point – the celestial pole itself – which lies on the horizon, and there are thus effectively no circumpolar stars at all.

As one travels south of the Equator, the opposite happens. The south celestial pole appears increasingly high in the sky, and all the stars lying within an increasingly large circle centered on that pole become circumpolar about it. This continues until one reaches the Earth's South Pole where, once again, all visible stars are circumpolar.

The north celestial pole is located very close to the pole star (Polaris or North Star), so from the Northern Hemisphere, all circumpolar stars appear to move around Polaris. Polaris itself remains almost stationary, always at the north (i.e. azimuth of 0°), and always at the same altitude (angle from the horizon), equal to the observer's latitude. These are then classified into quadrants.

Definition of circumpolar stars

Whether a star is circumpolar depends upon the observer's latitude.[1] Since the altitude of the north or south celestial pole (whichever is visible) equals the absolute value of the observer's latitude,[1] any star whose angular distance from the visible celestial pole is less than the absolute latitude will be circumpolar. For example, if the observer's latitude is 50° N, any star will be circumpolar if it is less than 50° from the north celestial pole. If the observer's latitude is 35° S, then all stars within 35° of the south celestial pole are circumpolar. Stars on the celestial equator are not circumpolar when observed from any latitude in either hemisphere of the Earth. "A star with its polar distance approximately equal to or less than the latitude of the observer".[2]

Whether a given star is circumpolar at the observer's latitude (θ) may be calculated in terms of the star's declination (δ). The star is circumpolar if θ + δ is greater than +90° (observer in Northern Hemisphere), or θ + δ is less than −90° (observer in Southern Hemisphere). "A star whose diurnal circle lies above the horizon never sets, even though it cannot be seen during the day. Designation of a star as circumpolar depends on the observer's latitude. At the equator no star is circumpolar. At the North or South Pole all stars that are visible at all are circumpolar, since only one half of the celestial sphere can ever be seen. For an observer at any other latitude a star whose declination is greater than 90° minus the observer's latitude will be circumpolar, appearing to circle the celestial pole and remaining always above the horizon. A constellation made up entirely of circumpolar stars is also called circumpolar. From most of the United States (above lat. 40°N) the Big Dipper is circumpolar".[2]

Similarly, the star will never rise above the local horizon if δ − θ is less than −90° (observer in Northern Hemisphere), or δ − θ is greater than +90° (observer in Southern Hemisphere). Thus, Canopus is invisible from such locations as San Francisco and Louisville, if marginally visible from Fresno, Tulsa, and Virginia Beach.

Some stars within the far northern constellation (such as Cassiopeia, Cepheus, Ursa Major, and Ursa Minor) roughly north of the Tropic of Cancer (+23½°) will be circumpolar stars, which never rise or set.[1]

For British observers, for example, the first magnitude stars Capella (declination +45° 59') and Deneb (+45° 16') do not set from anywhere in the country. Vega (+38° 47') is technically circumpolar north of latitude +51° 13' (just south of London); taking atmospheric refraction into account, it will probably only be seen to set at sea level from Cornwall and the Scilly Isles.

Some stars within the far southern constellations (such as Crux, Musca, and Hydrus) roughly south of the Tropic of Capricorn (-23½°) will also be circumpolar stars.[1]

Stars (and constellations) that are circumpolar in one hemisphere are always invisible in the high latitudes of the opposite hemisphere, and these never rise above the horizon. For example, the southern circumpolar star Acrux is invisible from most of the Contiguous United States, likewise, the seven stars of the northern circumpolar Big Dipper asterism are invisible from most of the Patagonia region of South America.

See also


  1. ^ a b c d Norton, A. P. "Norton's 2000.0 Star Atlas and Reference Handbook", Longman Scientific and Technical, (1986) p.39-40
  2. ^ a b star

External links

Alpha Apodis

Alpha Apodis (Alpha Aps, α Apodis, α Aps) is the brightest star in the southern circumpolar constellation of Apus, with an apparent magnitude of approximately 3.825. It had the Greek alpha designation as part of the constellation which Johann Bayer called Apis Indica in his 1603 Uranometria star atlas. With a declination of –79°, this is a circumpolar star for much of the southern hemisphere. It can be identified on the night sky by drawing an imaginary line through Alpha Centauri and Alpha Circini then extending it toward the south celestial pole.This is a giant star with a stellar classification of K2.5III, indicating that this star has consumed the hydrogen at its core and has evolved away from the main sequence. It has expanded to an estimated radius of about 48 times the radius of the Sun and is emitting 980 times the Sun's luminosity. The photosphere has an effective temperature of 4,256 K, giving the star the characteristic orange hue of a K-type star. Based upon parallax measurements, this star is 430 ± 20 light-years from the Earth. It is not known to have a companion.

Alpha Persei

Alpha Persei (Latinized from α Persei, abbreviated Alpha Per, α Per), formally named Mirfak (pronounced or ), is the brightest star in the northern constellation of Perseus, outshining the constellation's best known star, Algol. Alpha Persei has an apparent visual magnitude of 1.8, and is a circumpolar star when viewed from mid-northern latitudes.

Alpha Persei lies in the midst of a cluster of stars named as the eponymous Alpha Persei Cluster, or Melotte 20, which is easily visible in binoculars and includes many of the fainter stars in the constellation. Determined distance using the trigonometric parallax, places the star 510 light-years (160 parsecs) from the Sun.


Blitzars are a hypothetical type of astronomical object in which a spinning pulsar rapidly collapses into a black hole. They are proposed as an explanation for fast radio bursts (FRBs). The idea was proposed in 2013 by Heino Falcke and Luciano Rezzolla.

Bright giant

The luminosity class II in the Yerkes spectral classification is given to bright giants. These are stars which straddle the boundary between ordinary giants and supergiants, based on the appearance of their spectra.

CN star

A CN star is a star with strong cyanogen bands in its spectrum. Cyanogen is a simple molecule of one carbon atom and one nitrogen atom, with absorption bands around 388.9 and 421.6 nm. This group of stars was first noticed by Nancy G. Roman who called them 4150 stars.


Circumpolar may refer to:

Antarctic region

Antarctic Circle

the Antarctic Circumpolar Current


List of Antarctic and subantarctic islands

Antarctic Convergence

Antarctic Circumpolar Wave

Antarctic Ocean

Arctic region

Arctic Circle


Circumpolar peoples

Arctic Cooperation and Politics

Arctic Ocean

List of islands in the Arctic Ocean

Circumpolar constellation, a constellation that never rises or sets from the perspective of a given latitude on Earth

Circumpolar star, a star that never rises or sets from the perspective of a given latitude on Earth

Polar front in meteorology

circumpolar navigation, a global circumnavigation which traverses both poles

Infrared dark cloud

An infrared dark cloud (IRDC) is a cold, dense region of a giant molecular cloud. They can be seen in silhouette against the bright diffuse mid-infrared emission from the galactic plane.

Iron star

In astronomy, an iron star is a hypothetical type of compact star that could occur in the universe in the extremely far future, after perhaps 101500 years.

The premise behind iron stars states that cold fusion occurring via quantum tunnelling would cause the light nuclei in ordinary matter to fuse into iron-56 nuclei. Fission and alpha-particle emission would then make heavy nuclei decay into iron, converting stellar-mass objects to cold spheres of iron. The formation of these stars is only a possibility if protons do not decay. Though the surface of a neutron star may be iron, according to some predictions, it is distinct from an iron star.

Unrelatedly, the term is also used for blue supergiants which have a forest of forbidden FeII lines in their spectra. They are potentially quiescent hot luminous blue variables. Eta Carinae has been described as a prototypical example.

Lambda Boötis star

A Lambda Boötis star is a type of peculiar star which has an unusually low abundance of iron peak elements in its surface layers. One possible explanation for this is that it is the result of accretion of metal-poor gas from a circumstellar disc, a second possibility is the accretion of material from a hot Jupiter suffering from mass loss. The prototype is Lambda Boötis.

Lead star

A lead star is a low-metallicity star with an overabundance of lead and bismuth as compared to other products of the S-process.

List of hottest stars

This is a list of hottest stars so far discovered (excluding degenerate stars), arranged by decreasing temperature. The stars with temperatures higher than 60,000 K are included.

Photometric-standard star

Photometric-standard stars are a series of stars that have had their light output in various passbands of photometric system measured very carefully. Other objects can be observed using CCD cameras or photoelectric photometers connected to a telescope, and the flux, or amount of light received, can be compared to a photometric-standard star to determine the exact brightness, or stellar magnitude, of the object.A current set of photometric-standard stars for UBVRI photometry was published by Arlo U. Landolt in 1992 in the Astronomical Journal.


The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to it being a spherical surface that is perceived to emit light. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately 2/3, or equivalently, a depth from which 50% of light will escape without being scattered.

In other words, a photosphere is the deepest region of a luminous object, usually a star, that is transparent to photons of certain wavelengths.

Q star

A Q-Star, also known as a grey hole, is a hypothetical type of a compact, heavy neutron star with an exotic state of matter. The Q stands for a conserved particle number. A Q-Star may be mistaken for a stellar black hole.

Starfield (astronomy)

A starfield refers to a set of stars visible in an arbitrarily-sized field of view, usually in the context of some region of interest within the celestial sphere. For example: the starfield surrounding the stars Betelgeuse and Rigel could be defined as encompassing some or all of the Orion constellation.

Stellar atmosphere

The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone.

Stellar mass

Stellar mass is a phrase that is used by astronomers to describe the mass of a star. It is usually enumerated in terms of the Sun's mass as a proportion of a solar mass (M☉). Hence, the bright star Sirius has around 2.02 M☉. A star's mass will vary over its lifetime as additional mass becomes accreted, such as from a companion star, or mass is ejected with the stellar wind or pulsational behavior.

Supernova impostor

Supernova impostors are stellar explosions that appear at first to be a supernova but do not destroy their progenitor stars. As such, they are a class of extra-powerful novae. They are also known as Type V supernovae, Eta Carinae analogs, and giant eruptions of luminous blue variables (LBV).

Yellow giant

A yellow giant is a luminous giant star of low or intermediate mass (roughly 0.5–11 solar masses (M)) in a late phase of its stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature as low as 5,200-7500 K. The appearance of the yellow giant is from white to yellow, including the spectral types F and G. About 10.6 percent of all giant stars are yellow giants.

Luminosity class
Star systems
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