Circinus is a small, faint

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the earliest constellation ...

in the southern sky, first defined in 1756 by the French astronomer

Nicolas-Louis de Lacaille Abbé Nicolas-Louis de Lacaille (; 15 March 171321 March 1762), formerly sometimes spelled de la Caille, was a French astronomer and geodesist who named 14 out of the 88 constellations. From 1750 to 1754, he studied the sky at the Cape of Good ...

. Its name is

Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...

for

compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...

, referring to the drafting tool used for drawing circles (it should not be confused with Pyxis, a constellation that represents a mariner's compass which points north). Its brightest star is

Alpha Circini Alpha Circini (α Cir, α Circini) is a variable star in the faint, southern, circumpolar constellation of Circinus (constellation), Circinus. At an apparent visual magnitude of 3.19, it is the brightest star in the constellation and can ...

, with an

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...

of 3.19. Slightly variable, it is the brightest

rapidly oscillating Ap star Rapidly oscillating Ap stars (roAp stars) are a subtype of the Ap star class that exhibit short-timescale rapid photometric or radial velocity variations. The known periods range between 5 and 23 minutes. They lie in the δ Scuti instability stri ...

in the night sky.

AX Circini AX Circini is a binary star system in the southern constellation of Circinus. It has a nominal magnitude of 5.91, which is bright enough to be visible to the naked eye. Based upon an annual parallax shift of , it is located roughly 1 ...

is a Cepheid variable visible with the unaided eye, and

BX Circini BX Circini is a star in the constellation Circinus. Its variability was discovered in 1995, with its apparent magnitude ranging from 12.57 to 12.62 over a period of 2 hours 33 minutes. It is currently classified as a PV Telescopii vari ...

is a faint star thought to have been formed from the merger of two white dwarfs. Two sun-like stars have planetary systems:

HD 134060 HD 134060, also known by its Gould designation of 38 G. Circini, is a star in the southern constellation of Circinus. It is near the lower limit of stars visible to the naked eye, having an apparent visual magnitude of 6.29. The distance ...

has two small planets, and

HD 129445 HD 129445 is a G type star found in the Circinus constellation located approximately 220 light-years away from the Sun based on parallax. It is invisible to the naked eye with an apparent visual magnitude of 8.80. The star has been u ...

has a

Jupiter Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousand ...

-like planet.

Supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or whe ...

SN 185 appeared in Circinus in 185 AD and was recorded by Chinese observers. Two novae have been observed more recently, in the 20th century. The

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

runs through the constellation, featuring prominent objects such as the

open cluster An open cluster is a type of star cluster made of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, an ...

NGC 5823 and the planetary nebula NGC 5315. Circinus hosts a notable spiral galaxy, the Circinus Galaxy, discovered in 1977; it is the closest

Seyfert galaxy Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars. They have quasar-like nuclei (very luminous, distant and bright sources of electromagnetic radiation) with very high surface brightnesses whose spectr ...

to the Milky Way. The Alpha Circinids (ACI), a meteor shower also discovered in 1977, radiate from this constellation.

History

In 1756, French astronomer

introduced the

of Circinus with the French name ''le Compas'', representing a pair of dividing

es, on a chart of the southern sky. On that chart, Lacaille portrayed the constellations of Norma, Circinus, and

Triangulum Australe Triangulum Australe is a small constellation in the far Southern Celestial Hemisphere. Its name is Latin for "the southern triangle", which distinguishes it from Triangulum in the northern sky and is derived from the acute, almost equilateral p ...

, respectively, as a set square and ruler, a compass, and a surveyor's level in a set of draughtsman's instruments. Circinus was given its current name in 1763, when Lacaille published an updated sky map with Latin names for the constellations he introduced.

Characteristics

Bordered by Centaurus,

Musca Musca () is a small constellation in the deep southern sky. It was one of 12 constellations created by Petrus Plancius from the observations of Pieter Dirkszoon Keyser and Frederick de Houtman, and it first appeared on a celestial globe in d ...

Apus Apus is a small constellation in the southern sky. It represents a bird-of-paradise, and its name means "without feet" in Greek because the bird-of-paradise was once wrongly believed to lack feet. First depicted on a celestial globe by Pet ...

, Triangulum Australe, Norma and Lupus, Circinus lies adjacent to the

Alpha Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', or ell, άλφα, álfa) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter aleph , whi ...

and Beta Centauri stars. As it is at

declination In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of th ...

−50° to −70°, the whole constellation is only visible south of

latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north ...

30° N. The official constellation boundaries, as set by Belgian astronomer Eugène Delporte in 1930, are defined by a

polygon In geometry, a polygon () is a plane figure that is described by a finite number of straight line segments connected to form a closed '' polygonal chain'' (or ''polygonal circuit''). The bounded plane region, the bounding circuit, or the two ...

of 14 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between and , and the declination coordinates are between −55.43° and −70.62°. Circinus culminates each year at 9 p.m. on 30 July. The recommended three-letter abbreviation for the constellation, as adopted by the

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...

in 1922, is "Cir".

Features

Stars

Circinus is a faint constellation, with only one star brighter than fourth magnitude.

, a white main sequence star with an

of 3.19, is 54 light-years away and 4° south of

Alpha Centauri Alpha Centauri ( Latinized from α Centauri and often abbreviated Alpha Cen or α Cen) is a triple star system in the constellation of Centaurus. It consists of 3 stars: Alpha Centauri A (officially Rigil Kentaurus), Alpha Centa ...

. Not only the brightest star in the constellation, it is also the brightest example of a rapidly oscillating Ap (RoAp) star in the night sky. It has the unusual spectral type A7 Vp SrCrE, showing increased emissions of

strontium Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is e ...

chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hard ...

and europium. Stars of this type have oddly localised

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

s and are slightly variable. Alpha Circini forms a binary

star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a '' star cluster'' or ''galaxy'', although, broadly speaking ...

with an orange dwarf companion of spectral type K5 and magnitude 8.5, which with a separation of 5.7 arcseconds is only discernible with a telescope. The distance between the two stars is 260 AU and they take 2600 years to rotate around a common centre of gravity. The second brightest star is Beta Circini, a white main sequence star of spectral type A3Va and a magnitude of 4.07, about 100 light-years away. It has around 1.8 times the diameter of the Sun. Gamma Circini is a binary star 450 light-years away, whose components need a telescope of 150 mm to be seen, as they are only 0.8 arcseconds apart. The brighter component is a bluish

Be star Be stars are a heterogeneous set of stars with B spectral types and emission lines. A narrower definition, sometimes referred to as ''classical Be stars'', is a non-supergiant B star whose spectrum has, or had at some time, one or more Balmer e ...

of spectral type B5IV+ and magnitude 4.51, while the dimmer component is a yellow star of magnitude 5.5. They orbit each other every 180 years. Delta Circini is also a multiple star whose components have magnitudes of 5.1 and 13.4 and orbit around a common centre of gravity every 3.9 days. The brighter component is a close

eclipsing binary A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in ...

(specifically, a

rotating ellipsoidal variable Rotating ellipsoidal variables are a class of variable star. They are close binary systems whose components are ellipsoidal. They are not eclipsing, but fluctuations in apparent magnitude Apparent magnitude () is a measure of the brightne ...

), with a minor dip of magnitude (0.1). Both are hot blue stars of spectral types O7III-V and O9.5V, respectively, and are estimated to have around 22 and 12 times the Sun's mass. Over 3600 light-years away, this system would outshine

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...

at magnitude −4.8 if it were 32 light-years (10 parsecs) distant. The two main components are separated by 50 arcseconds, resolvable to the naked eye for individuals with good vision and easily discernible with a telescope. Eta Circini is a yellow giant of spectral type G8III and magnitude 5.17, located around 276 light-years distant, and Zeta Circini is a blue-white main sequence star of spectral type B3V and magnitude 6.09, located around 1273 light-years away. 493 variable stars have been recorded in Circinus, but most have a very small range or are quite dim. Three prominent examples are Theta Circini, T Circini, and

. Theta Circini is a B-class irregular variable, ranging in magnitude from 5.0 to 5.4. T Circini has a B-type spectrum, ranging in magnitude from 10.6 to 9.3 over a period of 3.298 days, although it is actually an eclipsing binary system rather than a pulsating star. AX is a Cepheid variable that varies between magnitudes 5.6 and 6.19 over 5.3 days. It is a yellow-white supergiant of spectral type F8II+, 1600 light-years away. BP Circini is another Cepheid variable with an apparent magnitude ranging from 7.37 to 7.71 over 2.4 days. Both cepheids are spectroscopic binaries, with companions that are blue-white stars of spectral type B6 and 5 and 4.7 solar masses respectively.

is a faint star that fluctuates between magnitudes 12.57 and 12.62 over a period of 2 hours 33 minutes. Over 99% of its composition appears to be helium. Its origin is unclear, but thought to be the result of the merger of a helium and a carbon/oxygen white dwarf. Several stars with planetary systems lie within the borders of Circinus, although none of the host stars are particularly prominent.

is a sun-like yellow dwarf star of spectral type G0VFe+0.4 and magnitude 6.29, around 79 light-years away. Its two planets were discovered in 2011 through the

radial velocity method Doppler spectroscopy (also known as the radial-velocity method, or colloquially, the wobble method) is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in th ...

: the smaller, HD 134060 b, has a mass of 0.0351 M_J (Jupiter masses) and orbits its star every 3.27 days, at 0.0444 AU; the larger, HD 134060 c (0.15 M_J), orbits farther out at 2.226 AU, with a period of approximately 1161 days. Even fainter, at magnitude 8.8,

is 220 light-years away and has 99% of the Sun's mass and a similar spectral type of G8V. HD 129445 b, a

-like planet (1.6 M_J) discovered in 2010 via the radial velocity method, orbits this star at a distance of 2.9 AU, approximately every 1840 days. As this constellation intersects the plane of the Milky Way, there are many massive stars located in this constellation, including GKF2010 MN18 (or simply MN18), a blue supergiant located in a bipolar nebula, as well as 9 Wolf-Rayet stars, a very high number for such a small constellation.

Deep-sky objects

Three

s and a planetary nebulae are found within the borders of Circinus, all visible with amateur telescopes of varying sizes. NGC 5823, also called

Caldwell Caldwell may refer to: People * Caldwell (surname) * Caldwell (given name) * Caldwell First Nation, a federally recognized Indian band in southern Ontario, Canada Places Great Britain * Caldwell, Derbyshire, a hamlet * Caldwell, Eas ...

88, is an 800-million-year-old open cluster, located 3500 light-years away and spanning a 12-light-year region along the constellation's northern border. Despite having an integrated magnitude of 7.9, the cluster can be seen by star hopping from Beta Circini or from Alpha Centauri. It contains 80–100 stars of 10th magnitude and fainter, which are spread out over a diameter of 10 arcseconds. The brighter stars, however, are not true members of the cluster, as they are closer to the Earth than the dimmer ones. NGC 5823 appears distinct to the observer, sometimes seen as a reversed "S", as described by

John Herschel Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor, experimental photographer who invented the blueprint and did botanical wo ...

, although it has also been described as "tulip-shaped" and "boxy". That cluster can be easily mistaken with a similar cluster, NGC 5822, nearby in Lupus. Comparatively, open cluster NGC 5715 is fainter (integrated magnitude of 9.8)—its brightest star is only 11th magnitude—and smaller (7.0 arcminutes), comprising only 30 stars. The third open cluster, Pismis 20, contains 12 stars in a diameter of 4.5 arcseconds but exhibits a magnitude similar to NGC 5823 (7.8). At 8270 light-years, it requires an amateur telescope with an aperture over 300 mm to be easily discerned. NGC 5315HSTfull

The planetary nebula NGC 5315 has a magnitude of 9.8 around a central star of magnitude 14.2, located 5.2 degrees west-southwest of Alpha Circini. It is only visible as a disc at magnifications over 200-fold. Bernes 145 is a dark and reflection nebula first listed in the 1971 Bernes Catalog. The dark nebula component is easily visible in a large amateur telescope, and it measures 12 by 5 arcminutes. The smaller reflection nebula component requires a larger instrument and averted vision to be seen. Circinus also houses ESO 97-G13, commonly known as the Circinus Galaxy. Discovered in 1977, it is a relatively unobscured galaxy (magnitude 10.6), which is unusual for galaxies located in constellations near the

, since their dim light is obscured by gas and dust. This oblong spiral galaxy with 6.9 by 3.0 arcminutes and 26,000 light-years in diameter, is located 13 million light-years away from Earth and lies 4 degrees off the galactic plane. It is the closest

to the Milky Way, and therefore hosts an active galactic nucleus.

Circinus X-1 Circinus X-1 is an X-ray binary star system that includes a neutron star. Observation of Circinus X-1 in July 2007 revealed the presence of X-ray jets normally found in black hole systems; it is the first of the sort to be discovered tha ...

is an

X-ray binary X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the ''donor'' (usually a relatively normal star), to the other component, called the ''accretor'', which ...

star system that includes a

neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...

. Observations of Circinus X-1 in July 2007 revealed the presence of X-ray jets normally found in black hole systems. Located at 19,000 light-years, the pulsar PSR B1509-58, also called the Circinus Pulsar, has expelled a 20-light-year-long jet of material from its southern pole, clearly visible in the

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

spectrum. Another supernova remnant in Circinus is that of SN 185. Recorded by Chinese observers in 185 AD, SN 185 was visible in the night sky for around eight months; its remnants, known as RCW 86, cover an area larger than the typical full moon. A white dwarf star in a close binary system can accumulate material from its companion until it ignites and blows off in a thermonuclear explosion, known as a nova. These stars generally brighten by 7 to 16 magnitudes. Nova Circini 1926, also known as X Circini, was observed at magnitude 6.5 on 3 September 1926, before fading and fluctuating between magnitudes 11.7 and 12.5, during 1928, and magnitude 13, in 1929. Nova Circini 1995 ( BY Circini) reached a maximum apparent magnitude of 7.2 in January 1995. BW Circini is a low mass X-ray binary system, comprising a black hole of around 8 solar masses and a yellow G0III-G5III subgiant star. X-ray outbursts were recorded in 1987 and 1997, and possibly 1971–72.

Meteor showers

Circinus is the

radiant Radiant may refer to: Computers, software, and video games * Radiant (software), a content management system * GtkRadiant, a level editor created by id Software for their games * Radiant AI, a technology developed by Bethesda Softworks for '' ...

of an annual meteor shower, the Alpha Circinids (ACI). First observed in

Queensland ) , nickname = Sunshine State , image_map = Queensland in Australia.svg , map_caption = Location of Queensland in Australia , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , establishe ...

in 1977, the meteors have an average velocity of 27.1 km/s and are thought to be associated with a

long-period comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are ...

. In 2011, Peter Jenniskens proposed that the debris trail of comet C/1969 T1 could intersect with the Earth's orbit and generate a meteor outburst coming from a radiant close to Beta Circini. The ACI shower peaks on 4 June, the day it was first observed.

References

External links

The Deep Photographic Guide to the Constellations: Circinus
{{DEFAULTSORT:Circinus Southern constellations Constellations listed by Lacaille