Not even light can escape a black hole's gravitational pull, so how do we actually know they exist? And what's the deal with the supermassive black hole at the center of the Milky Way?
We live in a fairly massive galaxy called the Milky Way. Galaxies are collections of stars, gas, and dust. Our Sun, while very special to us, is just one of ~200 billion stars that make up our galaxy’s stellar collection. It’s a bit challenging to get a sense of what the Milky Way looks like. Why? Because we're sitting inside it. (Imagine trying to take a picture of your house from inside your bedroom.)
Our Sun, while very special to us, is just one of ~200 billion stars that make up our galaxy’s stellar collection.
But we think our galaxy is shaped like a disk or frisbee with what we call a "bulge of extra material" at the center. And lurking in the center of that bulge is what we believe to be a supermassive black hole.
What is a black hole?
A black hole is something so compact, so dense that not even light, the fastest-traveling thing we know of, can escape the pull of its gravity. Technically, anything can be a black hole as long as you compress it enough. That means you, your couch, and even your apartment would all become black holes if we squeezed them down small enough.
You, your couch, and even your apartment would all become black holes if we squeezed them down small enough.
The Earth, which is just shy of 8,000 miles across, would be a black hole if we compressed it to 1.8 centimeters across —about the size of a penny. To turn the Sun into a black hole, it would have to be compressed down to about 6 km. That’s an incredible amount of mass shoved into a space that's only a little more than half of that charity 10k race I ran pre-pandemic, which took only an hour to finish.
Who discovered black holes?
In the 1700s, a man named John Michell theorized there could be dark stars, or stars so dense that even light could not escape them. Michell is described as a philosopher and a clergyman. The American Physical Society notes that he was "so far ahead of his scientific contemporaries that his ideas languished in obscurity until they were re-invented more than a century later.”
Now, haven’t we all felt like that at some point? No one seems to appreciate how brilliant my ideas are—I’m ahead of my time!
How are black holes formed?
We now understand fairly well how some of these dark stars form: through the collapse of massive stars nearing the ends of their lives. A star that’s more than 20-30 times the mass of our Sun will spend tens of millions of years battling the crushing weight of its gravity, mostly by converting mass into energy through fusion. But eventually, gravity wins and the star collapses in on itself. With nothing to support the star from the inside, all of the star’s mass is forced into a single point we call a singularity. Voila! You have a black hole.
Supermassive black holes are a different story
But our understanding of the formation of supermassive black holes, like the one at the center of our galaxy, is a different story.
The black holes formed from the collapse of a massive star end up in the ballpark of 10-100 times the mass of our Sun. Supermassive black holes have masses in the range of millions to billions of times the mass of our Sun. These objects are ubiquitous—every time we look for one of these behemoths at the center of another massive galaxy, we find one. But we’re not yet sure how they get there.
Nothing, not even light, can escape the intense gravitational field surrounding the supermassive black hole at the center of our galaxy.
Adding to the mystery, detections of intermediate-mass black holes—compact objects with masses between 100 and 1 million times the mass of our Sun—are rare.
If nothing, not even light, can escape the intense gravitational field surrounding the supermassive black hole (called Sagittarius A*) at the center of our galaxy, how do we know it’s there?
Well, we have the work of Andrea Ghez and the UCLA Galactic Center Group to thank for that knowledge. Her group has carefully tracked the motions of more than 3,000 stars that orbit close to Sagittarius A* (Sag A* for short) for 24 years. Over that time, their paths show the stars clearly orbiting something sitting at the galactic center … something we cannot see.
And that’s not all. The motions of the central stars reveal not only that they must be orbiting an invisible companion, but also that their companion must be an incredibly massive object crammed into a very small space.
To understand how, let’s first take a look at the Earth-Sun system. Our Earth, along with the other planets in our solar system, strikes a perfect balance of forces that keeps us settled in our orbit around the Sun. The Sun’s gravitational force wants to pull us inward but the centrifugal force, an outward force due to the inertia from our circular motion around the Sun, wants to push us outward. When the strengths of these two forces balance, we have a stable orbit.
This balance relies on three parameters: the distance between the Sun and the Earth, the velocity of the Earth in its orbit, and the mass of the Sun. For example, if the Earth were to suddenly slow down so that it takes far longer than one year to fully orbit the Sun, this decrease in speed would lower the strength of the centrifugal force. The balance of forces would be thrown off, and the gravitational force would win, pulling us into the Sun. Conversely, if the Earth were to suddenly speed up, this increase in speed would mean the centrifugal force would start to overpower gravity and we would fly off into space, leaving the solar system behind.
How do we know where black holes lurk?
We can generalize our knowledge of the Earth-Sun system to any orbiting objects, like the stars orbiting Sag A*. The stars are in stable orbits, so we know their forces must be balanced and that balance must depend only on the distance between each star and the supermassive black hole, the speeds of those stars in their orbits, and the mass of the central supermassive black hole. The first two parameters can be directly measured by watching the star’s motion: how big of an ellipse does it trace out? how long does it take to make a complete orbit? Then, with knowledge of two of the three unknowns, the third—the mass of Sag A*—can be calculated.
The stars near the center of the galaxy, tracked by Ghez and her group, are moving fast—really fast.
To think of this physically, the stars near the center of the galaxy, tracked by Ghez and her group, are moving fast—really fast. Astronomical timescales tend to be, well, astronomically large, so most events in the cosmos can’t be viewed over an astronomer’s lifetime. But some of the stars in the galactic center take only on the order of 10-20 years to complete an orbit. Since we know how fast they are moving, we know how much mass must be lurking in the center there to hold them in such a quick-moving orbit. If they were moving more slowly, then not as much mass would have to be there to hold them in place.
We can also estimate the size of the supermassive black hole simply because it can’t be bigger than the smallest stellar orbit we see. The size of whatever you’re orbiting can’t be bigger than your orbit around it. Once that ballpark estimate puts an upper limit on the size of the compact object in the center of our galaxy, we see that the only known objects with such high densities—such large amounts of mass crammed into such a small space—are black holes.
The Nobel Prize-winning work of Andrea Ghez
The work of Andrea Ghez and others have placed the mass of Sag A* around 4 million solar masses in a size of less than 0.2 Astronomical Units. That means that the mass equivalent of 4 million suns resides in a space less than a fifth of the distance between the Sun and the Earth.
In case those orbital calculations sound simple, the techniques required to measure those stellar orbits are incredibly challenging. For starters, the galactic center is shrouded in dust and thus requires that observers use telescopes that can detect light at near-infrared wavelengths, wavelengths too long for the human eye to see. Observing in the near-infrared comes with many challenges, like the fact that the night sky is bright at near-infrared wavelengths and so an observer has to calibrate—or assess this background level—frequently.
Only the highest resolution instruments can make out such small movements in the sky.
Also adding to the difficulty are the incredibly small sizes involved. Only the highest resolution instruments can make out such small movements in the sky. In fact, Ghez and her group had to push their instruments, already some of the best available at the Keck Observatory on Mauna Kea in Hawaii, to higher resolution through the very cool trick of adaptive optics. When this work first started in the 1990s, there was doubt among the astronomical community that it was even possible. In 2020, Ghez became only the fourth woman ever to win the Nobel Prize in Physics and the first woman astrophysicist to earn it, thanks to this work.
The Noble Prize being awarded for this work does not, however, mean that studies of Sag A* are a wrap. For starters, the supermassive black hole appears to be upping its appetite for surrounding gas and dust. Recent observations show an increase in the brightness of the light coming from the vicinity of the black hole, an increase that Ghez and her team haven’t seen in the 24 years they’ve been studying the massive object. This turning up of the brightness dial can happen when a black hole gains material from its surroundings. As Ghez herself notes, “The Nobel Prize is fabulous, but we still have a lot to learn.”