Sand and Stars

Gould’s Belt

Orion Belt ESA Hubble
Orion’s Belt. Image credit ESA Hubble

26 May 2017

In the vast Kalahari the sky stretches out in every direction to the very edge of forever, giving one an incredible panoramic view of the sky. And as both Orion and Scorpius are visible in the early evening at the opposite ends of the sky at this time of year, it makes this the perfect time and place to see Gould’s Belt – the ring of superstars that blaze across our night skies.

Starting with Orion setting in the west, not only could I trace Gould’s magnificent belt of bright blue-white stars right across the sky, but also at the same time see how the starry ring lies tilted toward the galactic plane by about 20 degrees. Orion…Canis Major… Puppis… Vela… Carina… the belt crosses the plane of the Milky Way in Crux… then continues into Centaurus… Lupus… Scorpius… before disappearing below my horizon.

It’s not hard to see why astronomer Ken Croswell wrote that if he were kidnapped by an alien spaceship and taken to some remote corner of the Galaxy, Gould’s Belt is what he’d look for to find his way back home. Imagine what a stupendous beacon this ring of brilliant blue-white stars would be from that remote corner of the Galaxy!

Gould's Belt overlayed on what to me is the most gorgeous of all panorama views of the sky -the drawing made in the 1950s under the supervision of astronomer Knut Lundmark at the Lund Observatory in Sweden. Image credit Lund Observatory
Gould’s Belt overlayed on what to me is the most gorgeous of all panorama views of the sky – the drawing made in the 1950s under the supervision of astronomer Knut Lundmark at the Lund Observatory in Sweden. Image credit Lund Observatory

Gould’s Belt was first described by Benjamin Gould in 1879 as a collection of bright and massive stars that formed a ring in a projection on the sky. It is composed of massive, young, hot O and B type stars, molecular clouds, and neutral hydrogen. The belt extends in an elliptical disk, about 2,400 x 1,500 light years across, around the Solar System, with the Sun well off-centre. Most studies put the centre of the belt around Perseus, several hundred light-years from the Sun.

Benjamin Apthorp Gould (1824-1896)

How did Gould’s Belt form? Astronomers don’t know for sure, and there are many conflicting theories. But clearly something of titanic proportions happened about 30 million to 50 million years ago, which is when astronomers believe the first stars of Gould’s Belt formed. Theories included a collision with an intergalactic gas cloud; others invoke a hypernova, a gamma-ray burst, or a series of supernovae sparked by a passing spiral arm. A recent theory is that about 30 million years ago a blob of dark matter collided with a molecular cloud in our region, triggering the formation of the belt’s stars.

Many of the first stars produced in the belt have already exploded as supernovae, triggering further star formation which still proceeds apace, notably in the Orion Nebula. Thus Rigel, just 10 million years old, is likely one of a later generation of stars formed from the residual gas and dust that formed the first Gould Belt stars. (I find it dumbfounding to trace the belt across the sky and wonder about the pulsars or black holes left behind by the explosions of the first brilliant stars that once illuminated Gould’s Belt.)

The Scorpius-Centaurus and Orion OB associations are the jewels of Gould’s belt, but it has plenty of other gems, among them the Orion Nebula and the Orion molecular clouds, the Rho Ophiuchi cloud, Cepheus OB2, Perseus OB2, and the Taurus-Auriga Molecular Clouds.

The ρ Oph cloud complex is a star-forming region in the Gould Belt. Image credit NASA/JPL-Caltech/WISE Team – WISE

The two prominent star clusters that reside in Taurus – the Pleiades and the Hyades – likely have nothing to do with Gould’s Belt, because they’re too old: the Pleiades, 435 light-years away, is twice as old as Gould’s Belt, and the Hyades, 150 light-years away, is even older. (Brilliant Vega which lies not far from other bright stars of Gould’s Belt, also cannot be a member because it’s more than 500 million years old.)

We all know that astronomy is all about looking back into time. But it’s not often one can put our personal human timescales into the vast enormity of galactic timescales. It was uncanny to stand out there and look at part of this incredibly beautiful ring of bright stars, and also look back, for while these brilliant fireworks were first lighting up the sky, our ape ancestors were on the cusp of splitting off from old world monkeys… and here, after an unbroken 30 million year chain of reproduction, stands one of their descendants, looking up… and looking back.

Copyright © Susan Young 2017