Circinus (the Compasses) is a small, faint constellation in the southern sky, introduced by French astronomer Nicolas-Louis de Lacaille during the 18th century. Its name refers to the drafting compass used to draw circles, rather than the magnetic compass used for navigation.
Its brightest star, Alpha Circini, is a rapidly pulsating white star approximately 54 light-years away. Although Circinus contains few bright stars, it is home to one of the nearest active galaxies, a remarkable X-ray binary, and several interesting star clusters.
Where Is Circinus and When Can You See It?
Circinus is the 85th-largest of the 88 recognized constellations, covering approximately 93 square degrees of the southern sky. It lies near Centaurus and Triangulum Australe, close to the bright stars of the Alpha Centauri system.
The constellation is best observed during May and June, particularly from the Southern Hemisphere. It is circumpolar from sufficiently southern latitudes, meaning it never sets below the horizon for those observers.
Circinus lies against the rich star fields of the Milky Way, although its own principal stars form a relatively inconspicuous pattern.
Who Created the Circinus Constellation?
Circinus was introduced by Nicolas-Louis de Lacaille, who mapped the southern sky while observing from the Cape of Good Hope in South Africa between 1751 and 1753. He named it after the drafting compass, a two-legged instrument used by mathematicians, engineers, and artists to draw circles and measure distances on diagrams. Its placement near Norma, the Carpenter’s Square, and Triangulum Australe reflects Lacaille’s interest in scientific and measuring instruments.
Unlike the ancient Greek constellations, Circinus has no traditional mythology.
Notable Stars in Circinus
- Alpha Circini – The constellation’s brightest star, a white main-sequence star approximately 54 light-years away with an apparent magnitude of 3.2. It belongs to the rapidly oscillating Ap (roAp) class, whose chemically peculiar stars undergo subtle brightness variations caused by pulsations lasting only a few minutes.
- Beta Circini – A white star approximately 100 light-years away, shining at magnitude 4.1. It is hotter and more luminous than the Sun.
- Gamma Circini – A binary system approximately 450 light-years away, with a combined apparent magnitude around 4.5. Its brighter component is a hot blue giant accompanied by a cooler, fainter star.
- Theta Circini – A variable star whose changing brightness makes it useful for studying stellar pulsations.
- HD 134060 – A Sun-like star known to host exoplanets, illustrating how even this small constellation contains planetary systems beyond our own.
Deep-Sky Objects in Circinus
Circinus contains no Messier objects, but its position along the Milky Way makes it home to several interesting stellar objects. It also contains a nearby galaxy whose active nucleus is largely hidden behind interstellar dust.
The Circinus Galaxy — A Nearby Active Galaxy
The Circinus Galaxy (ESO 97-G13) is a spiral galaxy approximately 13 million light-years away, making it one of the nearest galaxies with an active galactic nucleus. It is classified as a Seyfert galaxy, meaning its central region produces unusually intense radiation associated with material falling toward a supermassive black hole.
Much of the galaxy is obscured by dust in the foreground Milky Way, which explains why it was not identified until the 1970s. Observations reveal rings of gas and star formation around its center, along with evidence of energetic material flowing outward from the active nucleus.
The galaxy also hosted SN 1996cr, an unusual supernova that was recognized years after its explosion through observations at other wavelengths.
Circinus X-1 — A Neutron Star with Powerful Jets
Circinus X-1 is an X-ray binary system containing a neutron star that draws material from a companion star. As the material spirals inward, it becomes extremely hot and emits powerful X-rays. The system is particularly interesting because it produces jets of energetic matter, demonstrating that neutron stars, as well as black holes, can power these dramatic outflows.
Astronomers also study its unusual orbit and variable X-ray emission to understand how compact stellar remnants interact with their companions.
NGC 5823 — A Southern Open Cluster
NGC 5823, also known as Caldwell 88, is an open star cluster approximately 3,500 light-years away, with an apparent magnitude around 7.9. It contains a collection of stars that formed from the same cloud of gas and dust. Unlike tightly packed globular clusters, open clusters are relatively loose groups whose members gradually disperse over time.
NGC 5823 is one of the more accessible telescopic targets in Circinus.
PSR B1509−58 — A Young Pulsar
PSR B1509−58 is a young, rapidly rotating neutron star approximately 17,000 light-years away. It lies within a complex region of glowing gas associated with a supernova remnant and a pulsar wind nebula. The pulsar releases energetic particles that interact with the surrounding material, creating structures visible in X-ray observations.
Images from NASA’s Chandra X-ray Observatory reveal an extraordinary arrangement of filaments and energetic gas, sometimes described as resembling a cosmic hand.
Does Circinus Have a Meteor Shower?
The Alpha Circinids have been reported as a meteor shower associated with this region of the southern sky, with activity suggested around early June. However, the shower is poorly established compared with major annual showers such as the Perseids or Geminids. Claims of a reliable peak rate of 15 meteors per hour should therefore be treated cautiously.
Circinus is not generally considered a major meteor-watching destination.
How to Find Circinus
Begin by locating the bright Alpha and Beta Centauri stars, then look toward nearby Triangulum Australe. Circinus occupies a small area between these more recognizable southern landmarks. Its brightest star, Alpha Circini, can be seen with the naked eye under reasonably dark skies.
A detailed star chart is useful for identifying the constellation, while binoculars or a small telescope can reveal some of its richer Milky Way star fields.