Circinus

Circinus constellation is located in the southern sky. Its name means ‘compass’ in Latin, referring to the tool for drawing circles. It 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°. The neighboring constellations are Apus, Centaurus, Lupus, Musca, Norma, and Triangulum Australe.

[http://www.constellation-guide.com/constellation-list/circinus-constellation/]

Circinus from Chart XX of the Uranographia of Johann Bode, with Triangulum Australe to its left and Norma et Regula (the set square and ruler) just off the top of the picture. The bright star at the right is Alpha Centauri.

Circinus is an insignificant constellation representing a pair of dividing compasses as used by geometers, draughtsmen, and navigators for drawing circles and measuring distances; they are also known as dividers. It was introduced in the 1750s by the Frenchman Nicolas Louis de Lacaille, who fitted various figures into gaps between the existing constellations of the southern skies. In this case the gap seems to have been almost non-existent, and the compasses are squeezed in their folded position between the forefeet of Centaurus and Triangulum Australe. It is the smallest of Lacaille’s 14 inventions, and the fourth-smallest constellation in the entire sky.

The constellation first appeared under the French name le Compas on Lacaille’s preliminary chart of the southern skies published by the Académie Royal des Sciences in 1756. Its name was Latinized to Circinus on Lacaille’s chart of 1763.

The instrument is conveniently placed next to Triangulum Australe, a pre-existing constellation formed by Keyser and de Houtman which Lacaille visualized as a surveyor’s level, and Norma the set square, another of Lacaille’s inventions, thereby forming a related set of instruments.
[http://www.ianridpath.com/startales/circinus.htm]

Constellations of Circinus and Triangulum Australe

[http://www.davidmalin.com/fujii/source/TrA.html]

[http://astropixels.com/constellations/charts/Cir.html]

Circinus is a faint constellation, with only one star brighter than fourth magnitude, Alpha Circini. This is 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, it is also the brightest example of a rapidly oscillating star in the night sky. It has an unusual spectral type, showing increased emissions of strontium, chromium and europium. These stars have oddly localized 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, which is only discernible with small telescopes with a separation of 5.7 arcseconds. The distance between both stars is 260 AU and they take 2600 years to rotate around a common center 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.

Gamma Circini is a binary star 450 light-years away, whose components need a telescope to be seen. The brighter component is a bluish star of magnitude 4.51, while the dimmer component is a yellow star of magnitude 5.5. They orbit each other every 180 years.

Several stars with planetary systems lie within the borders of Circinus, although none of the host stars are particularly prominent. HD 134060 is a sun-like yellow dwarf star of magnitude 6.29, around 79 light-years away. Its two planets were discovered in 2011 through the radial velocity method: the smaller, HD 134060 b, has a mass of 0.0351 MJ (Jupiter masses) and orbits its star every 3.27 days, at 0.0444 AU; and the larger, HD 134060 c (0.15 MJ), orbits farther out at 2.226 AU, 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 via the radial velocity method, orbits this star at a distance of 2.9 AU, approximately every 1840 days.

493 variable stars have been 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 actually 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. BP Circini is another Cepheid variable with an apparent magnitude ranging from 7.37 to 7.71 over 2.4 days. Both cepheids are spectroscopic binaries, with companions that are blue-white stars of spectral type B6 and 5 and 4.7 solar masses respectively. BX Circini is a faint star that fluctuates between magnitudes 12.57 and 12.62 over a period of 2 hours 33 minutes. Over 99% of its composition appears to be helium. Its origin is unclear, but thought to be the result of the merger of a helium and a carbon/oxygen white dwarf.

PSR B1509-58 is a pulsar (rotating neutron star) approximately 17,000 light-years away in the constellation of Circinus discovered by the Einstein X-Ray Observatory in 1982. It is approximately 1700 years old and sits in a nebula that spans about 150 light years:

[https://en.wikipedia.org/wiki/PSR_B1509-58]

High-Energy X-ray View of ‘Hand of God’

Can you see the shape of a hand in this new X-ray image? The hand might look like an X-ray from the doctor’s office, but it is actually a cloud of material ejected from a star that exploded. NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR, has imaged the structure in high-energy X-rays for the first time, shown in blue. Lower-energy X-ray light previously detected by NASA’s Chandra X-ray Observatory is shown in green and red.

Nicknamed the ‘Hand of God,’ this object is called a pulsar wind nebula. It’s powered by the leftover, dense core of a star that blew up in a supernova explosion. The stellar corpse, called PSR B1509-58, or B1509 for short, is a pulsar: it rapidly spins around, seven times per second, firing out a particle wind into the material around it- material that was ejected in the star’s explosion. These particles are interacting with magnetic fields around the material, causing it to glow with X-rays. The result is a cloud that, in previous images, looked like an open hand. The pulsar itself can’t be seen in this picture, but is located near the bright white spot.

One of the big mysteries of this object is whether the pulsar particles are interacting with the material in a specific way to make it look like a hand, or if the material is in fact shaped like a hand.

NuSTAR’s view is providing new clues to the puzzle. The hand actually shrinks in the NuSTAR image, looking more like a fist, as indicated by the blue color. The northern region, where the fingers are located, shrinks more than the southern part, where a jet lies, implying the two areas are physically different.

The red cloud at the end of the finger region is a different structure, called RCW 89. Astronomers think the pulsar’s wind is heating the cloud, causing it to glow with lower-energy X-ray light.

In this image, X-ray light seen by Chandra with energy ranges of 0.5 to 2 kiloelectron volts (keV) and 2 to 4 keV is shown in red and green, respectively, while X-ray light detected by NuSTAR in the higher-energy range of 7 to 25 keV is blue.

[http://www.nasa.gov/jpl/nustar/B1509-pia17566]

SN 185 refers to an AD 185 transient astronomical event that was likely a supernova. The transient occurred in the direction of Alpha Centauri, between the constellations Circinus and Centaurus. This ‘guest star’ was observed by Chinese astronomers in the Book of Later Han, and might have been recorded in Roman literature. It remained visible in the night sky for eight months. This is believed to be the first supernova for which records exist:

[https://en.wikipedia.org/wiki/SN_185]

RCW 86: All Eyes on Oldest Recorded Supernova

Four space telescopes combined to make a new discovery on a very old supernova remnant. Chinese astronomers witnessed an event in that location in 185 AD, documenting a mysterious ‘guest star’ that remained for 8 months. The new data show that RCW 86 was created by a Type Ia supernova explosion. This type of supernova is caused when a white dwarf pulls material from a companion star until a thermonuclear reaction occurs.

This image combines data from four different space telescopes- Chandra, XMM-Newton, Spitzer, and WISE- to create a multi-wavelength view of all that remains of the oldest documented example of a supernova, called RCW 86. The Chinese witnessed the event in 185 A.D., documenting a mysterious ‘guest star’ that remained in the sky for eight months. X-ray images from NASA’ Chandra X-ray Observatory and the European Space Agency’s XMM-Newton Observatory are combined to form the blue and green colors in the image. The X-rays show the interstellar gas that has been heated to millions of degrees by the passage of the shock wave from the supernova.

Infrared data from NASA’s Spitzer Space Telescope, as well as NASA’s Wide-Field Infrared Survey Explorer (WISE) are shown in yellow and red, and reveal dust radiating at a temperature of several hundred degrees below zero, warm by comparison to normal dust in our Milky Way galaxy.

By studying the X-ray and infrared data together, astronomers were able to determine that the cause of the explosion witnessed nearly 2,000 years ago was a Type Ia supernova, in which an otherwise-stable white dwarf, or dead star, was pushed beyond the brink of stability when a companion star dumped material onto it. Furthermore, scientists used the data to solve another mystery surrounding the remnant- how it got to be so large in such a short amount of time. By blowing a wind prior to exploding, the white dwarf was able to clear out a huge ‘cavity,’ a region of very low-density surrounding the system. The explosion into this cavity was able to expand much faster than it otherwise would have.

This is the first time that this type of cavity has been seen around a white dwarf system prior to explosion. Scientists say the results may have significant implications for theories of white-dwarf binary systems and Type Ia supernovae.

RCW 86 is approximately 8,000 light-years away. At about 85 light-years in diameter, it occupies a region of the sky in the southern constellation of Circinus that is slightly larger than the full moon.

[http://chandra.harvard.edu/photo/2011/rcw86/index.html]

Three open clusters and a planetary nebula are found within the borders of Circinus, all visible with amateur telescopes of varying sizes.

 NGC 5823

NGC 5823, also called Caldwell 88, 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 from Alpha Centauri. It contains 80-100 stars of 10th magnitude and fainter, which are spread out over a diameter of 10 arcseconds. The brighter stars, however, are not true members of the cluster, as they are closer to the Earth than the dimmer ones.

A Hubble Space Telescope image of NGC 5315

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:

NGC 5315: When Serendipity Becomes Science

NGC 5315 is a planetary nebula about 7,000 light years from Earth in the constellation of Circinus. Planetary nebulas are gaseous clouds that are created in the last stages of the lifetime of a star like the Sun. The name of ‘planetary nebula’ is a misnomer, since these objects have nothing to do with planets. But the term was born because these objects look like planets when viewed through small optical telescopes. Chandra does not always see planetary nebulas in X-ray light. Rather, they may only become X-ray sources, like NGC 5315, when powerful winds from a particularly young star at the center collide with the ejected material.

This image of NGC 5315 shows the box-shaped outlines for two of the detectors on Chandra, plus the so-called aimpoint of the telescope, where the Chandra images are the sharpest (the spatial resolution of Chandra images, like those for other X-ray telescopes, decreases with distance from the aimpoint). For optimal imaging of Hen 2-99, Chandra was pointed so that this planetary nebula would fall near the aimpoint. Although Hen 2-99 was too faint to be detected, the planetary nebula NGC 5315 was serendipitously detected a large distance away from the aimpoint, where the image is not as sharp.

[http://chandra.harvard.edu/photo/2007/ngc5315/]

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 and lies 4 degrees off the galactic plane. It is the closest Seyfert galaxy to the Milky Way, and hosts an active galactic nucleus:

Active Galaxy Circinus

The Hubble telescope has taken a snapshot of a nearby active galaxy known as Circinus. This active galaxy belongs to a class of mostly spiral galaxies called Seyferts, which have compact centers and are believed to contain massive black holes. Seyfert galaxies are themselves part of a larger class of objects called Active Galactic Nuclei or AGN. AGN have the ability to remove gas from the centers of their galaxies by blowing it out into space at phenomenal speeds. Astronomers studying the Circinus galaxy are seeing evidence of a powerful AGN at its center.

What does this picture reveal to astronomers?

Much of the gas in the disk of the Circinus spiral is concentrated in two specific rings- a larger one of diameter 1,300 light-years, which has already been observed by ground-based telescopes, and a previously unseen ring of diameter 260 light-years. In the Hubble image, the smaller inner ring is located on the inside of the green disk. The larger outer ring extends off the image and is in the plane of the galaxy’s disk. Both rings are home to large amounts of gas and dust as well as areas of major ‘starburst’ activity, where new stars are rapidly forming on timescales much shorter than the age of the entire galaxy.

At the center of the starburst rings is the Seyfert nucleus, the believed signature of a supermassive black hole that is accreting surrounding gas and dust. The black hole and its accretion disk are expelling gas out of the galaxy’s disk and into its halo (the region above and below the disk). The detailed structure of this gas is seen as magenta-colored streamers extending towards the top of the image.

[http://hubblesite.org/newscenter/archive/releases/2000/2000/37/]

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:

Circinus X-1: X-ray Echoes Pinpoint Location of Distant Flaring Neutron Star

Light echoes around a neutron star have allowed a precise distance to be determined to it. These light echoes are produced when a burst of X-rays bounces off of clouds. Determining distances in astronomy is notoriously difficult so objects like this are valuable. The new distance measurement to Circinus X-1 is over twice that of one previously published value.

Data from NASA’s Chandra X-ray Observatory has helped provide a rare opportunity to determine the distance to an object on the other side of the Milky Way galaxy, as described in our latest press release.

The object is Circinus X-1, containing a neutron star- the collapsed core left behind after a star exploded- in orbit with a massive star. The Chandra data reveal a set of four rings that appear as circles around Circinus X-1. These rings can be seen in the composite image where X-rays from Chandra are red, green, and blue corresponding to low, medium, and high-energy X-rays respectively, which have been combined with a view in visible light from the Digitized Sky Survey. The sharp edges are caused by the large size of the X-ray rings compared to the relatively small field-of-view of the Chandra detectors, providing only partial coverage.

What are these rings and what information do they provide? These rings are light echoes, similar to sound echoes that we may experience here on Earth. Instead of sound waves bouncing off a canyon wall, the echoes around Circinus X-1 are produced when a burst of X-rays from the star system ricochet off of clouds of dust between Circinus X-1 and Earth.

This artist’s illustration shows in detail how the ringed structure seen by Chandra is produced. Each ring is caused by X-rays from the Circinus X-1 flare bouncing off of different dust clouds. If the cloud is closer to us, the ring appears to be larger. The result, as seen by Chandra, is a set of concentric rings with different apparent sizes depending on the distance of the intervening cloud from us. The physical sizes of the rings, using the labels given in the illustration, are 41 light years (ring a), 49 light years (ring b), 55 light years (ring c), and 52 light years (ring d).

By combining the light echoes that Chandra detects with radio data from the Mopra radio telescope in Australia, which determined the distance to the intervening clouds, astronomers can estimate the distance to Circinus X-1 using relatively simple geometry. The light echo method generates a distance of 30,700 light years. The observation thus settles a large difference amongst previous results, one similar to this work and one indicating a much smaller distance of about 13,000 light years.

Such a difference in distance estimate to Circinus X-1 would have implications for other properties that have been observed before in the system. For example, if it is over twice as far away as some have previously thought, then this means its light output is much greater. (Consider a light bulb that is moved farther away, it will appear dimmer.) Because Circinus X-1 has been known to flare strongly in X-ray light, including an outburst in 2013, this implies that the system has exceeded the so-called Eddington Limit. This threshold, which is the balance between the inward pull of gravity and the outward push of radiation from an object, is generally only exceeded by systems containing black holes, not neutron stars.

The researchers also determined that the speed of the jet of high-energy particles produced by the system is at least 99.9% of the speed of light. This extreme velocity is usually associated with jets produced by a black hole.

[http://chandra.harvard.edu/photo/2015/cirx1/]

J144547-5931 (Constellation Centaurus) and J144701-5919 (Constellation Circinus): Hunting for the Milky Way’s Heaviest Stars.

Like looking for Easter eggs in a lawn of long grass, the hunt for the Milky Way’s most massive stars takes persistence and sharp eyes. In their stellar search through our Galactic backyard, astronomers have used powerful telescopes sensitive to X-ray and infrared radiation to find evidence for a substantial population of X-ray emitting massive stars.

This image shows infrared data from NASA’s Spitzer Space Telescope near the plane of the Milky Way galaxy. Both outlined boxes contain an artificially darkened view of the Spitzer data, to highlight a bright X-ray source (blue) detected at the center of each square with NASA’s Chandra X-ray Observatory. Each X-ray source coincides with a strong infrared signal.

Analysis of the X-ray and infrared data, as well as optical and radio observations, reveals that these bright sources are, in fact, extremely massive stars. Two other massive stars have also been found near the plane of the Milky Way using similar methods. Deep observations from ESA’s XMM-Newton also provided valuable information for these other two objects. All four of these stars are thought to be at least 25 times more massive than the Sun and lie between 7,500 and 18,000 light years from Earth. These stars are expected to last only a few million years and will end their lives with supernova explosions.

Finding these very massive stars is not easy. Dust and gas throughout the Milky Way obscures much of the view from optical telescopes near the plane of the galaxy. Infrared images suffer less obscuration but are extremely crowded with stars. However, these stellar behemoths shine brightly in X-ray light and easily stand out from their neighbors in Chandra images.

Why are these massive stars so bright in X-rays? Some massive stars have winds that blow material away from their surface at over 2 million miles per hour. If this high-speed material collides with the wind from a companion star, it is decelerated so suddenly that it acts like it has collided with a Solar System-sized brick wall. The shock waves resulting from this enormous collision generate temperatures up to 100 million degrees, and produce copious amounts of X-rays.

These Chandra observations followed a survey of the plane of our Galaxy by the Advanced Satellite for Cosmology and Astrophysics (ASCA), a previous X-ray mission. This survey detected about 160 X-ray sources, but only a third of them could be definitively identified due to the limited spatial resolution of ASCA. Because Chandra’s ability to resolve sources is significantly greater, much more precise positions could be obtained. This has allowed scientists to identify counterparts to the X-ray sources in other wavelengths. There are many other unidentified Galactic X-ray sources with X-ray properties similar to these four sources, so a large population of massive stars may remain to be discovered with future Chandra observations.

[http://chandra.harvard.edu/photo/2011/massive/index.html]

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.

[https://en.wikipedia.org/wiki/Circinus]

[Return to the 88 Modern Constellations]