September 18, 2025
5 min read
How Do You Weigh a Black Hole?
Gauging the mass of a black hole is tricky, but astronomers have devised multiple methods to measure the heft of these galactic gluttons
This jagged jumble of pixels may look like a rainbow-colored thunderbolt, but it’s actually a spectrum of light gathered by the Hubble Space Telescope that shows the swirling motion of gas and stars around the heart of the nearby galaxy M84. The blue (left) and red (right) parts of the spectrum show motions toward and away from an observer, respectively; carefully measuring these motions allowed astronomers to weigh M84’s central supermassive black hole.
NASA, Gary Bower, Richard Green (NOAO), the STIS Instrument Definition Team
In a recent piece for my column The Universe, I wrote about the biggest black holes in the cosmos. These can tip the scales at many billions of times the sun’s mass, outweighing even entire galaxies.
But how do we know that? Black holes are rather famous for being, well, black because they can gobble down even light itself. So how can we figure out how massive they are?
There are several ways, actually, mostly depending on the kind of black hole we’re examining.
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The most common kind of black hole we know of is a stellar-mass black hole; one with a few to a few dozen times the mass of the sun. This type usually forms when a massive star explodes at the ends of its short life and its core collapses. The infalling material becomes so dense that the gravity skyrockets, becoming so strong that nothing, not even photons, can escape its grasp after getting too close. No light gets out of this object, so it’s black, and anything that falls in can’t get out, like an infinitely deep hole.
Naming such an object turns out not to be very difficult.
Because they’re forged from stellar cores, black holes must have a mass that’s close to that of a star. Theoretical calculations bear that out, yielding a minimum mass of about three times that of the sun. There’s no clear upper limit, but astronomers consider any black holes of less than 100 solar masses to be in the stellar-mass black hole category.
Of course, actually measuring that mass is difficult for an object you can’t see. But sometimes nature gives us a hand.
Massive stars are commonly found in binary systems, where they orbit with another star. When the massive star explodes and leaves behind a black hole, this remnant can stay gravitationally bound to its stellar companion, which can betray its presence. We could, for example, look for stars that appear to be orbiting a massive but unseen object. That method is difficult but has actually been successful in finding several black holes.
Imagine viewing one of these systems essentially “edge on” with respect to their co-orbital motion. As the two circle each other, the visible star spends half that orbital period moving toward Earth and the other half moving away. This induces a Doppler shift in its light, shortening the wavelengths when it moves toward us—a so-called blueshift—and lengthening them as a “redshift” when it moves away.
That’s the key to finding the black hole’s mass! Using the laws of motion derived by the German astronomer Johannes Kepler early in the 17th century, the total mass of the system can be calculated by just knowing the orbital period and stellar velocity. We can estimate the mass of the visible star using our understanding of stellar physics and subtract that from the total to weigh the black hole.
This method works even if the system is too distant from us to see the star physically move. Binaries like this can also be found in other ways: For example, if the black hole is stealing matter from its stellar companion, that material heats up so much as it falls into the black hole’s maw that it emits high-energy x-rays. If we see copious x-rays coming from what appears to be a normal star, we can be suspicious a black hole is at work there. The very first confirmed black hole, Cygnus X-1, was found exactly this way, and the Doppler method revealed its mass to be about 21 times that of the sun.
A variation of this method can be used on supermassive black holes, too. These objects are absolute beasts, millions or billions of times the mass of the sun, and are found in the cores of all big galaxies. They are far too beefy to orbit a single star, but in fact many stars can orbit them. The closer these stars are to that monster in the middle, the faster they circle it. Each of these stars will exhibit a large Doppler shift in their light, with roughly half moving toward us and half moving away. Measuring them en masse, we’ll see that characteristic duality between blueshift and redshift in their combined light. Again, the velocities of those stars depend on the mass of the object they orbit, so we can use that to weigh the black hole.
Based on nearly 20 years of observations by the European Southern Observatory’s Very Large Telescope in Chile, this time-lapse video shows stars orbiting around Sagittarius A*, the supermassive black hole at the heart of the Milky Way.
Astronomers have done this with many galaxies, thanks to instruments such as the Space Telescope Imaging Spectrograph (STIS), a camera that I worked on that’s onboard the Hubble Space Telescope. STIS was designed in part to make just this sort of observation. Not long after astronauts installed it on Hubble, STIS looked at the nearby galaxy M84 and easily detected a huge Doppler shift around the galactic core, chalking it up to a central black hole with at least 300 million solar masses—a behemoth indeed.
Current models of galaxy formation indicate that the mass of a galaxy is linked to the mass of its central black hole, too, with bigger galaxies tending to have a bigger central black hole. That’s not a hard-and-fast rule—our own sprawling Milky Way has a relatively undersize black hole, for instance—but if you measure enough galaxies the pattern becomes clear. While this trend won’t get you a hugely accurate measurement, it can be used to gauge a galaxy’s central black hole mass. Over a dozen elusive intermediate-mass black holes—ones with 100 to 100,000 solar masses—have possibly been found in dwarf galaxies this way.
There are many more indirect methods as well. The x-rays emitted by material as it falls into a black hole can be used to estimate its mass, for example. Sometimes, in the crowded chaos in a galactic center, a star can wander too close to the central supermassive black hole and get shredded by its powerful tidal force. This tidal disruption event creates a truly immense explosion, and the amount of time it takes to brighten and fade is related to the black hole mass, which in turn can be used to estimate the monster’s tonnage.
Also, when black holes collide and merge, they give off a staggering amount of energy in the form of gravitational waves—ripples in the fabric of spacetime. Encoded in those ripples is the mass of both converging black holes, as well as that of the final, somewhat larger black hole. The first such gravitational waves were detected just more than a decade ago, and to date, roughly 300 more have been found. Current detectors can only sense the mergers of stellar-mass black holes, but future space-based observatories such as LISA (Laser Interferometer Space Antenna) should also be able to “hear” the waves from colliding supermassive black holes.
Black holes themselves emit no light, but that doesn’t mean they’re entirely invisible. They reveal themselves in a myriad of ways, and if we’re clever—and we are—we can use that take the measure of them.
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