Circinus
“The Drawing Compasses” · Southern · best around June evenings
Real star positions and magnitudes (HYG database via D3-Celestial, BSD-3). Lines are the conventional stick figure; north is up.
The story
Lacaille's draftsman's compasses, squeezed beside Alpha Centauri. Tiny and faint, but it lends its name to the Circinus Galaxy, one of the nearest active galaxies, hidden behind Milky Way dust.
How to find it
Right beside Alpha Centauri.
✦ What lives inside it
- Circinus Galaxy
The deep dive
Researched for the Atlas from Wikipedia — Circinus (11,690 characters read) · updated Sep 20, 2026
01 Lacaille's Draughtsman's Toolkit
When Nicolas-Louis de Lacaille introduced Circinus in 1756, he was not simply filling blank space on a star map. He imagined the southern sky as a draughtsman's workshop, and Circinus was one tool among several. On his original chart, Norma represented a set square and ruler, Triangulum Australe a surveyor's level, and Circinus a pair of dividing compasses — the instrument used to step off precise distances on paper. The three constellations were grouped together as a coherent set of instruments rather than isolated inventions. Lacaille named it with the French le Compas in 1756, but the Latinised form Circinus did not appear until 1763, when he published a revised sky map standardising his constellation names into Latin. The International Astronomical Union later ratified the three-letter abbreviation 'Cir' in 1922. It is worth noting that Circinus represents a drafting compass, not a magnetic compass — that distinction belongs to Pyxis, a wholly separate constellation with no relation to Circinus either in position or in lore.
02 A Faint Sliver of Sky With Hard Borders
Circinus occupies a narrow wedge of the southern sky, hemmed in by six neighbours: Centaurus, Musca, Apus, Triangulum Australe, Norma, and Lupus. Its official boundaries were formalised in 1930 by Belgian astronomer Eugène Delporte, who defined them as a polygon of 14 segments. In the equatorial coordinate system, the constellation's right ascension runs from 13 hours 38.4 minutes to 15 hours 30.2 minutes, and its declination spans a band between −55.43° and −70.62°. Because it sits between declination −50° and −70°, the entire constellation clears the horizon only for observers south of latitude 30° N — roughly the latitude of Cairo or Houston. Observers in Melbourne, Cape Town, or Santiago see it ride reasonably high. Circinus culminates each year at 9 p.m. on 30 July, making mid-winter evenings in the southern hemisphere its prime viewing season. Its proximity to the brilliant pair of Alpha and Beta Centauri gives useful anchor stars for anyone attempting to identify it, since it lies immediately adjacent to those landmarks.
03 Alpha Circini: A Star That Rings Like a Bell Deeper
Alpha Circini carries the approved name Xami and sits 54 light-years from Earth, just 4 degrees south of the dazzling Alpha Centauri. At apparent magnitude 3.19 it is the only star in the constellation brighter than fourth magnitude, and it holds a remarkable distinction: it is the brightest rapidly oscillating Ap (RoAp) star in the entire night sky. Its spectral classification reads A7 Vp SrCrEu, meaning its atmosphere is anomalously enriched in strontium, chromium, and europium — elements concentrated in patches by its oddly localised magnetic field. That patchy field is a defining feature of Ap stars and is responsible for the slight but measurable variability in brightness. Alpha Circini is not alone: it forms a genuine binary system with an orange dwarf companion of spectral type K5 and magnitude 8.5. The two are separated by 5.7 arcseconds on the sky — close enough to require a telescope to split — but their true physical separation is 260 AU, roughly 260 times the Earth-Sun distance. At that wide orbit, they take an estimated 2600 years to complete a single revolution around their shared centre of gravity.
04 Delta Circini: Behemoths in Orbit Deeper
Delta Circini is one of the most physically extreme systems in this modest constellation. It is a multiple-star system located over 3600 light-years away, with two main components separated by 50 arcseconds — just enough that observers with sharp unaided eyes might resolve them, and easily split with any telescope. The brighter component is itself a close eclipsing binary classified as a rotating ellipsoidal variable, meaning the gravitational distortion between its component stars creates a slight dip of 0.1 magnitudes. The two stars within this tight inner pair are both hot blue giants, typed O7III-V and O9.5V, and carry estimated masses of roughly 22 and 12 times the Sun's mass respectively — among the most massive stars charted in Circinus. The outer companion has a magnitude of 13.4 and orbits the inner pair every 3.9 days. If the entire system were relocated to just 32 light-years away — the standard distance used for absolute magnitude comparisons — it would outshine Venus, blazing at magnitude −4.8 in our sky. As it stands, the great distance dims what would otherwise be one of the most spectacular stellar systems visible from Earth.
05 Gamma Circini and Beta Circini Up Close
Beta Circini, the second brightest star in the constellation at magnitude 4.07, is a white main sequence star of spectral type A3Va located about 100 light-years away. Its diameter is approximately 1.8 times that of the Sun, making it noticeably larger than our star but still modest by the scale of the galaxy. Gamma Circini, at magnitude 4.51, is a more challenging target. It is a binary system 450 light-years distant whose two components are only 0.8 arcseconds apart, requiring a telescope of at least 150 mm aperture to resolve them. The brighter star is a bluish Be star of spectral type B5IV+, while its companion is a yellow star of magnitude 5.5. The two orbit each other over a period of 180 years. Be stars are notable for their emission-line spectra, caused by a surrounding disc of gas ejected from the rapidly rotating star. These two stars illustrate the variety packed into Circinus despite its small footprint: one system is a straightforward, slightly oversized sibling of the Sun, while the other requires dedicated optics and offers an ongoing slow-motion orbital dance visible across centuries.
06 Variable Stars: 493 and Counting Deeper
Circinus has been found to contain 493 variable stars, a striking number for a constellation with only one star brighter than fourth magnitude. Most show very small amplitude changes or are too faint to observe casually. Among the notable ones, AX Circini is a Cepheid variable — the classic pulsating supergiant used as a cosmic distance marker — fluctuating between magnitudes 5.6 and 6.19 over a period of 5.3 days. It is a yellow-white supergiant of spectral type F8II+, lying 1600 light-years from Earth and visible to the naked eye. BP Circini is a fainter Cepheid, ranging from magnitude 7.37 to 7.71 over just 2.4 days. Both Cepheids are also spectroscopic binaries, each paired with a blue-white companion star. At the other extreme, BX Circini oscillates between magnitudes 12.57 and 12.62 in a period of just 2 hours 33 minutes. More than 99% of its composition appears to be helium, and its origin is thought to involve the merger of a helium white dwarf with a carbon/oxygen white dwarf — a theorised outcome that would make it one of the more unusual stellar remnants in this part of the sky. Its origin remains debated rather than settled.
07 Planets Around Sun-Like Neighbours
Two sun-like stars within Circinus have confirmed planetary systems, both discovered using the radial velocity method, which detects the subtle gravitational wobble a planet induces in its host star. HD 134060 is a yellow dwarf of spectral type G0VFe+0.4 and magnitude 6.29, located about 79 light-years away. Its two planets were announced in 2011. The inner planet, HD 134060 b, has a mass of 0.0351 Jupiter masses and orbits every 3.27 days at just 0.0444 AU — a scorchingly close orbit that would place it well inside the orbit of Mercury if transplanted to our solar system. The outer planet, HD 134060 c, is more substantial at 0.15 Jupiter masses and orbits at 2.226 AU with a period of around 1161 days, closer in character to a miniature version of Jupiter. HD 129445 is fainter, at magnitude 8.8 and 220 light-years away, with 99% of the Sun's mass. Its planet, HD 129445 b, has 1.6 Jupiter masses and orbits at 2.9 AU over a period of approximately 1840 days — a genuinely Jupiter-analogue world in terms of its orbital position.
08 SN 185: The Oldest Recorded Supernova
In 185 AD, Chinese observers recorded a 'guest star' that appeared in what we now call Circinus and remained visible in the night sky for approximately eight months. This event, catalogued as SN 185, is regarded as the oldest confirmed supernova observation in human history. Its remnant is known as RCW 86, a shell of expanding gas and dust that today covers an area on the sky larger than the typical full moon — a ghostly testament to the violence of the original explosion. Circinus also witnessed more recent stellar fireworks in the 20th century. Nova Circini 1926, now designated X Circini, was observed at magnitude 6.5 on 3 September 1926, before fading and fluctuating between magnitudes 11.7 and 12.5 by 1928 and settling near magnitude 13 in 1929. Nova Circini 1995, designated BY Circini, reached a peak apparent magnitude of 7.2 in January 1995. Both events fit the standard nova mechanism: a white dwarf in a close binary system accumulates material from its companion until a thermonuclear runaway ignites on the surface, temporarily brightening the star by 7 to 16 magnitudes.
09 The Circinus Galaxy: A Hidden Seyfert
The Circinus Galaxy, formally catalogued as ESO 97-G13, was discovered in 1977 and holds the distinction of being the closest Seyfert galaxy to the Milky Way, sitting just 13 million light-years away. A Seyfert galaxy is a type of spiral galaxy with an exceptionally luminous nucleus powered by an active galactic nucleus — in effect, a supermassive black hole actively consuming surrounding material. Its proximity makes it scientifically valuable, yet it went unnoticed until 1977 because it lies only 4 degrees off the plane of the Milky Way, a region where the gas and dust of our own galaxy typically blot out background objects. The Circinus Galaxy is unusual in that it is relatively unobscured despite this position, earning it a measured magnitude of 10.6. It is an oblong spiral spanning 6.9 by 3.0 arcminutes on the sky, with a physical diameter of 26,000 light-years — roughly a quarter the diameter of the Milky Way. For amateur observers, it is a genuine deep-sky target, though its low surface brightness and southerly position demand dark, transparent skies.
10 X-Ray Binaries and the Circinus Pulsar Deeper
Circinus contains several systems that reveal themselves not in visible light but in high-energy radiation. Circinus X-1 is an X-ray binary comprising a neutron star, and observations made in July 2007 revealed X-ray jets emanating from the system — the kind of jets previously associated almost exclusively with black hole systems, making Circinus X-1 a scientifically provocative object. The Circinus Pulsar, also known as PSR B1509-58, lies about 19,000 light-years away and has expelled a jet of material from its southern pole stretching 20 light-years in length, clearly visible in X-ray imaging. Then there is BW Circini, a low-mass X-ray binary pairing a black hole of roughly 8 solar masses with a yellow subgiant star classified as G0III-G5III. Outbursts from this system were recorded in 1987 and 1997, and a possible earlier event was noted in 1971-72. These X-ray systems sit in a constellation that optically appears sparse and unremarkable, a reminder that the sky as seen through X-ray telescopes can look very different from the one familiar to naked-eye observers.
11 The Alpha Circinids Meteor Shower
Circinus lends its name to an annual meteor shower, the Alpha Circinids, abbreviated ACI. The shower was first observed in Queensland, Australia, in 1977 — the same year that also saw the discovery of both the Circinus Galaxy and the Alpha Circinids themselves, making it a memorable year for Circinus research. The meteors strike Earth's atmosphere at an average velocity of 27.1 km/s, and they are thought to originate from the debris trail of a long-period comet, though the precise parent body remains uncertain. In 2011, astronomer Peter Jenniskens proposed that the debris trail of comet C/1969 T1 could intersect Earth's orbit and produce a notable meteor outburst, with the radiant located near Beta Circini. The shower peaks on 4 June, the calendar date of its first recorded observation. Because the radiant lies in the deep southern sky, the Alpha Circinids are best observed from southern hemisphere locations, where the constellation climbs to a useful altitude above the horizon — the same geographic constraint that applies to Circinus itself as a constellation target.
12 Wolf-Rayet Stars and Massive Stellar Residents Deeper
Because Circinus intersects the plane of the Milky Way, its sight lines plunge through dense star-forming regions of the galactic disc, revealing a disproportionate number of massive, short-lived stars. The constellation contains nine Wolf-Rayet stars — a notably high count for a constellation of its small size. Wolf-Rayet stars are extremely hot, luminous objects that have shed most of their outer hydrogen envelopes through powerful stellar winds, exposing deeper layers of nuclear-processed material. They represent late stages in the evolution of the most massive stars before their eventual supernova deaths. Circinus also contains GKF2010 MN18, also known simply as MN18, a blue supergiant embedded within a bipolar nebula — a double-lobed shell of gas and dust shaped by the supergiant's outflowing wind. The presence of so many extreme stars concentrated in this narrow strip of sky makes Circinus a surprisingly rich hunting ground for researchers studying stellar evolution and high-mass star formation, even though it is one of the least conspicuous constellations to a casual naked-eye observer in the southern sky.