Circinus is a small, faint constellation in the southern sky, first defined in 1756 by the French astronomer Nicolas-Louis de Lacaille. Its name is Latin for compass, 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). Bordered by Centaurus (Circinus is located at the bottom of Centaurus’ left hoof), Musca, Apus, Triangulum Australe, Norma and Lupus, Circinus lies adjacent to the Alpha and Beta Centauri stars. As it is at declination−50° to −70°, the whole constellation is only visible south of latitude 30° N. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 14 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 13h 38.4m and 15h 30.2m, and the declination coordinates are between −55.43° and −70.62°. Circinus culminates each year at 9 p.m. on 30 July.
With only one star brighter than the fourth magnitude, Alpha Circini, a white main sequence star with an apparent magnitude of 3.19, is 54 light-years away and 4° south of Alpha Centauri. Not only the brightest star in the constellation, but 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, chromium and europium. Stars of this type have oddly localised magnetic fields and are slightly variable. Alpha Circini forms a binary star system with an orange dwarf companion of spectral type K5 and magnitude 8.5. 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.
There are 493 variable stars recorded in Circinus, but most have a very small range or are quite dim. Three prominent examples are Theta Circini, T Circini, and AX Circini. 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 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.
Several stars with planetary systems lie within the borders of Circinus, although none of the host stars is particularly prominent. HD 134060 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; the smaller, HD 134060 b, has a mass of 0.0351 MJ (Jupiter masses) and orbits its star every 3.27 days; the larger, HD 134060 c (0.15 MJ), orbits farther out with a period of approximately 1161 days. Even fainter, at magnitude 8.8, HD 129445 is 220 light-years away and has 99% of the Sun’s mass and a similar spectral type of G8V. HD 129445 b, a Jupiter-like planet (1.6 MJ) discovered in 2010 orbits this star at a distance of 2.9 AU, approximately every 1840 days.
Three open clusters and a planetary nebula are found within the borders of Circinus, all visible with amateur telescopes of varying sizes. NGC 5823, also called Caldwell88, 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 Alpha Centauri. It contains 80–100 stars of 10th magnitude and fainter. Comparatively, open cluster NGC 5715 is fainter (integrated magnitude of 9.8)—its brightest star is only 11th magnitude—and smaller, comprising only 30 stars. The third open cluster, Pismis 20, contains 12 stars in a diameter of 4.5 arcseconds at 8270 light-years.
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. 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 Milky Way, 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. It is the closest Seyfert galaxy to the Milky Way and therefore hosts an active galactic nucleus.
Circinus X-1 is an X-ray binary star system that includes a neutron star. 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 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 brighten by 7 to 16 magnitudes.
Circinus is the radiant of an annual meteor shower, the Alpha Circinids (ACI). First observed in Queensland in 1977, the meteors have an average velocity of 27.1 km/s and are thought to be associated with a long-period comet. 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. Credit: Wikipedia.