The Circinus constellation is not home to any formally named stars, nor any stars brighter than magnitude 3.00 or located within 10 parsecs (32.6 light years) of Earth. The brightest star in the constellation is Alpha Circini. It is also the nearest star in Circinus, located at a distance of 53.50 light years from Earth.
Circinus has two stars with known exoplanets, HD 134060, which has two planets in its orbit, and HD 129445, which has one planet in its orbit. Circinus also has a number of notable deep sky objects, including the Circinus Galaxy, Circinus X-1 and Pismis 20. It does not contain any Messier objects, but is associated with one meteor shower: the Alpha Circinids (ACI). The Alpha Circinids peak on June 4 and were first observed in 1977.
History and Mythology
The constellation of Circinus was created and first catalogued by the French astronomer Nicolas Louis de Lacaille in the 18th century. It was first called le Compas, and represented a pair of dividing compasses. Lacaille portrayed the constellations of Norma, Circinus, and Triangulum Australe, respectively, as a set square and ruler, a compass, and a surveyor’s level in a set of draughtsman’s instruments.
In 1763, Lacaille published an updated sky map with Latin names for the constellations he introduced, which is when Circinus was given its current name.
Circinus is not associated with any myths.
The constellation of Circinus is the fourth smallest constellation in the sky and the 85th largest. It occupies an area of only 93 square degrees. It lies in the third quadrant of the southern hemisphere (SQ3) and can be seen at latitudes between +30° and -90°.
Its right ascension is 13h 38.4m to 15h 30.2m and its declination is −55.43° to −70.62°. It is best seen at 9pm during the month of July. Circinus culminates each year at 9 p.m. on 30 July.
Circinus’ neighboring constellations are Apus, Centaurus, Lupus, Musca, Norma and Triangulum Australe and it belongs to the Lacaille family of constellations, along with Antlia, Caelum, Fornax, Horologium, Mensa, Microscopium, Norma, Octans, Pictor, Reticulum, Sculptor and Telescopium.
Alpha Circini is the brightest star in the constellation with an apparent magnitude of 3.19. It is classified as a variable star, belonging to the class of rapidly oscillating Ap stars, and is the brightest example of a rapidly oscillating Ap (RoAp) star in the night sky. Alpha Circini is about 53.5 light years away from Earth.
It is a white main sequence star that forms a binary star system with an orange dwarf companion of spectral type K5 and magnitude 8.5. The two stars are separated by 5.7 arcseconds.
Beta Circini is the second brightest star in Circinus with an apparent magnitude of 4.069. It is a white main sequence star of spectral type A3Va and has around 1.8 times the diameter of the Sun. It is located around 100 light years away from us.
Gamma Circini is a binary star located around 450 light years away. The brighter component is a bluish Be star of spectral type B5IV+ with a magnitude of 4.51, while the dimmer component is a yellow star of magnitude 5.5. They have an orbital period of 180 days and are located 0.8 arcseconds apart, which means they need a telescope of 150 mm to be seen.
Delta Circini is 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 (specifically, a rotating ellipsoidal variable), 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. They are located around 3600 light years away.
Eta Circini is a yellow giant of spectral type G8III and has a magnitude of 5.17. It is located around 276 light years away from us.
Zeta Circini is a blue-white main sequence star of the spectral type B3V. It has a magnitude of 6.09 and is located around 1273 light years away.
Theta Circini is a B-class irregular variable, ranging in magnitude from 5.0 to 5.4.
T Circini is a variable star that has a B-type spectrum, ranging in magnitude from 10.6 to 9.3 over a period of 3.298 days.
AX Circini 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+ and is located around 1600 light years away.
BP Circini is a Cepheid variable with an apparent magnitude ranging from 7.37 to 7.71 over 2.4 days. It is a yellow-white supergiant of spectral type F2 or F3II.
HD 129445 has an apparent magnitude of 8.8 and is around 220 light years away from us. It belongs to the spectral class G6V and has 99% of the Sun’s mass. The star is notable for having a planet in its orbit, named HD 129445 b.
HD 134060 is a sun-like yellow dwarf star of spectral type of G0VFe+0.4. It has a magnitude of 6.29 and is around 79 light years away.
Deep Sky Objects
Circinus Galaxy is a spiral galaxy with a magnitude of 10.6. It is located 13 million light years away from Earth and is the closest Seyfert galaxy to the Milky Way. It, therefore, hosts an active galactic nucleus with a black hole-powered core. The Circinus Galaxy was first discovered in 1977.
Circinus X-1 is an X-ray double star composed of a neutron star and a main sequence star, with the former orbiting the latter. It is located around 30,700 light years away from us.
NGC 5315 is a bright planetary nebula with an apparent magnitude of 9.8. The nebula’s central star has a magnitude of 14.2 and it is located around 5.2 degrees west-southwest of the star Alpha Circini.
NGC 5715 is an open cluster with a visual magnitude of 9.8. It occupies an area 7 arc minutes in size and contains only 30 stars.
NGC 5823, also known as Caldwell 88, is an open cluster with an apparent magnitude of 7.9. It was discovered by the Scottish astronomer James Dunlop in 1826 and is located around 3,500 light years away from Earth, on the border with the constellation Lupus. It spans about 12 light years and is about 800 million years old.
Pismis 20 is an open cluster and has an apparent magnitude of 7.8. It has an apparent size of 4.5 arc seconds and only contains 12 stars. It is located around 8,270 light years away from Earth.
PSR B1509-58 is a pulsar that was discovered in 1982 and first detected by the Einstein X-Ray Observatory, NASA’s first fully imaging X-ray telescope to be launched into space. It is located around 17,000 light years away from Earth and is believed to be about 1,700 years old. It spans about 150 light years across.
- The recommended three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is “Cir”.
- The official constellation boundaries, as set by Belgian astronomer Eugène Delporte in 1930, are defined by a polygon of 14 segments.
- The whole constellation is only visible south of latitude 30° N.
- Some Images created with the NightVision app – https://www.nvastro.com/nvj.html
- Some Images created with the Stelvision Sky Map https://www.stelvision.com/en/sky-map/
- Circinus 1 – https://starregistration.net/constellations/circinus-constellation.html
- Circinus 2 – https://in-the-sky.org/data/constellation.php?id=24
- Circinus 3 – https://en.wikipedia.org/wiki/Circinus
- Circinus Galaxy – https://en.wikipedia.org/wiki/Circinus_Galaxy
- Pismis 20 – https://commons.wikimedia.org/wiki/File:Pismis_20_large.png
- Circinus X-1 – https://www.nasa.gov/mission_pages/chandra/multimedia/xray-binary-circinus-x1.html