Mapping a black hole
Ken Tapping, 26th January, 2016
In the 1960s, Canada was the first country to succeed with a radio astronomical technique called Very Long Baseline Interferometry, or VLBI. By means of some rather exotic clocks and signal recording technology it became possible to use two or more radio telescopes, separated by thousands of kilometres, as one huge radio telescope, potentially as big as the Earth. By putting one or more antennas in space, we can achieve an even larger radio telescope. The reason we need to do such things is because radio waves are much longer than light waves.
Our ability to discern detail depends on the size of the lens or mirror we use to form an image compared with the length of the waves we are using to make that image. The pupils of our eyes are typically a few millimetres across. However, the wavelength of the light we use to image our surroundings is around a few hundred nanometres (billionths of a metre). Compared with that, our pupils are huge. The result is that our eyes can discern details as small as an arc-minute, a sixtieth of a degree, or roughly one thirtieth of the diameter of the Full Moon. The Synthesis Radio Telescope here at our observatory, operating at a radio wavelength of 21 cm, can map the same level of detail as the human eye; but to do this we need a line of antennas 600 metres long. That is why we need a technique such as VLBI to make detailed radio images of many of the distant but fascinating things we are finding in space. In many cases these objects do not produce light at all, or if they do it is by completely different processes from how they make radio waves, so we need the radio observations. In other cases clouds of gas and dust block out the light, so the only way to see some objects is by means of radio telescopes. That brings us to a new experiment that is being set up. Radio telescopes all over the world are going to be used together as a huge VLBI system to have our first really close look at a black hole. This is something very exciting. Calculations tell us what these objects should be like, and we see hints of objects in the distance that almost certainly have to be black holes, but we have never before been able to have a really good, close look.
Black holes are among the most bizarre things we have so far found in the universe. We know they can be formed when the core of a giant star collapses to the point where its gravity becomes so intense that even light cannot get out, and the fabric of space and time become severely distorted. The collapsed object becomes enclosed in a roughly-spherical boundary known as an event horizon. Anything we see dropping in through the event horizon will take forever to get back out
It looks as though most large galaxies have black holes in their centres, including ours. These might be the result of black holes from collapsed stars gorging on their surroundings for billions of years, or they might be objects dating back to the beginning of the universe or when the galaxies first formed. The black holes in the centres of galaxies are now massive. Our one has a mass of about a million suns, but is so compressed that it should be only about 12km in diameter. It is surrounded by a disc of very hot material, obtained by pulling apart stars, planets and dust clouds that happen to get too close. The material spirals in, getting hotter and hotter, until it vanishes through the event horizon. It gets hot enough to produce light, X-rays and radio waves. The emissions are so intense that any planets close by would be sterilized long before they are pulled apart and join the disc.
However, the core of our galaxy lies behind about 30,000 light years of gas and dust. Light from it is completely blocked, which is where radio astronomy and VLBI come to the rescue.
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