Half-man, half-horse, Centaurus strides across the southern sky. The constellation is big and bright, and its two brightest stars, Alpha and Beta Centauri, are pointers to Crux, the Southern Cross. Crux aids navigators as Polaris does in the northern hemisphere.
History and mythology
The Greeks named the constellation in the third century BCE. However, well over a thousand years before that, it was the Babylonian constellation MUL.GUD.ALIM, a being that was half-man, half-bison. (Babylon was located in present-day Iraq.)
It seems odd that you can't see this important constellation from today's Greece or Iraq, because they're too far north. This isn't due to any change in Earth's geography over a few thousand years, but to the precession of Earth's axis, changing our view of the sky. (For an explanation, click the link for “Ecliptic and Equinoxes” at the end of this article.)
In mythology, the centaurs were loutish, drunken, lecherous and violent. Chiron was the exception. Wise, kind, virtuous and learned, he was the tutor to many heroes, including Heracles, who has his own constellation, and Asclepius the healer, represented by the constellation Ophiuchus. Alas for Chiron, he was the accidental victim of a poisoned arrow for which there was no remedy. As an immortal in agonizing pain, he could only suffer eternally. Fortunately, Zeus was moved to release him from his immortality and when he died, place him among the stars.
Of its nearly three hundred stars visible to the unaided eye, the brightest star in Centaurus is Alpha Centauri, also known as Rigil Kentaurus (centaur's foot). It's the third brightest star in the sky - no surprise since it's just over four light years away from us. But it's not a single star, it's a star family. Alpha Centauri AB has been recognized as a binary star since the seventeenth century. Alpha Centauri A and B are stars similar to the Sun. There is even an Earth-sized planet orbiting Alpha Centauri B.
A third member of the Alpha Centauri system, Proxima, wasn't discovered until 1915. It's a red dwarf star which is widely separated from the binary and can only been seen through a telescope. In 2016 it was discovered to have a rocky planet in the star's habitable zone where water can be liquid on the surface.
Beta Centauri (traditionally, Hadar) is also a first magnitude star that turns out to be double. It's made up of two nearly identical stars, blue giants about 55,000 times more luminous than the Sun. They will evolve into red supergiants and eventually explode as supernovae. Beta Centauri is over 350 light years away, so it looks fainter than Alpha Centauri even though it gives out more light.
If the old song is right about diamonds being a “girl's best friend” then BPM 37093, also known as Lucy, has lots of friends. Lucy is a white dwarf whose core is crystallized carbon, i.e., a massive diamond, about ten billion trillion trillion carats worth. If you're a Beatles fan, you've probably guessed that the nickname comes from another song, “Lucy in the Sky with Diamonds.”
The Milky Way has over 150 globular star clusters, all more than twice as old as the Sun. They're called globular clusters because the gravity of the densely-packed stars pulls them together into a sphere.
In Centaurus we find the biggest globular cluster visible from Earth, containing some ten million stars. NGC 5139 is known as Omega Centauri, which is actually a star designation. In the second century Ptolemy catalogued it as a star and only in 1677 did Edmond Halley show that it was some kind of “nebula”. It took a further 150 years to determine that it was a globular cluster.
Usually, stars in a cluster are of a similar age and chemical composition, but not so for Omega Centauri. A Hubble Space Telescope image of the core shows a whole range of star ages. One theory is that the cluster is the remnant of a dwarf galaxy pulled apart by the Milky Way.
The Blue Planetary (NGC 3918) is a planetary nebula, a term coined by William Herschel, because in a telescope they showed a planet-like disk. In fact, they're not related to planets, but are created when dying stars slough off their outer layers. NGC 3918, which was discovered by John Herschel, does rather resemble Uranus, the planet his father discovered.
Galaxies of all sorts are abundant in Centaurus. Here are three unusual ones.
Blue compact dwarf galaxies, such as the irregularly-shaped NGC 4253, are rare. They're also starburst galaxies, meaning they have very active star-formation regions. Two supernovae have been recorded in this small galaxy.
Centaurus A (NGC 5128) was discovered in 1826 and it was evident even in the eighteenth century that it was an odd shape. In the twentieth century, radio astronomers discovered that it was a strong radio source. The name Centaurus A means that it was the first radio source known in Centaurus. The radio emission comes from a central supermassive black hole. The irregularity of Centaurus A is created by the merger of two galaxies.
John Herschel discovered the polar-ring galaxy NGC 4650A in 1831. It has a group of stars in the center, surrounded at a distance by a revolving ring of young stars. Ring galaxies are rare and may be the result of a collision of two galaxies. The ring revolves more quickly than expected from the visible matter in the galaxy. This means that some kind of dark matter is implicated and astronomers can estimate the amount of dark matter from studying the ring's velocity.