“Just So Science” – Chiron’s Secrets

By Elf Eldridge 29/06/2011

One of the lesser known, yet most spectacular constellations in our southern sky is currently doing the rounds about the south celestial pole for almost the entire evening; the celestial centaur, Chiron a.k.a. Centaurus.  His front two hooves are marked by the two “pointers”, the two bright stars that point to the Southern Cross, which used to mark  out his back legs (but now is a constellation in its own right). The furthest away of the two pointers stars to the cross is Alpha Centauri, a triple star system (A, B and Proxima) containing the closest star to our own sun at about 4.3 light years away. Deep within Centaurus however, is the dusty elliptical galaxy, Centaurus A.

Image of dust around Centaurus A
Close up of the active galactic centre

Centaurus A is one of the brightest radio-objects in the sky, probably due to a supermassive black hole at the galactic centre that’s slowly consuming the galaxy.

There are several other bizarre objects within Centaurus as well, the most well known being Omega Centauri, the brightest globular cluster in our night skies – but more details about that in another post. The two that really interest me are the Boomerang Nebula, and BPM 37093.

Boomerang Nebula - the coldest place in the universe?

Whilst the above looks nothing like a boomerang (it’s also known as the bow-tie nebula for slightly more obvious reasons) we have measured it as the coldest place in the universe, at -272.15 degrees centigrade, just a single degree above absolute zero (though we have created colder things in the lab here on earth). It’s colder the the background radiation left throughout the universe by the big bang – so something deeply weird must be going on here! So far our best guess is that the low temperature is due to the speed at which this nebula is emitting gas, which is between 10 and 100 times faster than what is observed in other nebula similar to this, estimated at 500,000 kilometers per hour.

Cold White Dwarf
Cold White Dwarf

BPM37093 is just a variable white dwarf star, a little bit more massive than our sun and probably around the same size as the earth, so quite dense. White dwarves are formed at the end of a stars evolution after they have swelled to red giants through their helium fusion phase, but are not massive enough to fuse carbon and oxygen.These stars inevitably cool down as they run out of fuel and their fusion ebbs – and here’s why BPM37093 is interesting, because it’s cool enough to have crystallized. Observations suggest that the carbon and oxygen at the centre of this star have cooled sufficiently to solidify into a cubic crystal, similar in structure to diamond  – hence this star is now known affectionately by astronomers as ‘Lucy’ after the Beatles’ song “Lucy in the Sky with Diamonds“. Importantly, this is still only the second-best nickname for an astronomical object, after one of Saturn’s moons, Mimas, takes the top spot after being affectionately dubbed the “Death Star” due to a rather impressive crater on its surface.

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