Key Takeaways on the Middle of the Milky Way
The supermassive black hole named Sagittarius A* lies in the middle of the Milky Way.
Observations of S2 and other stars also helped astronomers calculate the extraordinary size and mass of Sagittarius A* and it weighs in at about 4.3 million solar masses.
Though the entire Milky Way — our solar system included — orbits Sagittarius A*, we’re in no danger of being sucked in.
If you point a telescope at just the right sector of the night sky, in the constellation Sagittarius near where it borders Scorpio, you’ll be looking into the heart of the Milky Way. At 26,000 light-years away, it takes quite a telescope to make out anything distinct.
But there, smack in the middle of our galaxy, lies one of the most awe-inspiring objects in the universe: a supermassive black hole, or SMBH, named Sagittarius A* (pronounced “Sagittarius A star”).
The Center of the Milky Way
Of course, there’s one inescapable challenge for anyone who wants to probe its mysteries. “The problem with trying to understand supermassive black holes is we can’t see them,” says Brian McNamara, an astronomer at the University of Waterloo.
The gravitational pull of Sagittarius A* is too strong for light to escape and reach us — it’s invisible by definition. What you can see is its accretion disk, a whirling ring of gas, dust, and plasma that circles the central void.
In May 2022, the Event Horizon Telescope Collaboration released the first image of Sagittarius A*, which looks eerily like a blurred Eye of Sauron.
Read More: Here’s What Would Happen If You Walked Through a Black Hole
How We Know There’s a Black Hole
Nevertheless, the scientific community knew about our friendly neighborhood SMBH long before they got this visual confirmation. Sagittarius A* was discovered indirectly in 1974, when the astronomers Bruce Balick and Robert L. Brown detected an unusually strong radio signal coming from that region of space.
Over the next couple decades, scientists inferred its existence from the behavior of nearby stars — especially one called S2, which whips around Sagittarius A* at astonishing speeds.
How Far Away Is Sagittarius A*?
To put things in perspective with an example from our own solar system: Jupiter is roughly 5 times farther from the Sun than Earth, and its orbit takes about 12 Earth years. S2, by contrast, lies 120 times farther from Sagittarius A* than Earth does from the Sun. Yet it completes its orbit in 16 years, only a bit longer than Jupiter.
At its closest approach to Sagittarius A*, S2 zips along at nearly 5,000 miles per second, almost 3 percent the speed of light (Earth travels at a measly 18.5 miles per second). Long before the Event Horizon Telescope provided proof, just about everyone was convinced that S2’s orbit — the fastest known — could only be due to the tremendous gravitational pull of an SMBH.
The Size of Supermassive Black Holes
Observations of S2 and other stars also helped astronomers calculate the extraordinary size and mass of Sagittarius A*. Most black holes form when enormous stars — many times the size of our Sun — collapse. But even those giants are put to shame by SMBHs, which are thought to form when garden-variety black holes spiral into each other.
After many iterations these mergers can result in a behemoth like Sagittarius A*, weighing in at about 4.3 million solar masses.
In terms of volume, SMBHs are still miniscule compared with the galaxies they inhabit — “akin to an object the size of a grape […] compared to an object the size of the Earth,” McNamara says.
But they’re unbelievably massive, accounting for about a thousandth of their galaxy’s total mass. As such, these cosmic heavyweights exert enormous force on everything around them, their sphere of influence extending thousands of light years.
Read More: This is What a Black Hole Sounds Like
Could We Get Pulled into the Black Hole?
When matter falls into orbit around an SMBH, friction raises its temperature to trillions of degrees. The energy generated in that process sometimes launches superheated jets of plasma outward into the galaxy, where they circulate, preventing clouds of hydrogen and helium from cooling enough to collapse into stars.
In that way, an SMBH is like the dark overlord at the center of its domain. Though the entire Milky Way — our solar system included — orbits Sagittarius A*, we’re in no danger of being sucked in.
Despite a supermassive black hole’s immense power, objects can maintain a stable orbit around it, just like Earth does around the Sun. That said, black holes do have a reputation for gobbling matter that strays too close, and Sagittarius A* is no exception.
For years scientists referred to it as “dormant,” but a Nature paper published in 2023 showed that it does, on occasion, wake up to swallow some unfortunate cloud of gas and dust. "Imagine a bear going into hibernation after devouring everything around it,” lead author Frederic Marin said at the time.
An Active Sagittarius A*
When enough matter enters an SMBH’s accretion disk it becomes, rather paradoxically, one of the most dazzling sights in the universe — a quasar.
In defiance of the black hole’s gravity, these hyperactive objects emit blazing plasma jets that can outshine entire galaxies.
At present, Sagittarius A* is dimmer than a quasar by many orders of magnitude. But Marin’s team found that just 200 years ago — no time at all on an astronomical scale — it flared brightly enough to rival some of the most luminous galaxies. Who knows when its current slumber will end?
Read More: The Oldest Black Hole Could Wreak Havoc on a Faraway Galaxy
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
Event Horizon Telescope. Astronomers Reveal First Image of the Black Hole at the Heart of Our Galaxy
arXiv Cornell University. High Proper Motion Stars in the Vicinity of Sgr A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy
Astronomy & Physics. Polarimetry and astrometry of NIR flares as event horizon scale, dynamical probes for the mass of Sgr A*
National Library of Medicine. X-ray polarization evidence for a 200-year-old flare of Sgr A
Cody Cottier is a contributing writer at Discover who loves exploring big questions about the universe and our home planet, the nature of consciousness, the ethical implications of science and more. He holds a bachelor's degree in journalism and media production from Washington State University.
