Circinus (“the compass”) commemorates the drawing instrument used by navigators, and is one of 14 southern sky constellations created by French astronomer Nicolas Louis de Lacaille in the 18th century. The constellation is circumpolar, with its brightest stellar object, Alpha Circini, a white variable star found about 53.5 light years distant of magnitude 3.19. There is one meteor shower associated with the constellation, namely the Alpha Circinids which peaks on June 4th.
Circinus is the 85th largest constellation in the sky, making it the 4th smallest of the 88 recognized constellations. It can be seen by observers located between +30° and -90° of latitude, although best seen from March to August. Circinus lies in the direction of the Milky way, and can be found west of Triangulum Australe, and to the south-east of Centaurus, which contains the bright star Alpha Centauri.
Lacaille Constellation Family
Circinus is a member of the Lacaille family of constellations, together with Antlia, Caelum, Fornax, Horologium, Mensa, Microscopium, Norma, Octans, Pictor, Reticulum, Sculptor, and Telescopium.
French astronomer Nicolas-Louis de Lacaille created Circinus during his 1751-1752 expedition to South Africa to study the southern skies. Its name Circinus refers to a drafting compass, with the constellation placed in the celestial heavens near to Norma (carpenter’s square) and Triangulum Australe (surveyor’s level).
– Alpha Circini, the constellation’s brightest star, is a white main sequence star located 53.5 light years from our solar system with a visual magnitude of 3.19. It has around twice our sun’s size and mass, and 10 times its luminosity. Alpha Circini is also an example of an oscillating Ap variable star, which exhibit rapid non-radial pulsation cycles of between 5 and 23 minutes.
– Beta Circini, the second brightest star in Circinus, is a blue-white subgiant (A3Va) found 97 light years from the Sun with a magnitude of 4.07. This 370 million year old star has around twice the Sun’s size and mass.
– Gamma Circini, the constellation’s third brightest star, is a binary system situated 450 light-years away that shines with a magnitude of 4.48. Its primary component is a blue giant (B5III + F8) around 10 times bigger than the Sun, and six times more massive. Its companion is a dimmer yellow star, and together the system has an orbital period of 258 years.
Other stars of interest in the constellation of Circinus includes the yellow giant Eta Circini; the yellow dwarf HD 134060; the blue dwarf Zeta Circini; and more than 493 variable stars, the most prominet of which are Theta Circini, T Circini, and AX Circini.
Notable Deep Sky Objects
There are no Messier objects in Circinus, although its location along the Milky Way means that it does contains a number of notable deep-sky objects.
– Circinus Galaxy (ESO 97-G13) is an example of a Seyfert galaxy, which have extremely bright nuclei and usually have a supermassive black hole at their centres. It is 26,000 light-years wide, shines with a visual magnitude of 10.6, and at 13 million light-years distant is one of the closest active galaxies to our own Milky Way. Its outer ring is located around 700 light-years from the galactic core, and is an active region for star formation. The Circinus Galaxy was also home to the supernova explosion named SN 1996cr, which despite being one of the nearest such explosions in three decades, went undiscovered for several years.
– Circinus X-1 is an X-ray producing binary star system consisting of a neutron star orbiting a main sequence star. It is around 30,700 light years away, and produces X-ray jets usually associated with black hole systems.
– NGC 5823 (Caldwell 88) is an open cluster situated around 3,500 light years distant with an apparent visual magnitude of 7.9. It is about 800 million years old, 12 light years across and can be found near the border of the constellation Lupus.
– PSR B1509-58 is a pulsar found inside a 150 light years wide nebula 17,000 light years from our sun. Pulsating radio stars are actually rotating neutron stars, or dense objects left over after a supernova explosion, which emit a focused beam of electromagnetic radiation.
Other objects of interest in Circinus includes the planetary nebula NGC 5315; and the open clusters NGC 5715 and Pismis 20.
Circinus is home to the Alpha Circinids meteor shower, which each year peaks on June 4th when around 15 meteors per hour can be seen. From northern latitudes, however, the radiant lies very low towards the south around midnight, meaning only its brightest meteors can be seen. The Alpha Circinids were discovered by Australian astronomers in 1977, and in 2011 American astronomer Peter Jenniskens proposed that the shower may be caused by a debris trail left behind by the long-period comet C/1969 T1.