This month, we saw the first-ever photograph of the supermassive black hole at the center of our own Milky Way galaxy. We had seen stars orbiting around something invisible and massive but this image was the first direct visual evidence of our black hole. But what are we really seeing in this image? Bob Berman shares insight.
As you may have heard, astronomers unveiled the first-ever image of the black hole at the center of our galaxy on the 12th of May, 2022. It was quite a scientific achievement involving many collaborators, but I won’t get into the complexities of what it takes to capture an image of a black hole.
There appears to be a supermassive black hole in the heart of every galaxy. Some black holes like our own galaxy’s are called “supermassive” because they contain millions or billions of times the mass of our Sun. In one galaxy, a truly weighty one has the mass of nine billion suns. But the black hole in the exact center of our own Milky Way galaxy is a relative lightweight, with the weight of “just” four million suns. It goes by the odd name of Sagittarius A* (Sgr A*, pronounced “sadge-ay-star”).
What Are We Seeing in the Black Hole Image?
This was the first-ever photograph of our black hole. But beware here, too. It looks like a black glob, but that’s not the black hole.
We cannot see the black hole itself because it is completely dark, but we can see a dark central region called a shadow, surrounded by a glowing and somewhat eerie-looking orange-red ring of gas, which measures around 72 million miles across.
It may be hard for some folks to think about a picture of a black hole since it’s invisible. What we are really seeing in this picture is a radio image (not a picture taken in visible light like we might get from a telescope like Hubble) and depicts a much bigger space surrounding the actual black hole, where the distorting effects of extreme gravity have prevented radio waves from heading in our direction.
So, we’re not seeing the central black hole. But if we ever do, it will appear far smaller and still look inky black—an apt visual metaphor for all the mysteries it represents.
No matter. We now have visible evidence that our galaxy’s black hole exists. According to the European Southern Observatory, “Scientists had previously seen stars orbiting around something invisible, compact, and very massive at the center of the Milky Way. This strongly suggested that this object is a black hole, and today’s image provides the first direct visual evidence of it.”
This is the first image of our Milky Way galaxy’s black hole and the second-ever image of any black hole. The first image, taken in 2019, was of the massive black hole at the center of the massive Messier 87 galaxy. The two black holes share many similar properties and there’s no doubt having two images will lead to many new learnings about how galaxies are formed.
First image of black hole at the heart of M87. April 10, 2019. Credit: NASA/Hubble.
For one thing, the black hole discoveries confirm Albert Einstein’s general theory of relativity! The black hole is precisely the size that Einstein’s equations dictate. It is about the size of the orbit of Mercury around our sun. See full ESO press release.
More About Collapsed Suns (aka Black Holes)
The wild concept of a collapsed sun (black hole) started in 1783 with the British natural philosopher John Mitchell. He proposed the seemingly preposterous idea of an object collapsing so completely that it would occupy no space at all. The invisible puppeteer would be gravity, a universal force with patience that transcends time itself.
Our sun, like most stars, has a stable size and shape. But gravity is always waiting in the wings like a greedy relative hovering over a deathbed. Its inheritance can’t be denied, and the mere matter of waiting 10 billion years presents not the slightest obstacle.
Only by consuming four million tons of itself every second, and converting this into energy via Einstein’s E=MC squared, can the sun produce enough outward pushing force to forestall gravity’s foreclosure. Each clump of solar material the size of a person is converted into an H-bomb worth of energy each second. Given the sun’s mass in tons (2 followed by 27 zeroes) there is plenty of fodder for a mass-into-energy lifespan of billions of years. Eventually, however, the Sun loses its gravity battle and shrinks to our planet’s size while still weighing the same as 300,000 Earths. It’s crushed down to be 40,000 denser than steel. But at this point the action eternally stops—with the Sun a dark “black dwarf” the size of Earth.
It’s a different story for stars with ten times the sun’s mass. This puts tremendous pressures on their cores, forcing their nuclear fuel to be squandered in explosive frenzies. So much heat bursts from their centers that these stars always shine with an arc welder blue-white intensity. Instead of a stable 10 billion year sabbatical of leisurely hydrogen-burning like the sun, massive blue-white stars have longevities measured in the mere millions of years, the cosmic equivalent of mayflies. Then gravity steps in with a true vengeance.
The melodramatic details of giant suns imploding was first set down by the Indian astrophysicist Subrahmanyan Chandrasekhar in 1930. His surprising calculations described how high-mass objects behave under enormous pressure. For when stars have more than 1.4 times the mass of the sun, their collapse becomes so overwhelming that gravity is not satiated when their atoms are merely pushed into a crowded state like a rush hour subway car. Instead, atoms are merged into each other and the star keeps collapsing. The smaller it gets, the stronger the gravity at its new surface becomes, which pulls it yet smaller, and on it goes. This is runaway collapse.
The collapse stops when the star is only ten miles across. Such freakish curiosities are neutron stars. If their fast spin aims beams of energy our way, like a lighthouse, it’s called a pulsar. Harder than a zircon, smaller than Brooklyn, and spinning dozens or even hundreds of times a second, hundreds of such objects have been catalogued.
The most famous—the Crab Pulsar—is now up at dawn in the east. You wouldn’t want to visit one. Anyone reaching a neutron star’s solid surface would experience such awesome gravity that their body would be crushed down to the height of a single atom, their organs promptly spreading themselves evenly over the entire surface like a thin film of lubricating oil. This isn’t good for you.
But even smaller and more crushed than pulsars are stars weighing more than about a dozen of our Suns. Then runaway gravity doesn’t merely push atomic protons and electrons into each other to form a solid ball that is essentially a single neutron, but the shrinkage makes its surface gravity so strong that at one point in its collapse the velocity needed for escape equals and then exceeds the speed of light. At that moment it becomes a black hole, because if light cannot escape, nothing else can, either. It blinks out of existence.
The public has always been fascinated with this idea. But people carry many wrong ideas about them. Yes, a black hole is black because it cannot emit nor even reflect light. But it’s not a hole. A hole suggests emptiness, yet here is a place where matter is present and packed so tightly it is the exact antithesis of a hole.
Because its size is meaningless because it keeps changing—it may even crush itself down to where it occupies no space at all—and because its time is so warped that while anyone in the black hole would witness the crush-down to have happened in a flash, everyone outside of it, meaning the entire rest of the universe including us, views its time as having frozen. So to us, it doesn’t budge, it doesn’t shrink, it doesn’t have a reliable size we can quantify—and it never even quite reaches black hole status, not even in a billion years.
If its frozen time makes notions of its mutating size meaningless, the only way to characterize it is by stating its mass, or weight. A single massive old-aged star collapsing toward being a black hole, for example, might have the mass of 30 of our Suns. We base that on its effect on stars that may be orbiting it.
Do Not Fear Black Holes
One more thing. Black holes get bad press because people think they go around gobbling things up. Yes, black holes do gobble up galactic material but this one is eating very little. One astronomer said it’s equivalent to a person eating a single grain of rice over millions of years.
Further, in truth, you’d have to venture right up to one in order to reach its event horizon and thereby imperil yourself, because you’d then be on the very edge of where escape is impossible unless you have powerful rocket engines and lots of fuel.
If our Sun collapsed to become a black hole in the next five minutes, our planet Earth would not be pulled into it. We wouldn’t even be nudged an inch in its direction. That’s because its mass and therefore its overall gravity would still be the same as it always was, so we’d keep orbiting it just as we’ve been doing all these years.