Circinus constellation is located in the southern sky. Its name means “the compass” in Latin, referring to the tool for drawing circles.
Circinus was created and first catalogued by the French astronomer Nicolas Louis de Lacaille in the 18th century. Lacaille created the constellation to bridge the void between Triangulum Australe and the stars marking Centaurus‘ forefeet.
Circinus contains several notable deep sky objects, including the Circinus Galaxy, the X-ray source Circinus X-1, the pulsar PSR B1509-58, the open clusters NGC 5823 and NGC 5715, and the planetary nebula NGC 5315.
FACTS, LOCATION & MAP
Circinus is the fourth smallest constellation in the sky, 85th in size, occupying 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°.
Circinus does not have 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, which 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 (spectral class G0VFe+0.4) and HD 129445 (G6V). HD 134060 has two planets in its orbit, both discovered in 2011, and HD 129445 has one, a Jupiter-like planet discovered in 2010. Circinus does not contain any Messier objects. It is associated with a meteor shower that peaks on June 4, the Alpha Circinids (ACI), first observed in 1977.
There are no myths associated with Circinus. The constellation was named after the drafting tool used for drawing circles (not after a mariner’s compass, which is represented by the constellation Pyxis).
It is depicted as a pair of dividing compasses used by draughtsmen to measure distances.
Lacaille originally named the constellation le Compas and placed it next to Triangulum Australe, which he depicted as a surveyor’s level, to fill the void between several existing constellations in the south.
MAJOR STARS IN CIRCINUS
α Circini (Alpha Circini)
Alpha Circini is the brightest star in the constellation, with an apparent visual magnitude of 3.19. It is a visual binary, with a faint companion. It is classified as a variable star, belonging to the class of rapidly oscillating Ap stars. It lies about 53.5 light years from Earth.
β Circini (Beta Circini)
Beta Circini is the second brightest star in Circinus, with an apparent magnitude of 4.069. It is a main sequence star belonging to the spectral class A3Va. It is approximately 97 light years distant.
γ Circini (Gamma Circini)
Gamma Circini is another binary star in the constellation. It is composed of a pair of blue and yellow stars.
HD 129445 is a star belonging to the spectral class G6V, notable for having a planet in its orbit. The planet was named HD 129445 b. Its existence was confirmed by 17 doppler velocity tests conducted as part of the Magellan Planet Search Program. The star has an apparent magnitude of 8.8 and is 220 light years distant.
DEEP SKY OBJECTS IN CIRCINUS
The Circinus Galaxy is a spiral galaxy approximately 13 million light years distant. The galaxy is notable for the motions of gas that form two rings inside it. One of the rings is a site of massive star forming activity.
The galaxy was discovered in 1975. It is an active galaxy with a black hole-powered core.
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. The system is located approximately 30,700 light years away. The X-ray source was discovered on June 14, 1969, during a scan of the region of the sky occupied by the constellations Circinus, Norma and Lupus.
NGC 5315 is a bright planetary nebula in Circinus. Like all other planetary nebulae, it was created when a star in the final stages of its life shed its outer layers and cast them off into space.
The nebula is located 5.2 degrees west-southwest of the star Alpha Circini. It has an apparent magnitude of 9.8. The nebula’s central star has a magnitude of 14.2.
NGC 5823 (Caldwell 88) is an open cluster in Circinus, located on the border with the constellation Lupus. The cluster is about 800 million years old. It has an apparent magnitude of 7.9 and lies at a distance of 3,500 light years from Earth.
NGC 5823 was discovered by the Scottish astronomer James Dunlop in 1826. It spans about 12 light years.
NGC 5715 is another open cluster in Circinus. It has a visual magnitude of 9.8 and is considerably smaller than NGC 5823. The cluster occupies an area 7 arcminutes in size and contains only 30 stars.
Pismis 20 is an open cluster that contains only 12 stars. It has an apparent size of only 4.5 arcseconds but, at magnitude 7.8, is almost as bright as NGC 5823. Pismis 20 lies at a distance of 8,270 light years from Earth.
PSR B1509-58 is a pulsar located about 17,000 light years from Earth, discovered in 1982. The pulsar is embedded in a nebula about 150 light years across. It is believed to be about 1,700 years old.
PSR B1509-58 was first detected by the Einstein X-Ray Observatory, NASA’s first fully imaging X-ray telescope to be launched into space. The pulsar has a spin rate of almost 7 times per second.