Centaurus is the ninth largest constellation, and takes up an area of 1,060 square degrees of the southern hemisphere night sky, where it can be seen by any observer located between +25° and -90°of latitude. It is home to two of the top ten most luminous stars in the sky, namely Alpha Centauri (-0.27), and Beta Centauri (+0.6), and also contains Omega Centauri, one of the Milky Way’s largest attendant globular clusters, which is a perennial favourite among southern observers, and once seen through a telescope is never forgotten.
Although Centaurus contains no Messier objects, it does have at least eleven stars with confirmed planets, and several notable deep sky objects, such as the famous blue planetary nebula known as the Southerner, the equally famous Boomerang Nebula, and one of the most active galaxies yet discovered called Centaurus A.
Hercules Family of Constellations
Centaurus belongs to the large Hercules family of constellations, which includes the constellations Aquila, Ara, Centaurus, Corona Australis, Corvus, Crux, Crater, Cygnus, Hercules, Hydra, Lupus, Lyra, Ophiuchus, Scutum, Serpens, Sextans, Sagitta, Triangulum Australe, and Vulpecula.
The origins of the constellation can be traced back all the way to ancient Babylon, when it was known as “the Bison Man” and associated with Shamash, the sun-god. In ancient Greece and Rome, the constellation was linked to a race of half-human, half-horse creatures known as centaurs, with the constellation thought to depict the noble centaur called Chiron, a wise, high-born creature that mentored many famous heroes in the study of music, medicine, and hunting, including Jason, Theseus, and Heracles. Unfortunately, Heracles accidentally shot the centaur with a poisoned arrow, but since he was immortal, Chiron could not die from the deadly poison. Instead, he suffered terrible agony until Cronus turned his son into a mere moral, thus allowing him to die before placing him in the sky. The Centaurus constellation depicts a mythological scene involving three constellations, with Chiron (Centaurus) impaling a wolf (Lupus) on his spear in order to sacrifice it on an altar (Ara) for the gods.
– Rigil Kentaurus (Alpha Centauri) has a visual magnitude of -0.27, making it the night sky’s third brightest star, behind Sirius (-1.44) in Canis Major, and Canopus (-0.62) in Carina. At a distance of just 4.4 light years, Alpha Centauri is also our closest stellar neighbour, but it actually a triple star system with its primary component Alpha Centauri A, a yellow-white (G2V) main sequence star about 10% more massive than the Sun, with its slightly smaller companion, Alpha Centauri B, an orange (K1V) main sequence star situated 11 AU away, with the pair having an orbital period of 80 years. Alpha Centauri and Beta Centauri are believed to be roughly the same age at about 4.85 billion years, making them around 450 million years older than the Sun.
The third star in the Alpha Centauri system, Proxima Centauri (Alpha Centauri C), is a red dwarf (M5Ve) found 13,000 AU, or 0.2 light-years, from the Alpha Centauri AB pairing, and is thought to be gravitationally bound to the bigger stars. It only about 12% as massive as the Sun, and is expected to spend at least four trillion years on the main sequence.
– Hadar (Agena, Beta Centauri), the constellation’s second brightest star, is a trinary system found 349 light years distant with an apparent magnitude of +0.6, making it the 10th most luminous star in the entire sky. Its primary components, Beta Centauri Aa and Ab, are a spectroscopic binary system consisting of two similar blue-white giants (B1 III) with an orbital period of 357 days. They are also both Beta Cephei variables. The companion star in the system, Hadar B, is located 1.3 seconds of arc (210 AU) from the primary pair, and takes about 1,500 years to complete one orbit.
– Menkent (Theta Centauri, Haratan), the third brightest star in Centaurus, is an orange giant (K0IIIb) found 60.9 light years from our solar system with a visual magnitude of +2.06. Its name derives drom the Arabic for “shoulder of the Centaur”. Theta Centauri is around 10.6 times bigger than the Sun, with 60 times its brightness.
Other stars of interest in Centaurus includes the separate binary systems of Muhlifain (Gamma Centauri), Alnair (Zeta Centauri), Ke Kwan (Kappa Centauri); the blue-white giant Epsilon Centauri; the blue-white subgiant Nu Centauri; the blue dwarf Eta Centauri; and the white dwarf BPM 37093.
Notable Deep Sky Objects
– Centaurus A (NGC 5128) is not only the closest radio source to the solar system, it is also the fifth most luminous galaxy in the entire sky with an apparent magnitude of 6.84, despite being located between 10 and 16 million light years away at the centre of the Centaurus A subgroup of the very big Centaurus A/ M83 Group of Galaxies. More than 100 active star forming regions have been identified within the galaxy, with most, if not all, being the result of an ongoing collision with a large spiral galaxy.
– Omega Centauri (NGC 5139) is located just four degrees to the southward of Centaurus A, and is the largest, and most massive of the globular clusters surrounding the Milky Way. It has an apparent magnitude of 3.7, making it bright enough to be spotted without optical aid, even at a distance of about 15,800 light years. The cluster is estimated to be about 12 billion years old, and while its origin and formation is not certain, it is believed to be the remains of a dwarf galaxy that was tidally stripped by the Milky Way.
– The Blue Planetary (NGC 3918) is a spectacularly beautiful planetary nebula about 4,900 light years distant, and with an apparent magnitude of 8.5 is the brightest planetary nebula in the southern skies. NGC 3918 is approaching Earth at about 17 km/second, while it is expanding at around 24 km/second. Note that the central bright dot in the nebula is a foreground object and not the progenitor star, which is not visible to optical observers due to the nebula’s sheer brightness.
– Alpha Centaurids; while this meteor shower can be active during most of February, the peak usually occurs on the 6th/7th of the month. Observers in the southern hemisphere usually get a better view of the shower, whose maximums rarely exceeds 20 meteors per hour.
– Omicron Centaurids, another weak shower, is best viewed from the southern hemisphere from late January to early February, with the peak on February 14th. Best views of the shower is from around 2:00 AM local standard time, when between five and ten meteors per hour can be expected
– Theta Centaurids occurs from around January 23rd to March 12th, with a peak on February 14th, although the shower is only visible from the southern hemisphere. Its meteors are usually fast at around 60 km/second, with maximum rates reaching as few as 2-4 meteors per hour.