Supermassive black holes are greedy gravitational monsters that weigh-in at millions to billions of times the mass of our Sun. Indeed, astronomers now propose that perhaps every large galaxy in the observable Universe hosts one of these bizarre objects in its secretive dark heart–and our own barred-spiral Milky Way Galaxy is no exception. Our Galaxy is haunted by its own hungry heart of darkness, enshrouded in a cloak of mystery, and it has managed to keep its myriad secrets very well hidden from the prying eyes of curious astronomers. But, despite their enormous mass and huge numbers, supermassive black holes are notoriously camera-shy, and have managed to escape having their pictures taken–until now. On April 10, 2019, the Event Horizon Telescope (EHT) unveiled the historic, first-ever image of a supermassive black hole’s event horizon, which is the region beyond which not even light can escape from the powerful, merciless gravitational grip of the voracious dark-hearted beast. Even though the existence of black holes has been theorized for more than two centuries, it was generally thought to be impossible to observe them directly. The EHT is an international collaboration whose support in the U.S. includes the National Science Foundation (NSF).
The recently unveiled supermassive black hole weighs-in at 6.5 billion times the mass of our Sun. In contrast, our own Galaxy’s dark heart is a relative light-weight– at least, by supermassive black hole standards–and weighs-in at mere millions (as opposed to billions) of times solar-mass. Our Milky Way’s resident gravitational beast has been named Sagittarius A* (pronounced Sagittarius–A-Star ), and it is a quiet, elderly gravitational beast now, only arousing from its peaceful slumber occasionally to nibble on a doomed wandering star or cloud of unfortunate gas that has managed to travel too close to its maw. When the Universe, our Galaxy and Sagittarius A* were young, our resident beast glared brightly as a quasar (the accretion disk surrounding a black hole), as it dined hungrily and sloppily on whatever managed to travel too close to where it lay in wait. The ill-fated banquet swirled down, down, down into the waiting gravitational claws of the then-young black hole, tumbling to its inevitable doom from the surrounding, glaring accretion disk. Sagittarius A* is considered to be dormant now, but occasionally it awakens to dine with the same greed as it once did, long ago, when it was a brilliant quasar lighting up the ancient Universe during its flaming youth. Sagittarius A* is elderly and quiet now–but it can still remember.
The camera-shy black hole, whose picture was recently taken, is situated in the elliptical galaxy Messier 87 (M87). An earlier image obtained from NASA’s Spitzer Space Telescope shows the entire M87 galaxy in infrared light. In contrast, the EHT image relied on radio wavelengths to unveil the black hole’s secretive shadow against the backdrop of high-energy material swirling around it.
The Nature Of The Gravitational Beast
Black holes come in different sizes. Some are the supermassive kind, residing in the center of galaxies, while those of “only” stellar mass are much smaller. A stellar mass black hole is born when a very massive star blasts itself to smithereens in a supernova conflagration–thus ending its life as a main-sequence (hydrogen-burning) star on the Hertzsprung-Russell Diagram of Stellar Evolution There are also intermediate-mass black holes that are much heavier than their stellar mass siblings, but much less massive than their supermassive kin. The gravitational collapse of a very massive star is a natural process. It is inevitable that when a heavy star comes to the end of that long stellar road–meaning that all of its sources of energy have been used up–it will collapse under the merciless crush of its own mighty gravity. This catastrophic event ia heralded by the brilliant, blazing grand finale of a supernova explosion. The most massive stars in the Universe perish this way, ultimately collapsing into a black hole of stellar mass.
Intermediate-mass objects weigh-in at hundreds of solar masses. Some astronomers have proposed that intermediate mass black holes collided and merged in the ancient Universe, thus creating the enormous supermassive variety that haunt the hearts of galaxies.
Our Milky Way’s Sagittarius A* has plenty of smaller company. Theoretical studies suggest that a large population of stellar-mass black holes–perhaps as many as 20,000–could be dancing around our own Galaxy’s resident dark heart. A 2018 study, using data gathered by NASA’s Chandra X-ray Observatory, indicates the existence of just such a bevy of bewitching black holes of stellar mass in the heart of our Milky Way.
Despite their name, black holes are not merely empty space. Squeeze enough matter into a small enough area, and a black hole will be born every time. Nevertheless, black holes are really simple objects. A black hole of any mass has only three properties: electric charge, mass and spin (angular momentum).
Many astronomers think that supermassive black holes already existed when the Universe was very young. During that ancient epoch, clouds of gas and ill-fated stars swirled down into the black hole’s fatal gravitational embrace, never to return from the churning, whirling maelstrom surrounding this voracious entity. As the captured material swirled down to its doom, it created a brilliant and violent storm of glaring material around the black hole–the accretion disk (quasar). As the material grew hotter and hotter, it hurled out a violent storm of radiation, particularly as it traveled closer to the event horizon–the point of no return.
In the 18th century, John Michell and Pierre-Simon Laplace considered the possibility that there could really be bizarre black holes in the Universe. In 1915, Albert Einstein, in his Theory of General Relativity (1915) predicted the existence of objects sporting such strong gravitational fields that anything unfortunate enough to travel too close to the hungry beast would be consumed. However, the idea that such weird objects could really exist in the Cosmos seemed so outlandish at the time that Einstein rejected the idea–even though his own calculations suggested otherwise.
In 1916, the physicist Karl Schwarzschild formulated the first modern solution to the Theory of General Relativity that described a black hole. However, its interpretation as a region of space from which absolutely nothing could escape–as a result of the object’s powerful gravitational grip–was not adequately understood until almost 50 years later. Until that time, black holes were thought to be mere mathematical oddities. It was not until the middle of the 20th century that theoretical work showed that these strange objects are a generic prediction of General Relativity.
The Dark Heart Of M87
Astronomers have been observing M87 for over a century, and it has been imaged by numerous NASA observatories, including the Hubble Space Telescope, the Chandra X-ray Observatory and NuSTAR. In 1918, the American astronomer Heber Curtis (1872-1942) was the first to detect “a curious straight ray” reaching out from the galaxy’s center. This dazzling jet of high-energy material formed a rapidly spinning disk, encircling the black hole, that could be observed in multiple wavelengths of light–from radio waves all the way through X-rays. When the particles in the jet struck the interstellar medium, they formed a shockwave that radiated in the infrared and radio wavelengths of the electromagnetic spectrum–but not in visible light. Spitzer images show a shockwave that is more prominent than the jet itself.
The brighter jet is situated to the right of the galaxy’s center, and it is traveling almost directly toward Earth. The jet’s brightness is intensified both because of its high speed in our direction, and its “relativistic effects” that arise because the jet is zipping along close to the speed of light. The jet’s trajectory is slightly out of our line of sight with respect to the galaxy. This means that astronomers can observe some of the length of the jet. The shockwave begins around the point where the jet appears to curve down, thus highlighting the regions where fast-moving particles are bumping into gas in the galaxy and are therefore slowing it down.
In contrast, the second jet is flying so quickly away from Earth that relativistic effects cause it to be invisible at all the wavelengths of the electromagnetic spectrum. However, the shockwave it creates in the interstellar medium can nevertheless be observed from here.
The shockwave is situated on the left side of M87’s center, and it looks like an inverted letter “C”. Even though it cannot be seen in optical images, the lobe can be observed in radio waves, as seen in an image obtained from the National Radio Astronomy Observatory’s Very Large Array.
By combining observations obtained in the infrared, radio waves, visible light, X-rays and extremely energetic gamma rays, astronomers are able to study the physics of these powerful jets. Astronomers are still trying to attain a solid theoretical understanding of how gas being consumed by black holes forms outflowing jets.
Infrared light at wavelengths of 3.6 and 4.5 microns are rendered in blue and green in the revealing image of the camera-shy dark heart of M87–thus revealing the distribution of stars. Dust features that shine brightly at 8.0 microns are shown in red in the image. The picture was obtained during Spitzer’s initial “cold” mission.
The Event Horizon Telescope, that captured the historic image of a black hole, is a planet-scale array composed of eight ground-based radio telescopes that were designed to attain images of a camera-shy black hole. EHT project director Dr. Sheperd S. Doelman of the Harvard-Smithsonian Center for Astrophysics (CfA), noted in an April 10, 2019 EHT Press Release that “We have taken the first picture of a black hole. This is an extraordinary scientific feat accomplished by a team of more than 200 researchers.”
This historic scientific breakthrough was announced in a series of six papers published on April 10, 2019 in a special issue of The Astrophysical Journal Letters.
Dr. Doelman continued to comment that “We have achieved something presumed to be impossible just a generation ago. Breakthroughs in technology, connections between the world’s best radio observatories, and innovative algorithms all came together to open an entirely new window on black holes and the event horizon.”