There’s a hole lot of love
IT is news that spread around the world very quickly: astronomers last week released the first ever picture of a black hole.
It represents the first direct evidence that black holes exist, and the subject is the supermassive black hole in the galaxy M87, in the constellation of Virgo.
In a nutshell, a black hole is a region of space whose gravity is so strong that not even light can escape. Inside, matter is compressed into a point called a singularity. Very massive stars, when they explode, can have their cores collapse so violently that they form a black hole.
In the centres of galaxies, supermassive black holes are common, though astronomers are still debating exactly how such massive ones form. M87’s black hole has been found to have a mass of about 6.5 billion times that of our sun.
There really has never been much doubt about the existence of black holes. They fit well into the physics developed by Albert Einstein, and we have gathered a lot of other evidence for their existence over the years.
However, this picture, based on observations in 2017, has verified astronomers’ ideas. It will, no doubt, go
down in history as one of the most famous astronomical pictures ever made.
I say “made’’ because this is far from being a simple snapshot taken with a powerful telescope.
Astronomical imaging has always been a more specialised task than the everyday pictures we take of the family and our holidays, but the complexity in the production of this picture was extreme.
In addition, in this case, we are not looking at a “visible light’’ image, but a representation of the emissions surrounding the black hole. These have been detected using a set of radio telescopes located at very different locations on Earth, including Antarctica.
The collection of instruments is called the Event Horizon Telescope, or EHT. By combining the output of the telescopes using a technique known as Very Long Baseline Interferometry, or VLBI, it was possible to construct an “image’’ with the resolution, or sharpness, of a single telescope with a diameter of many thousands of kilometres.
As avid sky-watchers with telescopes well know, a telescope with a larger lens or mirror will gather more light, but it will also produce sharper images. This is because of the wave nature of light. It means that, for example, two stars that appear very close together will be easier to see separately with larger-diameter telescopes.
Of course, we can’t build a radio telescope as large as the Earth, but the VLBI technique simulates this, at least with regard to resolution. In fact, the resolution of this radio telescope set-up is far better than obtainable with the world’s largest optical telescopes, such as the twin 10-metre-diameter Keck telescopes atop Mauna Kea in Hawaii.
This resolution is essential, because the galaxy M87 is about 53 million light years away. We can’t picture this sort of distance in our minds: it’s the distance that light, travelling at 300,000 kilometres per second, travels in 53 million years in a straight line. It also means that the image that astronomers have put together is effectively looking back in time to only about 13 million years after the dinosaurs were wiped out.
Optical telescopes cannot be combined in this way (except over very short distances), but the output from radio telescopes can. An extremely accurate time base forms part of the radio observations, allowing computer drives with the data on them to be physically transported across the world to a central point for their data to be matched with all of the others.
And what a lot of data! For this image to be constructed, there were approximately five petabytes of data, equal to 5000 terabytes. My largest, and probably your largest, hard drive holds just a few terabytes, so it’s easy to see that this is a huge amount of information.
The picture looks relatively simple, but the reason for the “ring of light’’ is actually quite complicated. The light comes from superheated gas that is orbiting the hole, emitting a wide range of wavelengths, including X-rays and radio. The dark region in the middle is the black hole, or, more correctly, what astronomers call the “shadow’’ of the hole.
Light behaves in strange ways around a black hole, and the ring is a result of material, and light itself, orbiting the hole and travelling in complex paths due to the hole’s enormous gravity.
This picture will be my computer wallpaper for quite a while! Martin George is manager of the Launceston Planetarium (QVMAG).