The constellation Centaurus is found in the southern sky, not visible in its entirety north of 20 degrees latitude, and it is one of the most important constellations in the sky, containing the closest three stars to Earth - the Alpha Centauri star system. The constellation represents the classic Centaur (half man, half horse) of mythology named Chiron, an association often wrongly attributed to the nearby Sagittarius. According to legend Chiron was wise and benevolent, and tutored Hercules, the son of Jupiter (Zeus). When Hercules accidentally shot and killed Chiron with a poisoned arrow, he asked Zeus to immortalise his beloved teacher by placing his image in the sky. Centaurus is seen straddling that famous southern landmark, Crux, The Southern Cross, and brandishing a spear, with which he threatens the hapless wolf, Lupus, placed in the sky beside him.
The Alpha Centauri star system is the closest known star system to Earth, and that is no small thing. Sooner or later we will find a way to travel to other stars, and when we do, the Alpha Centauri star system is the first one we'll visit. It won't be anytime soon, of course. Even though Alpha Centauri is closer than any other star by far, it is still 4.37 light years away. That's about 26 trillion miles (42 trillion kms), and our fastest twenty-first century spacecraft whizzing along at 40,000 mph would take 60,000 years to reach it. To cover such a distance in any kind of reasonable time frame, we would have to find a way to travel faster than light, which would entail nothing less than changing the laws of physics. Clearly we have our work cut out for us. But sometime in the far off future it will undoubtedly happen, and our descendants will finally venture out of our home solar system, and their first port of call will be Alpha Centauri.
Because it is so close, Alpha Centauri is the third brightest star in the sky. With an apparent magnitude of -0.27, only Sirius (mag -1.46) and Canopus (mag -0.72) are brighter. Appearing as one star to the naked eye, Alpha Centauri is actually a triple star system, consisting of Alpha Centauri A, Alpha Centauri B, and Alpha Centauri C (also known as Proxima Centauri, and technically the closest of the three to Earth). Alpha Centauri is also known as Rigel Kentaurus, from the Arabic for the foot of the Centaur.
Alpha Centauri A is a G2V yellow main sequence star, slightly larger than the Sun. Alpha Centauri B is a K1V orange main sequence star, slightly smaller than the Sun. Proxima Centauri is an M5VI red subdwarf, only about one seventh the size of the Sun. Centauri A and B orbit each other every 80 years at an average distance of 2.5 billion miles (4 billion kms). Proxima Centauri is very far away from its two companions, orbiting them at an average distance of 1.38 trillion miles (2.2 trillion kms). The time it would take to complete such a large orbit is uncertain, and estimated between 100,000 and 500,000 years.
When we finally get around to visiting our closest stellar neighbour, what will we find? Will there be planets? Will there be life? To help answer these questions, astronomers at the European Southern Observatory in Chile studied variations in the orbit of Alpha Centauri B for four years, and in October, 2012, announced evidence of a planet in orbit around the star. The planet was named Alpha Centauri B-b, calculated to be slightly larger than Earth, and orbiting extremely close to the star at a distance of only about 4 million miles (Mercury orbits the Sun at about 36 million miles). At such a close distance the planet would orbit the star every three days, and have a mostly molten surface, certainly not conducive to any form of life as we know it.
Despite its apparently hostile environment, the discovery of a planet around our closest stellar neighbour is of enormous consequence, and naturally leads to the question of life evolving on some as yet undetected planet or moon of this star system. Unfortunately, Earth size planets orbiting at a safe, life sustaining distance from their star are almost impossible to detect with present technology. Our radio, television and communications signals have been washing over the Alpha Centauri star system for decades now, but we have yet to receive any signals in return. This would indicate the lack of a technological civilization similar to our own. Or not. When it comes to extraterrestrial life, we are working in the dark both figuratively and literally, and just about anything is possible.
The second brightest star in Centaurus, Beta Centauri, is named Agena, although even Allen admits to not knowing the name's origin or meaning. With a magnitude of 0.6, it is the tenth brightest star in the sky. Like Alpha Centauri, Beta Centauri is a triple star system, but it is very much farther away, at a distance of 350 light years. The system is classified as a B1III blue giant.
Theta Centauri is the only other named star, called Menkent, from the Arabic for shoulder of the centaur. Classified as a K0III orange giant, it is 60 light years away with a magnitude of 2.06.
Altogether there are 19 stars with confirmed planetary systems discovered so far in Centaurus. The big news of course, is the discovery of a planet around the closest star to Earth, Alpha Centauri, as mentioned above, but there are two more planet hosting stars that are visible to the naked eye. The next closest is HD 102365, at a distance of 30.12 light years, with a magnitude of 4.91. It is a G2V yellow main sequence star almost identical to our Sun, with a planet only 16 times larger than Earth. The planet is in a 122.1 day orbit, 0.46 AU (42.8 million miles) from the star, putting it on the inside edge of the star's habitable zone.
The next closest, naked eye star with planets is HD 114613, at a distance of 67 light years, and a magnitude of 4.85. It is a G3V yellow main sequence star, once again a near twin to our Sun, with a planet almost half the size of Jupiter. The planet takes more than 10 years to orbit the star, at a distance of about 500 million miles. Although this places it outside the star's calculated habitable zone, it does not necessarily preclude the existence of life, especially on any moons the planet may possess.
In 2004, the European Southern Observatory captured an infrared image of a very hot object orbiting the brown dwarf star, 2M1207, about 170 light years away. The object was calculated to be a gas giant about five times the size of Jupiter, well below the size where fusion would transform it into a star, and it was hailed as the first direct image of an extrasolar planet. For more information on these and other extrasolar planets, visit NASA's New Worlds Atlas, and The Open Exoplanets Catalogue.
Centaurus contains the brightest, largest, most densely populated globular cluster in our galaxy, NGC 5139. With an apparent magnitude of 3.9, it is easily visible to the naked eye, and was considered a star throughout most of history, and given the designation of a star, Omega Centauri, by which it is referred to this day. It is 15,800 light years away, and contains tens of millions of stars.
With an apparent magnitude of 6.8, NGC 5128 is the fifth brightest galaxy in the sky, making it a favourite target for small telescopes. Also known as Centaurus A, or Caldwell 77, it is 15 million light years away. The image below shows the extraordinary jets and plumes produced by a supermassive black hole at its centre.
NGC 4945 is a large spiral galaxy very similar to our own Milky Way galaxy. It is 13 million light years away, with an apparent magnitude of 8.6.
|Winter: Orion Canis Major Canis Minor Monoceros Lepus Eridanus Taurus Auriga Camelopardalis Lynx Gemini Cancer|
|Spring: Hydra Sextans Crater Corvus Leo Leo Minor Ursa Major Ursa Minor Canes Venatici Coma Berenices Virgo Bootes|
|Summer: Draco Corona Borealis Hercules Ophiuchus Serpens Libra Scorpius Sagittarius Scutum Aquila Sagitta Vulpecula Lyra Cygnus|
|Autumn: Andromeda Perseus Pegasus Cassiopeia Cepheus Cetus Lacerta Delphinus Equuleus Capricornus Aquarius Pisces Aries Triangulum|
|Southern Skies: Centaurus Crux Lupus Corona Australis Piscis Australis Sculptor Tucana Fornax Dorado Columba Puppis Carina Vela|