“Researchers argue that it’s of utmost importance to unravel the nature of black holes, lest we someday begin to worship them,” said Harvard astrophysicist, Eric Chaisson. In April 2019 an event took place that was as epic as the Apollo 11 landing on the Moon, and may make Chaisson’s warning seem prescient.
For the first time the world viewed an image of what had once been only theoretical: a supermassive black hole at the heart of galaxy M87. At a distance of ~55 million light years, we were seeing this object as it appeared in the distant past. The size of the event horizon for this black hole is comparable to our solar system but it has a mass of six and a half billion suns packed into the same volume.
Over the eons that it took light to reach the solar system from M87 we emerged on Earth along with our myths, differentiated cultures, ideologies, languages and varied beliefs.
The image was captured by a telescope array that has a combined aperture equivalent to the radius of planet Earth. The capability of this array reaches an imaging resolution that is 4,000 times more powerful than the Hubble Space Telescope. Over the eons that it took light to reach the solar system from M87, said astrophysicist Janna Levin at Columbia University, “we emerged on Earth along with our myths, differentiated cultures, ideologies, languages and varied beliefs.”
“The gates of hell, the end of space and time”
“The gates of hell, the end of space and time.” That was how the black hole was described at the press conference in Brussels where the first ever photograph of one was revealed to an excited audience. And this black hole, a super-massive object at the center of the galaxy Messier 87 (M87 shown above), really is a monster, observed Ellie Mae O’Hagan for The Guardian. “Everything unfortunate enough to get too close to it falls in and never emerges again, including light itself. It’s the point at which every physical law of the known universe collapses. Perhaps it is the closest thing there is to hell: it is an abyss, a moment of oblivion.”
Astronomers have theorized that the black hole at the center of M87 grew to its massive size by merging with several other black holes. M87 is the largest, most massive galaxy in the nearby universe and it is thought to have formed from the merging of 100 or so smaller galaxies. The size of the black holes at the center of M87 along with its relative proximity to Earth, led astronomers to think that it could be the first black hole that they could actually “see.”
“The Planet Earth Telescope”
The Event Horizon Telescope that imaged the black hole is actually 10 telescopes, linked across four continents in the United States, Mexico, Chile, Spain, and Antarctica, and designed to scan the cosmos in radio waves. For a few days in April 2017, the observatories studied the skies in tandem, creating a gargantuan telescope nearly the size of the planet.
A Galaxy Fell Through It
“A medium-sized galaxy fell through the center of M87, and as a consequence of the enormous gravitational tidal forces, its stars are now scattered over a region that is 100 times larger than the original galaxy!” said Ortwin Gerhard, head of the dynamics group at the Max Planck Institute for Extraterrestrial Physics. Observations in July 2018 with ESO’s Very Large Telescope revealed that the giant elliptical galaxy likely swallowed the entire medium-sized galaxy over the last billion years.
Located about 55 million light-years from Earth, M87 has been a subject of astronomical study for more than 100 years and has been imaged by many NASA observatories, including the Hubble Space Telescope, the Chandra X-ray Observatory and NuSTAR.
In 1918, astronomer Heber Curtis first noticed “a curious straight ray” extending from the galaxy’s center. This bright jet of high-energy material, produced by a disk of material spinning rapidly around the black hole, is visible in multiple wavelengths of light, from radio waves through X-rays. When the particles in the jet impact the interstellar medium (the sparse material filling the space between stars in M87), they create a shockwave that radiates in infrared and radio wavelengths of light but not visible light. In the Spitzer image, the shockwave is more prominent than the jet itself.
Harvard history of science professor Peter L. Galison, a collaborator on the Event Horizon Telescope (EHT), said that scientists proposed theoretical arguments for black holes as early as 1916. It was not until the 1970s, however, that researchers substantiated the theory by observing extremely dense areas of matter. Scientists announced in 2016 that, for the first time, they had detected gravitational waves — which many argued were produced by black holes merging, and therefore were evidence that black holes exist.
Like Trying to Photograph a Golf Ball on the Moon
In contrast to M87’s monster, the Milky Way’s central supermassive black hole called Sagr A* has four million times the mass of our sun and is 44 million kilometers across. That may sound like a big target, but for the telescope array on Earth some 26,000 light-years (or 245 trillion kilometers) away, it’s like trying to photograph a golf ball on the Moon.
“More than 50 years ago, scientists saw that there was something very bright at the center of our galaxy,” Paul McNamara, an astrophysicist at the European Space Agency and an expert on black holes, told AFP’s Marlowe Hood. It has a gravitational pull strong enough to make stars orbit around it very quickly—as fast as 20 years, compared to our Solar System’s journey, which takes about 230 million years to circle the center of the Milky Way.
“We are sitting in the plain of our galaxy—you have to look through all the stars and dust to get to the center,” said McNamara.
We hope to see images of the Sgr A* supermassive black hole at the center of our galaxy from EHT in the near future but for now galaxies beyond our own like M87 are the easier targets.
Editor, Jackie Faherty, astrophysicist, Senior Scientist with AMNH. Jackie was formerly a NASA Hubble Fellow at the Carnegie Institution for Science. Aside from a love of scientific research, she is a passionate educator and can often be found giving public lectures in the Hayden Planetarium. Her research team has won multiple grants from NASA, NSF, and the Heising Simons foundation to support projects focused on characterising planet-like objects. She has also co-founded the popular citizen science project entitled Backyard Worlds: Planet 9 which invites the general public to help scan the solar neighbourhood for previously missed cold worlds. A Google Scholar, Faherty has over 100 peer reviewed articles in astrophysical journals and has been an invited speaker at universities and conferences across the globe. Jackie received the 2020 Vera Rubin Early Career Prize from the American Astronomical Society, an award that recognises scientists who have made an impact in the field of dynamical astronomy and the 2021 Robert H Goddard Award for science accomplishments.