Black Holes - a new show at the planetarium


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A new daily planetarium show all about Black Holes is now available at the Peter Harrison Planetarium, here at the Royal Observatory Greenwich.
The show touches on several areas relevant to my previous research into astrophysics, which I will discuss in this blog.
The show covers observations made by NASA's Swift space-telescope. Swift has a collection of cameras each sensitive to different wavelengths. It has an optical camera (made in the UK at University College London's Mullard Space Science Lab), an X-ray camera (built in the UK at the University of Leicester's Space Research Centre) and a gamma-ray detector (the burst alert telescope, or BAT, built at NASA's Goddard Space Flight Centre in America).
When a black hole forms at the centre of a collapsing star, we see an intense burst of high-energy gamma-rays. As the exploding gas cools down to a few million degrees (!), the gas then begins to give off X-ray radiation. Eventually, the gas cools down to a few thousand degrees, giving off optical light.
So, the three cameras on board Swift can study all three of these stages in detail.

The X-ray camera on board Swift has a single 600x600 pixel detector. My previous job was calibrating the 14 identical detectors on board the European XMM-Newton space telescope – Swift's much bigger brother!
As a sanity check, both XMM-Newton and Swift are often pointed at the same object, and the two observations are compared, making sure that the two telescopes see exactly the same colours (colour is simply the energy of a photon, a packet of light).
This was part of my previous job as a XMM-Newton calibration scientist. Even a 0.1% difference in colour could lead to wrong conclusions being made about the speed of the gases involved, and that could lead to misunderstandings of how black holes form.
While Swift is small and nimble, allowing it to swiftly turn around to spot the formation of new black holes, XMM-Newton is much bigger, and spends its time studying existing black holes in detail.
When gas, or even a star, falls towards a black hole, it gets very hot – up to 10 million degrees centigrade! And gas this hot emits X-ray radiation.

In September 2001, XMM-Newton spotted a star being swallowed up by the black hole at the centre of our Milky-way galaxy, and took photographs of the event.
So, even though we cannot see black holes directly (finding a black star against the black background of space remains difficult!), we can still look for the tell-tale signs of their presence.
One project, called AXIS (An XMM-Newton International Survey) involved astronomers working together from around Europe. When XMM-Newton looks at a known object, it discovers hundreds of other objects around the target. (It's a bit like taking a photograph of a friend in the street, and you end up also photographing other people in the background. But we want to know more about those other people. What colour hair do they have? How tall are they?)
The easiest way to find out what those other, unknown objects were was to look using another telescope. In June 2001 I, along with two colleagues, travelled to the Nordic Optical Telescope on the Canary Island of La Palma.
About 10 of the objects we observed turned out to be super-massive black holes at the centre of distant galaxies – black holes a thousand million times more massive than our Sun.