Ara is a small constellation located in the southern sky. Its name means ‘altar’ in Latin. It is among the smaller constellations (63rd in size), occupying an area of 237 square degrees. It lies in the third quadrant of the southern hemisphere (SQ3) and can be seen at latitudes between +25° and -90°. The neighboring constellations are Apus, Corona Australis, Norma, Pavo, Scorpius, Telescopium, and Triangulum Australe.
[http://www.constellation-guide.com/constellation-list/ara-constellation/]
Johann Elert Bode’s illustration of Ara, from his Uranographia (1801)
In illustrations, Ara is usually depicted as an altar with its smoke ‘rising’ southward. In ancient Greek mythology, Ara was identified as the altar where the gods first made offerings and formed an alliance before defeating the Titans. The nearby Milky Way represents the smoke rising from the offerings on the altar.
However, depictions of Ara often vary in their details. In the early days of printing, a 1482 woodcut of Gaius Julius Hyginus’s classic Poeticon Astronomicon depicts the altar as surrounded by demons. Johann Bayer in 1603 depicted Ara as an altar with burning incense; the flames rise southward as in most atlases. Hyginus also depicted Ara as an altar with burning incense, though his Ara featured devils on either side of the flames. However, Willem Blaeu, a Dutch uranographer active in the 16th and 17th centuries, drew Ara as an altar designed for sacrifice, with a burning animal offering. Unlike most depictions, the smoke from Blaeu's Ara rises northward, represented by Alpha Arae. A more unusual depiction of Ara comes from Aratus, a Greek uranographer, in 270 BCE. He drew Ara as a lighthouse, where Alpha. Beta, Epsilon, and Zeta Arae represent the base, and Eta Arae represents the flames at the lighthouse’s light.
In Chinese astronomy, the stars of the constellation Ara lie within The Azure Dragon of the East (Dōng Fāng Qīng Lóng). Five stars of Ara formed Guī, a tortoise, while another three formed Chǔ, a pestle.
Constellation of Ara
[http://www.davidmalin.com/fujii/source/Ara.html]
[http://astropixels.com/constellations/charts/Ara.html]
Beta Arae is the brightest star in the constellation Ara, with an apparent visual magnitude of 2.8. It is at a distance of roughly 650 light-years (200 parsecs) from Earth. The spectrum of this star matches a stellar classification of K3 Ib-IIa, with the luminosity class notation ‘Ib-IIa’ indicating that the star lies part way between a higher luminosity bright giant (IIa) and a lower luminosity supergiant (Ib). This represents two of the evolutionary stages that a massive star passes through after it has exhausted the hydrogen at its core. Beta Arae is radiating energy from its outer envelope at an effective temperature of 4,200 K, which causes it to take on the orange hue of a K-type star. The abundance of elements other than hydrogen and helium, what astronomers term the star’s metallicity, is more than three times the abundance in the Sun.
[https://en.wikipedia.org/wiki/Beta_Arae]
Alpha Arae is the second brightest star in the southern constellation of Ara. With an average apparent visual magnitude 2.93, it is readily visible to the naked eye from the southern hemisphere. It is around 270 light-years (83 parsecs) away. The visual magnitude of the star is diminished by 0.10 magnitudes as a result of extinction from intervening gas and dust.
Alpha Arae has a stellar classification of B2 Vne, indicating that it is a massive B-type main sequence star. The ‘n’ notation in the suffix indicates that the absorption lines in the star’s spectrum appear spread out and nebulous because of the Doppler effect from rapid rotation. The measured projected rotational velocity has been measured as high as 375 km s−1. The rapid rotation is causing a pronounced equatorial bulge of about 2.4-2.7 times the polar radius.
It is a Be star, as indicated by the ‘e’ notation in the star’s classification. This indicates that emission lines are observed in the spectrum, which is coming from a disk of material ejected from the star because of its rapid rotation. This star has around 9.6 times as much mass as the Sun and an average of 4.5 times the Sun’s radius. It is emitting 5,800 as much luminosity as the Sun from its outer envelope at an effective temperature of 18,044 K. This heat gives Alpha Arae the blue-white hue that is characteristic of B-type stars. It is a variable star with a magnitude that varies between 2.76m and 2.90m.
Alpha Arae has a visual companion star, CCDM J17318-4953B, located approximately 50 arcseconds away along a position angle of 168°, with an apparent visual magnitude of about 11. The two stars appear close to each other by coincidence and are not physically close in space.
[https://en.wikipedia.org/wiki/Alpha_Arae]
Zeta Arae is the third brightest star in the constellation Ara. The apparent visual magnitude of this star is 3.1, which can be seen from suburban skies in the southern hemisphere. It is located at a distance of 490 light-years (150 parsecs) from Earth.
The spectrum of this star matches a stellar classification of K3 III. The luminosity class of ‘III’ indicates this is a giant star that has exhausted the hydrogen at its core and evolved away from the main sequence. It is radiating energy from its outer atmosphere at an effective temperature of 4,350 K, which is what gives it the orange hue of a K-type star. This star displays an excess of infrared emission that may indicate circumstellar matter.
[https://en.wikipedia.org/wiki/Zeta_Arae]
Gamma Arae is is the fourth brightest star in the constellation in the constellation of Ara. With an apparent visual magnitude of 3.3, it and is readily visible to the naked eye. The distance to this star is 1,110 light-years (340 parsecs) from Earth.
This is an enormous star with 23 times the radius of the Sun. It is radiating 120,000 as much energy as the Sun from its outer envelope at an effective temperature of 21,500 K. This heat gives the star the blue-white glow of a B-type star. The spectrum shows it to match a stellar classification of B1 Ib, with the luminosity class of ‘Ib’ indicating this is a lower luminosity supergiant star. It is a relatively young body, with an estimated age of around 15.7 million years.
Gamma Arae has an optical companion located at an angular separation of 17.9 arcseconds, which is an A-type main sequence star with an apparent magnitude of 10.5.
[https://en.wikipedia.org/wiki/Gamma_Arae]
Delta Arae is a double star with an apparent visual magnitude of 3.62, and is visible to the naked eye. It is about 198 light-years (61 parsecs) distant from the Earth. Delta Arae is massive B-type main sequence star with a stellar classification of B8 Vn. It has a magnitude 9.5 companion G-type main sequence star that may form a binary star system with Delta Arae. There is also a 12th magnitude optical companion located 47.4 arcseconds away along a position angle of 313°.
[https://en.wikipedia.org/wiki/Delta_Arae]
Theta Arae has an apparent visual magnitude of +3.67, which is bright enough to be seen with the naked eye. It is 810 light-years (250 parsecs) distant from the Earth. This is a supergiant star with a stellar classification of B2 Ib. It has nearly nine times the mass of the Sun and is over 20 times the Sun’s radius. The outer atmosphere of this star has an effective temperature of 17,231 K; much hotter than the surface of the Sun. At this heat, the star shines with the characteristic blue-white hue of a B-type star.
[https://en.wikipedia.org/wiki/Theta_Arae]
Eta Arae is approximately 299 light-years (92 parsecs) from Earth and is visible to the naked eye with an apparent visual magnitude of 3.76. The star has a stellar classification of K5 III, indicating that, at an estimated age of seven billion years, it has reached the giant star stage of its evolution. With a mass nearly the same as the Sun, it has an outer envelope that has expanded to nearly 56 times the Sun’s radius. The star is now spinning so slowly that it takes more than eleven years to complete a single rotation. Eta Arae is radiating energy into space at an effective temperature of 3,886 K, giving it the orange-hued glow of a K-type star. It has a 14th magnitude optical companion, located 25.7 arcseconds away.
[https://en.wikipedia.org/wiki/Eta_Arae]
Epsilon1 Arae is visible to the naked eye with an apparent visual magnitude of +4.1. This star is around 360 light-years (110 parsecs) distant from the Earth. It is an evolved giant star with a stellar classification of K3 III.[3] It is around 74% more massive than the Sun. At an age of about 1.7 billion years, the outer envelope of the star has expanded to almost 34 times the Sun’s radius. It is radiating energy into space at an effective temperature of 4,176 K, giving it the orange-hued glow of a K-type star.
[https://en.wikipedia.org/wiki/Epsilon1_Arae]
Mu Arae, often designated HD 160691, also named Cervantes, is a main sequence G-type star approximately 50 light-years away from the Sun in the constellation of Ara. The star has a planetary system with four known extrasolar planets (designated Mu Arae b, c, d and e; later named Quijote, Dulcinea, Rocinante and Sancho, respectively), three of them with masses comparable to that of Jupiter. The system’s innermost planet was the first ‘hot Neptune’ or ‘super-Earth’ to be discovered.
This star seen from Earth has an apparent magnitude of +5.12 and is visible to the naked eye. It is approximately 10% more massive than the Sun and significantly older, at around 6.34 billion years. The radius of the star is 36% greater than that of the Sun and it is 90% more luminous. The star contains twice the abundance of iron relative to hydrogen of our Sun and is therefore described as metal-rich. Mu Arae is also more enriched than the Sun in the element helium.
The Mu Arae star with distance relationships for its four planets
The Mu Arae system consists of an inner Uranus-mass planet in a tight 9-day orbit and three massive planets, probably gas giants, on wide, near-circular orbits, which contrasts with the high-eccentricity orbits typically observed for long-period extrasolar planets. Searches for circumstellar discs show no evidence for a debris disc similar to the Kuiper belt around Mu Arae. If Mu Arae does have a Kuiper belt, it is too faint to be detected with current instruments. The gas giant planet ‘b’ is located in the liquid water habitable zone of Mu Arae. This would prevent an Earth-like planet from forming in the habitable zone, however large moons of the gas giant could potentially support liquid water. On the other hand, it is unclear whether such massive moons could actually form around a gas giant planet, thanks to an apparent scaling law between the mass of the planet and its satellite system. In addition, measurements of the star's ultraviolet flux suggest that any potentially habitable planets or moons may not receive enough ultraviolet to trigger the formation of biomolecules. Planet ‘d’ would receive a similar amount of ultraviolet to the Earth and thus lies in the ultraviolet habitable zone, however, it would be too hot for any moons to support surface liquid water.
[https://en.wikipedia.org/wiki/Mu_Arae]
Gliese 674 is a pre-main-sequence red dwarf approximately 15 light years away in the southern constellation of Ara. An intermediate mass planet has been found orbiting close to the red dwarf star in an unusually eccentric orbit. This planet is a sub-Neptune-or-Uranus-mass planet either gaseous or rocky. It orbits as close as 0.039 AU from the star and takes only 4.6938 days to orbit. This planet has a similar eccentricity to Mercury (e=0.2). The discovery of the planet was announced on January 7, 2007 by using the HARPS spectrograph mounted on the ESO’s 3.6 meter telescope at La Silla, Chile.
[https://en.wikipedia.org/wiki/Gliese_674]
[https://en.wikipedia.org/wiki/Gliese_674_b]
Westerlund 1-26 or Wd 1-26 is a red supergiant or hypergiant within the outskirts of the Westerlund 1 super star cluster. It is one of the largest known stars discovered so far, with a size of approximately 1,530 solar radii. If placed at the center of the Solar System, its photosphere would engulf the orbit of Jupiter. This star is located 11,500 light-years from Earth. Its spectral type (M2-M6Ia) identifies it a red star with a high luminosity. At radio wavelengths it is 310,000 times brighter than the Sun, making its luminosity range somewhere around 380,000 times brighter than the Sun. It has a surface temperature of about 3000 K.
[https://en.wikipedia.org/wiki/Westerlund_1-26]
Westerlund 1 (also called Ara Cluster, designated Wd1) is a compact young super star cluster in the Milky Way galaxy, about 3.5–5 kpc away from Earth. It is one of the most massive young star clusters in the Milky Way, and was discovered by Bengt Westerlund in 1961. The cluster contains a large number of rare, evolved, high-mass stars, including: 6 yellow hypergiants, 4 red supergiants including Westerlund 1-26, one of the largest known stars, 24 Wolf-Rayet stars, a luminous blue variable, many OB supergiants, and an unusual supergiant sgB[e] star which has been proposed to be the remnant of a recent stellar merger. In addition, X-ray observations have revealed the presence of the anomalous X-ray pulsar CXO J164710.2-455216:
[https://en.wikipedia.org/wiki/Westerlund_1]
Magnetar Formation Mystery Solved?
Magnetars are the bizarre super-dense remnants of supernova explosions. They are the strongest magnets known in the Universe- millions of times more powerful than the strongest magnets on Earth. A team of European astronomers using ESO’s Very Large Telescope (VLT) now believe they’ve found the partner star of a magnetar for the first time. This discovery helps to explain how magnetars form and why this particular star didn’t collapse into a black hole as astronomers would expect.
When a massive star collapses under its own gravity during a supernova explosion it forms either a neutron star or black hole. Magnetars are an unusual and very exotic form of neutron star. Like all of these strange objects they are tiny and extraordinarily dense- a teaspoon of neutron star material would have a mass of about a billion tonnes- but they also have extremely powerful magnetic fields. Magnetar surfaces release vast quantities of gamma rays when they undergo a sudden adjustment known as a starquake as a result of the huge stresses in their crusts.
The star cluster Westerlund 1 and the positions of the magnetar and its probable former companion star.
The Westerlund 1 star cluster, located 16 000 light-years away in the southern constellation of Ara (the Altar), hosts one of the two dozen magnetars known in the Milky Way. It is called CXOU J164710.2-455216 and it has greatly puzzled astronomers.
Astronomers proposed a solution to this mystery. They suggested that the magnetar formed through the interactions of two very massive stars orbiting one another in a binary system so compact that it would fit within the orbit of the Earth around the Sun. But, up to now, no companion star was detected at the location of the magnetar in Westerlund 1, so astronomers used the VLT to search for it in other parts of the cluster. They hunted for runaway stars- objects escaping the cluster at high velocities- that might have been kicked out of orbit by the supernova explosion that formed the magnetar. One star, known as Westerlund 1-5, was found to be doing just that.
“Not only does this star have the high velocity expected if it is recoiling from a supernova explosion, but the combination of its low mass, high luminosity and carbon-rich composition appear impossible to replicate in a single star- a smoking gun that shows it must have originally formed with a binary companion,” says team member Ben Ritchie.
This discovery allowed the astronomers to reconstruct the stellar life story that permitted the magnetar to form, in place of the expected black hole. In the first stage of this process, the more massive star of the pair begins to run out of fuel, transferring its outer layers to its less massive companion- which is destined to become the magnetar- causing it to rotate more and more quickly. This rapid rotation appears to be the essential ingredient in the formation of the magnetar’s ultra-strong magnetic field.
In the second stage, as a result of this mass transfer, the companion itself becomes so massive that it in turn sheds a large amount of its recently gained mass. Much of this mass is lost but some is passed back to the original star that we still see shining today as Westerlund 1-5.
“It is this process of swapping material that has imparted the unique chemical signature to Westerlund 1-5 and allowed the mass of its companion to shrink to low enough levels that a magnetar was born instead of a black hole — a game of stellar pass-the-parcel with cosmic consequences!” concludes team member Francisco Najarro.
It seems that being a component of a double star may therefore be an essential ingredient in the recipe for forming a magnetar. The rapid rotation created by mass transfer between the two stars appears necessary to generate the ultra-strong magnetic field and then a second mass transfer phase allows the magnetar-to-be to slim down sufficiently so that it does not collapse into a black hole at the moment of its death.
[https://www.eso.org/public/usa/news/eso1415/]
NGC 6193 is another open cluster in Ara, containing approximately 30 stars with an overall magnitude of 5.0 and a size of 0.25 square degrees, about half the size of the full Moon. It has one bright member, a double star with a blue-white hued primary of magnitude 5.6 and a secondary of magnitude 6.9. NGC 6193 is surrounded by NGC 6188, a faint nebula only normally visible in long-exposure photographs:
NGC 6188 and NGC 6193
NGC 6188 is a large emission nebula located some 4000 light years distant in the Southern constellation Ara. This giant molecular cloud is being illuminated by the young star cluster NGC 6193 which in this image appears in the center almost as if suspended in front of a blanket of nebulosity littered with thousands of other dimmer stars in a multitude of colors.
At the center of this cluster lies an intensely bright pair of stars; the O-type giants HD 150135 and HD 150136. The latter is in fact itself an extremely close binary with a confirmed third component in an outer eccentric orbit of between 2950 and 5500 days. This triple star system is one of the most massive in our galaxy with the three components containing 64, 40 and 35 solar masses respectively. The cluster is very young, not older than 3 million years, and the intense radiation from the massive stars is slowly eroding the surrounding gas and dust away, sculpting it into the striking forms seen in this image. Within these clouds new stars are currently being born and in places some have already begun to light up the nebula from within.
[http://www.pbase.com/rolfolsen/image/144166552/original]
Hubble Space Telescope photograph of the The Stingray Nebula
The Stingray Nebula (Hen 3-1357) is the youngest known planetary nebula, and is located 18,000 light-years away. Although it is some 130 times the size of the Solar System, the Stingray Nebula is only about 1/10 the size of most other known planetary nebulae. Forty years ago it was still a protoplanetary nebula in which the gas had not yet become hot and ionized. The image of the Stingray Nebula here shows how the outer shells of gas are collimating the continuing outflow of gas from the central star- an important observation, as the process of how these outflows become collimated has not been well understood.
Prior to the discovery of the nebula, the central star was classified as an A or B type Hα emission line star in 1967. It was observed in 1971 to be a pre-planetary nebula when it seemed to be an asymptotic giant branch B1 supergiant. Planetary nebula emission lines were identified in this star in 1989 by the IUE (International Ultraviolet Explorer). In 1995 the central planetary nebula nucleus was observed as a DA white dwarf, having seemingly faded by a factor of three between 1987 and 1995. The nucleus has an estimated mass of 0.6 solar masses, and has an observed companion star separated by 0.3 arcsec. Nebula mass is estimated as 0.015 solar masses. Luminosity is estimated to be 3000 times that of the sun.
[https://en.wikipedia.org/wiki/Stingray_Nebula]
A star’s colourful final splash
The Hubble Space Telescope captured this beautiful image of NGC 6326, a planetary nebula with glowing wisps of outpouring gas that are lit up by a central star nearing the end of its life. When a star ages and the red giant phase of its life comes to an end, it starts to eject layers of gas from its surface leaving behind a hot and compact white dwarf. Sometimes this ejection results in elegantly symmetric patterns of glowing gas, but NGC 6326 is much less structured. This object is located in the constellation of Ara, the Altar, about 11000 light-years from Earth.
Planetary nebulae are one of the main ways in which elements heavier than hydrogen and helium are dispersed into space after their creation in the hearts of stars. Eventually some of this outflung material may form new stars and planets. The vivid red and blue hues in this image come from the material glowing under the action of the fierce ultraviolet radiation from the still hot central star.
This picture was created from images taken using the Hubble Space Telescope’s Wide Field Planetary Camera 2. The red light was captured through a filter letting through the glow from hydrogen gas (F658N). The blue glow comes from ionised oxygen and was recorded through a green filter (F502N). The green layer of the image, which shows the stars well, was taken through a broader yellow filter (F555W). The total exposure times were 1400 s, 360 s and 260 s respectively. The field of view is about 30 arcseconds across.
[https://www.spacetelescope.org/images/potw1010a/]
[https://en.wikipedia.org/wiki/Ara_%28constellation%29]
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