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The discovery of a new black hole in the galaxy

But just how extraordinary this source was only became clear three decades later, when it was identified as a supermassive black hole with the mass of whopping four million suns. Theory predicts that such supermassive black holes, which reside at the centre of most large galaxies, should be surrounded by a cluster of smaller black holes and other objects.

But decades of searches have revealed nothing — until now. These could follow the motion of stars very close to the unseen object as they orbited around it. This work had to be done by looking at infrared light. Some infrared light, radio waves and X-rays, however, can penetrate the obstacles in the galaxy to the discovery of a new black hole in the galaxy our detectors on Earth.

One possible explanation is that they are created as galaxies merge or interact — trapping their central supermassive black holes into orbits about each other and eventually merging into an even bigger supermassive black hole. We know that active star formation is going on in the region of our galactic centre. As the normal star evolves, its matter can get sucked up by the black hole or fall onto the neutron star — reaching extremely high temperatures.

This makes the system emit X-rays, which we can detect. It is not surprising then that the unique and very high resolution of the space-based Chandra X-ray Observatory has detected hundreds of X-ray sources in and around our galactic centre.

You can view some superb images of these objects here. Unlike the X-rays from binary systems involving neutron stars, these are relatively weak. However, there are millions of white dwarfs in the galaxy. Both types binary systems and pulsars undergo occasional X-ray outbursts, but their properties differ. Pulsars, however, could account for about half of the sources — they are very steady and quiet. But that means that the remaining half at least must be binary systems involving black holes — a class that have much rarer outbursts usually many decades between them and properties generally similar to those seen in the study.

How We Discovered the Black Hole at the Center of Our Galaxy

The team suggests there could be hundreds of such black hole binaries at the centre of our galaxy and thousands of black holes without a companion star. Future observations are needed to confirm this finding. It can be particularly tricky to distinguish between binary systems involving quiescent minimally accreting black holes and millisecond pulsars.

But the better the technology gets, the more accurately will we be able to do this. If these really are black holes, it is extremely exciting — showing that we are on the right track in understanding how supermassive black holes impact the behaviour of stars around them.

It might even be important for future observations using gravitational waves ripples in the fabric of space itself. Clearly, we still have a lot to learn about our own galaxy.