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The Story of How Scientists Discovered the Black Hole at the Milky Way’s Centre

The Story of How Scientists Discovered the Black Hole at the Milky Way’s Centre

One of the twin Keck Telescopes at Mauna Kea, Hawaii, fires a laser guide to create an artificial star image to help correct for atmosphere-induced blur. Photo: Keck Observatory website.

On October 6, the Nobel Prize for physics was awarded to Reinhard Genzel and Andrea Ghez for the discovery of a supermassive compact object at the centre of our galaxy, and to Roger Penrose for the discovery that black hole formation is a robust prediction of the general theory of relativity.

Black holes are one of the most enigmatic objects in the universe, and they have fascinated scientists and non-scientists alike. In the 18th century, based on Isaac Newton’s work, John Mitchell and Pierre-Simon Laplace first proposed the idea of objects so heavy that even light couldn’t escape their gravitational pull. In 1915, Albert Einstein proposed an ‘upgraded’ theory of gravity called the general theory of relativity. He postulated that gravity as a force was created when objects with mass bent the space-time continuum. The heavier the object, the more it bends space-time around itself, and so more its gravitational pull is felt to be.

Einstein’s theory contained a set of equations that could be used to determine the strength and direction of the force of gravity exerted in any natural situation. The German physicist Karl Schwarzschild, while serving in the army during the First World War, published the first exact solutions to these equations. In his calculations, he determined the curvature of space-time around a spherical object, which physicists later found in nature – in the form of black holes. Curiously, Schwarzschild found that at two locations, the theory breaks down and becomes unable to predict what could happen there. One was at the centre of the sphere, and the other at a certain radius from the centre, called the Schwarzschild radius.

Following the work of the American physicists Robert Oppenheimer and Hartland Snyder in 1939 and of David Finkelstein in 1958, we now understand these ‘locations’ a bit differently. The centre of the object is known as the singularity. The surface of the sphere described by the Schwarzschild radius is called the event horizon.

The singularity at the centre of a black hole is formed when too much matter is crammed into too small a space, and the density becomes infinite. The event horizon is the ‘point of no return’: once something, including light, has crossed beyond this point in the black hole, there is no escape. However, based on his own work Einstein had previously speculated that such ultra-compact masses can’t exist. Several other physicists also thought that such ‘singularities’ might be artefacts of approximations and assumptions in the theory itself, and not something we might observe in the natural universe.

angular resolution, black hole, black holes, Event Horizon Telescope, interstellar dust, M87 galaxy, Milky Way galaxy, radio waves, Sagittarius A*, very-long baseline interferometry, Virgo constellation,
The black hole at the centre of the M87 galaxy, as observed by the Event Horizon Telescope in 2017. Image: Event Horizon Telescope Collaboration

Roger Penrose, a mathematician and physicist at the University of London, wanted to analyse the Einstein equations without assuming a spherical geometry, like Schwarzschild had. For this, he employed a branch of advanced geometry called topology and introduced a new concept to aid his calculations, called trapped surfaces. His work expanded on the ideas of the Indian physicist Amal Kumar Raychaudhuri and the Soviet physicist Lev Landau – specifically, the Raychaudhuri-Landau equation.

If you have a bunch of particles sitting at rest with respect to each other, they will eventually come together and form a singularity. This is because gravity is an attractive force. In reality, there are other forces in play between particles that prevent them from collapsing into a singularity every time they come close enough (‘like charges repel’ is one of them). Penrose showed that if light becomes trapped inside some region and cannot escape, then a singularity must occur and the path of light will lead to the singularity. This trapped surface is the event horizon of a black hole.

When the core of a sufficiently massive star collapses at the end of its life, the gravity at its centre is so strong that no other force can prevent it from imploding into a small, ultra-dense region, bending the space-time continuum infinitely at its centre and trapping light within its event horizon. Penrose proved that some stars will inevitably collapse into a singularity surrounded by an event horizon, forming a black hole. His theory, however, doesn’t account for quantum physics, which describes how physics works at very tiny scales.

Also read: A Black Hole Paradox Where Relativity and Quantum Physics Meet

Heart of darkness

In the 1950s and 1960s, astronomers working with radio telescopes found tiny dots in their data that seemed to be the source of strong radio waves. When they observed these dots with visible-light telescopes, the dots appeared to be blue in colour and seemed to be stars in our galaxy. They were thus named quasi-stellar objects, or quasars. (The prefix ‘quasi-‘ means ‘almost’.)

A Hubble space telescope image of quasar 3C 273, the first quasar to be identified. It is around 2.5 billion light-years away. Photo: ESA/Hubble & NASA/Wikimedia Commons, CC BY 4.0

Later, astronomers found that these were not stars in our galaxy but objects associated with distant galaxies, many of which were more than a billion light-years away. Even at such awesome distances, their light was thousands of times brighter than all the light originating from the Milky Way. Their brightness also appeared to flicker across a matter of days and months. In astronomical terms, this is a blink of the eye. Only a supremely dense object could produce such extreme brightness and rapid flickering. No wonder then that astronomers quickly suspected quasars could be supermassive black holes surrounded by superhot, radiation-emitting plasma.

Donald Lynden-Bell, a physicist at the Royal Greenwich Observatory, provided one of the first theoretical descriptions of quasars, and suggested that most galaxies contain supermassive black-holes at their centres. In 1971, Lynden-Bell and Martin Rees, of the University of Cambridge, compared a map of quasars to radiation coming from the Milky Way. Based on their analysis, they predicted that the Milky Way should also host a massive black hole at its centre.

If you looked at the centre of the Milky Way through a visible-light telescope, you’ll notice a large amount of dust blocking your view. But while dust blocks visible light, radio-waves can penetrate it, so astronomers prefer radio telescopes. In 1974, two astronomers named Bruce Balick and Robert Brown used the US National Radio Astronomy Observatory’s radio telescope to study the Milky Way’s centre. They found a small ‘core’ in this region that was powerfully emitting radio waves. Brown called it Sagittarius A* (pronounced ‘Sagittarius A-star’, and shortened as Sgr A*).

In this image of the neighbourhood of Sgr A*, high-energy X-ray emission captured by NASA’s Chandra X-Ray Observatory appears in green and blue, and low-energy radio emission captured by SARAO’s ground-based MeerKAT telescope array is coloured red. Caption and photo: X-Ray: NASA, CXC, UMass, D. Wang et al.; radio: NRF, SARAO, MeerKAT

Astronomers subsequently used infrared telescopes to study this region and also found a large cluster of stars. The compact radio source Sgr A* was at the centre of this cluster. They studied the apparent movement of Sgr A* (by comparing its position against the background of faraway galaxies) and concluded that Sgr A* was part of the galactic centre, and not a distant background object. All of this evidence pointed to this region being the Milky Way’s nucleus. Estimates in the late 1970s suggested Sgr A* had a mass of 5 million times that of the Sun, clumped together in a very small volume of space. But one question remained: was Sgr A* a supermassive black hole or something else?

Despite the weight of data, answering this question turned out to be quite difficult.

Since the early 1990s, two international teams – one at the Max Planck Institute for Extraterrestrial Physics, Germany, led by Reinhard Genzel and another at the University of California, Los Angeles, led by Andrea Ghez – have been studying the galactic centre for this purpose.

Both teams studied the infrared radiation coming from the Sgr A* region, which the dust couldn’t block, for information about what could be going on there. Genzel’s team used the European Southern Observatory’s (ESO) infrared telescopes in Chile, and Ghez’s team used the Keck Telescope in Hawaii. The first few light-years from the galaxy’s centre contain hundreds of stars, all orbiting the nuclear point. By studying their orbits, astronomers can tell if Sgr A* is a single, massive object or a large number of stars or stellar remnants, like neutron stars or small black holes, close to each other.

Observing these stars is not easy. Turbulence in Earth’s atmosphere blurs the starlight, so it’s hard to determine their position accurately using ground-based telescopes. The long observation time required makes the use of space telescopes unfeasible. To overcome this challenge, the two teams developed a technique called speckle imaging: taking many short-exposure pictures of stars and stacking them together to improve the quality of images. But this technique worked only for the brighter stars.

They had their next breakthrough when the telescopes they were working with were upgraded with adaptive optics. This technology creates ‘artificial stars’ in the field of view by firing strong lasers; then, by using the lasers’ light as a reference, the telescope can correct for the blur (see image on top). This way, the teams discovered dozens of stars within 0.1 light-years of Sgr A*. These are called the S-stars. One of them, S2, has an orbital period of only 16 years, and it comes within 17 light-hours,  or 120-times the Earth-Sun distance, at its closest approach.

The teams studied S2 and several other stars in the S-cluster for many years, particularly their distances, speeds and orbits. In the end, they were able to determine that Sgr A* contains a mass of 4 million Suns within a region of space the size of our Solar System, and the only way this is possible is if Sgr A* is a supermassive black hole.

In 2008, both teams presented the results of their long observations and analyses, confirming the presence of a black hole at the centre of the Milky Way. For this work, Genzel and Ghez together received one-half of the 2020 Nobel Prize for physics.

Physicists have subsequently further observed SgR A* using a new technique called infrared interferometry, with the GRAVITY  instrument aboard the ESO’s Very Large Telescope. They have improved Genzel’s and Ghez’s measurements a hundred-fold, and have also reported that S2’s orbit shows effects predicted by the general theory of relativity, further confirming the results. The Event Horizon Telescope is poised to study the shadow of this supermassive black hole in the coming years.

Abhijeet Borkar is a postdoctoral researcher at the Astronomy Institute of the Czech Academy of Sciences, Ondřejov.

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