By Dr Marc Sarzi, Head of Research
After the first image of a supermassive black hole more than 60 million light years away from the Earth1 amazed the whole World last year, black holes are back on the news this week following the award of the 2020 Nobel prize in physics to UK mathematician Roger Penrose, for its theoretical work proving that black holes are a direct consequence of Einstein’s theory of general relativity, and to German and American astronomers Reinhard Genzel and Andrea Ghez for their discovery of a supermassive black hole at the centre of the Milky Way.
Black holes occur whenever mass gets compressed in such a small volume that on its surface gravity becomes so strong that nothing, not even light can escape. To turn the Sun into a black hole you would have to compress it into a 6 km wide sphere, for the Earth you will have squeeze down to the size of a marble ball. You should not think of these Sun- or Earth-size black holes just as very dense solid objects, however, since gravity has a runaway effect once a black hole forms: all matter collapses toward a central point of immense density called the singularity. This is hidden from us since no light can escape beyond a certain distance from the singularity, a limit that is commonly referred to as the event horizon and which you can picture as the dark surface of our ideal 2 cm wide Earth black hole.
In 1965 Penrose was the first to predict that the formation of a singularity is in fact inevitable once matter gets sufficiently compressed for gravity to take over any other force. Previous models for the formation of black holes were working only under very special conditions. Incidentally, Penrose – who found stargazing truly inspiring as a young kid – became interested in black holes at a time when the discovery of a new class of bright astronomical objects called quasars in the 1960s prompted speculations about the existence of supermassive black holes packing up to several billions Suns inside them. Indeed, quasars were found to be so distant and therefore so intrinsically luminous that researchers could only explain them as the gleam of material falling into ultra-compact, supermassive objects.
It took a long time to prove that such monstrous objects existed at all, however, which is where Genzel and Ghez come in play. Before I explain their role, however, it is only fair to mention that Genzel and Ghez had not been to first looking for a supermassive black hole at the centre of the Milky Way, or in several other galaxies for that matter. In fact, the discovery in 1974 of a compact radio source with no visible counterpart (called Sagittarius A* from the name of the constellation where the Galaxy centre appears to be) already hinted at the presence of a dark object that is currently accreting gleaming material. Furthermore, in 1995 the observed motion of gas clouds in the immediate proximity of Sag A* already gave away the presence of something much more massive (and sinister!) than the central stars of the Milky Way which was pulling these clouds around. Finally, the launch of the Hubble Space Telescope in 1990 opened up the possibility of tracking the motion of gas and stars at the very centre of external galaxies2, allowing to infer the presence and gravitational pull of several compact, dark and very massive objects3.
None of these early studies, however, could unequivocally prove that these central objects were truly supermassive black holes, which is exactly what Genzel and Ghez did with their respective teams. For this, Genzel and Ghez had to wait until the advent of modern infrared-light detectors allowing to peer through the dust-enshrouded central regions of the Milky Way4 and had to pioneered techniques to compensate for the blurring of the Earth atmosphere which made it possible achieve the necessary imaging quality to observed the stars orbiting in the immediate vicinity of Sag A*. Following the orbit of these stars for several years between the mid 1990s and early 2000s using the Very Large Telescope in Chile and the Keck Observatory in Hawai’i in Genzel and Ghez case, respectively, both teams could establish the mass of Sag A* as being around 4 million solar masses and – even more crucially – observe one particular star called S2 as it swung around Sag A* like a Sun-grazing comet at perihelion. Indeed, passing within just 100 times the distance between the Earth and the Sun from Sag A* at a neck breaking speed of 22 million miles per hour, this single daring star unequivocally demonstrated how fantastically concentrated the dark object at the centre of the Galaxy is, and that indeed Sag A* could only be a supermassive black hole.
Animation of Objects Orbiting the Centre of the Milky Way
1More precisely, the famous image taken from the Event Horizon Telescope (EHT) shows the emission of the hot gas circling and falling into the supermassive black hole.
2Actually, the full potential of Hubble for discovering the central massive dark objects in external galaxies was only achieved after the first STS-61 Space Shuttle servicing mission, which fixed an initial problem with Hubble mirror.
3This includes a 1994 study that looked at the centre of M87, the massive galaxy in the constellation of Virgo whose central supermassive black hole was finally imaged last year by EHT.
4Infrared light is not as effectively absorbed by dust as light at optical wavelengths, which is why it can be used to look at stars beyond interstellar screens of dust. This is also why sunsets are red, as the blue light from the Sun is absorbed by dust (and water vapour) as it passes through a long column of atmosphere when the Sun grazes across the Earth’s surface