Supermassive black holes are the most mysterious objects in the universe. They are giant, invisible engines found at the heart of nearly every large galaxy, including our own Milky Way. These objects are not just heavy; they are mind bogglingly massive, often weighing millions or even billions of times more than our Sun. Because they are “black,” they do not shine or reflect light, so we cannot see them directly. They are invisible vacuums in space.
This creates a fascinating puzzle for scientists. If you cannot see an object, and you certainly cannot travel to it to put it on a scale, how do you find out how much it “weighs”? Measuring a black hole’s mass is one of the most important jobs in modern astronomy. The mass of a black hole tells us how it formed, how it grew so large, and how its immense power shapes the entire galaxy around it. It controls the orbits of stars, feeds on gas, and can even stop new stars from being born.
Scientists have become incredibly clever “cosmic detectives” to solve this problem. They have developed several different methods to weigh these invisible giants, not by looking at the black hole, but by looking at everything around it. Each method is like a different tool in a toolbox, perfect for different types of galaxies. So, how exactly do we measure the pull of an object we can never hope to see?
What Is the Main Way to Weigh a Black Hole?
The main way to weigh any object in space, whether it is a planet, a star, or a black hole, is to use its gravity. Gravity is the one thing a black hole cannot hide. While the object itself is invisible, its gravitational pull is enormous and affects everything in its neighborhood. We can weigh a black hole by watching how it makes other things move.
Think about our own solar system. We know the mass of our Sun not because we put it on a scale, but because we can measure the Earth’s orbit. We know that the Earth is about 93 million miles away from the Sun, and we know it takes one year (365 days) to complete one full orbit. Using these two pieces of information and the laws of gravity (first figured out by Johannes Kepler and later perfected by Isaac Newton), scientists can calculate the Sun’s mass. If the Sun were more massive, its gravity would be stronger, and the Earth would have to orbit much faster to avoid falling in.
This exact same principle applies to a supermassive black hole at the center of a galaxy. We find something that is orbiting the black hole, like a star or a cloud of gas. Then, we need to measure two things: how fast that object is moving, and how far away it is from the central black hole. Once we have those two numbers, the laws of gravity give us the mass of the object in the middle. This is the most direct and accurate method we have, and it forms the foundation for almost every other technique.
How Do We ‘See’ Stars Orbiting a Black Hole?
This first method, called “stellar dynamics,” is the gold standard for weighing a black hole. It is the most direct proof we have, and it was used to prove without a doubt that a supermassive black hole exists in our own Milky Way galaxy. Our galaxy’s giant black hole is named Sagittarius A* (pronounced “Sagittarius A-star”). It is about 26,000 light years away from Earth, which is very close in cosmic terms. Because it is so close, we can use the world’s most powerful telescopes to see individual stars orbiting it.
For more than 30 years, teams of astronomers have been pointing telescopes at the very center of our galaxy. They have been patiently tracking the movement of a small group of stars, known as the “S-stars,” that are looping around the invisible center. This is incredibly difficult work. These stars are far away, and they are blurred by Earth’s atmosphere. Scientists use a special technology called “adaptive optics,” which shoots a laser into the sky to measure the air’s wobbling. A computer then corrects for this wobble in real time, allowing the telescopes to take pictures that are incredibly sharp.
Over decades, scientists mapped the full orbits of these stars. One star, called S2, completes a full orbit in just 16 years. It loops around the black hole at breathtaking speeds, at one point reaching over 17 million miles per hour (or 7,650 kilometers per second). By tracking this star’s precise path and speed, astronomers calculated the mass of the object it was orbiting. The answer was clear: the object at the center must be 4.3 million times the mass of our Sun. Furthermore, all that mass is crammed into a space smaller than our solar system. Only a black hole could be that massive and that small at the same time. This work was so important it won the Nobel Prize in Physics in 2020.
What If We Cannot See Individual Stars?
The stellar dynamics method is amazing, but it only works for our own galaxy and a few very nearby neighbors. In most galaxies, even giant ones, they are simply too far away for us to see individual stars at their core. All the stars in the center just blur together into a single blob of light. So, how do we weigh black holes in these distant galaxies? We use the exact same principle, but instead of tracking stars, we track the movement of gas.
Many supermassive black holes are surrounded by a massive, spinning pancake of gas and dust called an “accretion disk.” This disk is formed from material that is slowly spiraling into the black hole. As the gas gets closer to the center, it moves faster and faster, just like the stars near Sagittarius A*. We can measure the speed of this gas using a technique called spectroscopy. A spectrograph is an instrument that splits light into its full range of colors, like a rainbow.
When an object moves, the light it gives off changes. You may know this as the Doppler effect, which is why an ambulance siren sounds high pitched as it comes toward you and low pitched as it moves away. The same thing happens with light. Gas moving towards us looks slightly bluer (this is called “blueshift”). Gas moving away from us looks slightly redder (this is called “redshift”). One side of the spinning disk is moving toward Earth, and the other side is moving away. By measuring this redshift and blueshift with a spectrograph, we can calculate the rotation speed of the gas. Telescopes like the Hubble Space Telescope can measure this speed at different distances from the center, giving us a full “rotation curve.” This gives us the speed and the distance, and just like with the stars, we use the laws of gravity to calculate the mass of the black hole.
What Is a ‘Megamaser’ and How Does It Help?
There is a special, high precision version of the gas dynamics method that gives us some of the most accurate black hole weights ever measured. This method uses something called a “megamaser.” A maser is like a laser, but instead of shooting visible light, it shoots out a powerful, focused beam of microwave light. In some rare galaxies, there are clouds of water (H2O) orbiting the central black hole in a thin disk. These water clouds get energized and act as natural, giant masers.
These maser signals are incredibly bright and sharp. The best part is that we can observe them with radio telescopes. By linking radio telescopes all across the globe (a technique called Very Long Baseline Interferometry or VLBI), scientists can create a “virtual telescope” the size of the entire Earth. This gives them vision so sharp it is like being able to read a newspaper in Los Angeles while standing in New York.
With this incredible precision, astronomers can pinpoint the exact location and measure the precise speed of individual maser clouds in the disk. They can map the disk’s rotation with pinpoint accuracy. The classic example is a galaxy called NGC 4258. Scientists were able to track these water masers and found they were orbiting a central object at over 2 million miles per hour (1,000 kilometers per second). The measurements were so good that they proved the gas was following a perfect Keplerian orbit, exactly as the laws of gravity predicted. This allowed them to “weigh” the black hole with amazing confidence. The result was 39 million times the mass of our Sun. This method is considered another “gold standard” because it is so direct and precise, but it only works for the few special galaxies that happen to have these water masers.
How Does ‘Reverberation Mapping’ Weigh a Black Hole?
The methods we have discussed so far work best for “quiet” black holes, like the one in our galaxy. But what about the most extreme and powerful black holes in the universe? These are known as “quasars” or “Active Galactic Nuclei” (AGN). These black holes are “active” because they are in the middle of a feeding frenzy, swallowing enormous amounts of gas and dust. This process makes the center of the galaxy shine brighter than all its billions of stars combined. These objects are so bright we can see them from billions of light years away.
For these black holes, we use a very clever and complex sounding technique called “reverberation mapping.” The name sounds fancy, but the idea is simple: it is all about timing a “light echo.” The process works like this:
- The Flicker: The very center of the accretion disk, right next to the black hole, is extremely hot and chaotic. It doesn’t shine with a steady light; it “flickers” randomly, getting brighter and dimmer all the time.
- The Clouds: Further away from the black hole (perhaps light days or light weeks away) are huge, fast moving clouds of gas. These clouds are too far away to see individually, but we know they are there. This whole area is called the “Broad Line Region.”
- The Echo: When the center of the disk “flickers” and gets brighter, it sends out a flash of light in all directions. Some of this light travels straight to us on Earth. But some of it travels outwards and hits those distant gas clouds.
- The Glow: When the light flash hits the gas clouds, it “excites” the gas and makes the clouds glow brightly for a short time. This glow is the “echo” of the original flash.
Scientists use telescopes to watch this all happen. They measure the light from the central flicker. Then, they wait. It might take 10 days, 30 days, or even a few months, but eventually they will see the “echo” as the Broad Line Region lights up. This time delay, or “time lag,” tells them the distance between the black hole and the gas clouds. If the echo takes 30 days to arrive, they know the clouds must be 30 light days away.
So, now they have the distance. How do they get the speed? They use spectroscopy again. They measure the light from the glowing gas clouds. Because the clouds are orbiting the black hole at high speed, their light is “smeared out” by the Doppler effect. By measuring how much the light is smeared, they can calculate the average speed of the clouds.
Now they have both key ingredients: the distance (from the time delay) and the speed (from the light’s smearing). They plug these two numbers into the gravity formula and get the mass of the black hole. This clever method is our only way to weigh the most distant and active black holes in the early universe.
Is There an ‘Easier’ Way to Estimate the Mass?
All the methods we have covered are “direct” measurements. They are very accurate, but they are also incredibly difficult. They require decades of observation, the most powerful telescopes in the world, or very lucky alignments like a maser disk. Scientists cannot possibly do this for the billions of galaxies in the universe. They needed a faster, “good enough” way to estimate a black hole’s mass. And they found one, in a very surprising place.
Around the year 2000, astronomers discovered a shocking connection: the mass of a supermassive black hole is almost perfectly linked to the properties of its host galaxy. Specifically, the black hole’s mass is related to the “velocity dispersion” of the galaxy’s central bulge. The bulge is the big, round, tightly packed ball of stars at the very center of a spiral galaxy (or in the case of an elliptical galaxy, the entire galaxy is a bulge).
“Velocity dispersion” is just a scientific way of saying how fast and randomly the stars in the bulge are moving. In a small, lightweight bulge, the stars move slowly and calmly. In a giant, massive bulge, the stars are whizzing around chaotically at very high speeds. You can measure this average speed easily, even from far away, just by looking at the combined, “smeared out” light from the bulge. Scientists call this measurement sigma ($\sigma$).
This discovery, known as the “M-sigma relation,” is a powerful shortcut. It shows that if a galaxy has a massive bulge with fast moving stars, it is guaranteed to have a massive black hole. If it has a small bulge with slow moving stars, it will have a small black hole. This relationship is so tight and reliable that if you can measure a galaxy’s bulge sigma (which is much easier than weighing the black hole directly), you can get a very good estimate of the black hole’s mass. This was a revolutionary discovery, as it proves that a galaxy and its black hole must “grow up” together, influencing each other’s size over billions of years.
How Does the Famous Black Hole Picture Help?
In 2019, science changed forever when the Event Horizon Telescope (EHT) collaboration released the first ever picture of a black hole. We finally “saw” the unseeable. The image was of the supermassive black hole at the center of a giant galaxy called Messier 87 (M87), which is 55 million light years away. In 2022, they released the second picture: our own Sagittarius A*. These pictures show a bright, glowing ring of orange light surrounding a perfect circle of pure blackness. That black circle is the “black hole shadow.”
These incredible images provide a brand new, completely independent way to weigh a black hole. Albert Einstein’s Theory of General Relativity, which is our modern theory of gravity, makes a very precise prediction. It says that the size of a black hole’s shadow is directly proportional to its mass. A more massive black hole will have a larger shadow.
The EHT is a global network of radio telescopes that work together as one planet sized observatory, giving it the power to measure the size of this shadow on the sky. Once they measured the size of M87’s shadow, they just had to look at Einstein’s equations. The equations told them exactly how massive a black hole would need to be to create a shadow of that exact size.
The answer they got was 6.5 billion times the mass of our Sun. This was an amazing moment for science. Years before, astronomers had used the gas dynamics method (measuring the spinning gas disk) to weigh M87’s black hole. Their answer was in the same ballpark. The EHT’s new measurement, using a completely different method based on Einstein’s theory, confirmed the old one. It was like weighing a person on a bathroom scale, getting a number, and then weighing them on a different, high tech scale and getting the exact same answer. It proved that both methods work and that our understanding of gravity is correct, even in the most extreme place in the universe.
Conclusion
Weighing an invisible object millions of light years away sounds like science fiction, but it is a real and active part of modern astronomy. We cannot put a supermassive black hole on a scale, so we use the next best thing: we use gravity as our scale. By patiently watching the universe, scientists can measure the dance of stars and gas caught in the grip of these cosmic giants.
Whether it is by tracking a single star’s 30 year journey, measuring the Doppler shift of a spinning gas disk, timing the precise echo of a light flash, or even just measuring the size of the shadow it casts, each method gives us a piece of the puzzle. These different techniques all work together, allowing us to be confident in our answers. Each time we weigh a black hole, we learn more about how these monsters formed and how they grew to become the powerful engines that shape the galaxies we see today. What do you think these cosmic scales will teach us next about the history of our universe?
FAQs – People Also Ask
What is a supermassive black hole?
A supermassive black hole (SMBH) is the largest type of black hole, with a mass that is millions to billions of times greater than our Sun. They are found at the center of almost every large galaxy, including our own Milky Way.
How massive is the black hole in our Milky Way?
The supermassive black hole at the center of our Milky Way galaxy is named Sagittarius A*. Based on the orbits of stars around it, scientists have calculated its mass to be about 4.3 million times the mass of our Sun.
What is the most massive black hole ever found?
One of the most massive black holes discovered so far is TON 618, which is estimated to have a mass of around 66 billion times the mass of our Sun. There are other candidates, like the Phoenix Cluster black hole, that may be even larger, but their masses are less certain.
Can a black hole’s mass change over time?
Yes, a black hole’s mass can and does change. It grows by “eating” or accreting material from its surroundings, such as gas, dust, and even entire stars. They can also grow by merging with other black holes.
Why is it important to know a black hole’s mass?
A black hole’s mass is its most fundamental property. Knowing its mass helps us understand how the black hole formed, how it affects the stars and gas in its host galaxy, and how galaxies and their central black holes grow and evolve together over billions of years.
What is the M-sigma relation in simple terms?
The M-sigma relation is a surprisingly tight connection between a black hole’s mass (M) and the average speed of stars in its galaxy’s central bulge (sigma, or $\sigma$). It means that galaxies with bigger, faster bulges always have bigger black holes, which allows us to estimate a black hole’s mass by just measuring its galaxy.
Does every galaxy have a supermassive black hole?
Scientists believe that almost every large galaxy, like spiral galaxies (e.g., Milky Way) and elliptical galaxies, has a supermassive black hole at its center. Smaller dwarf galaxies may not, or they may have smaller “intermediate-mass” black holes.
How is reverberation mapping different from stellar dynamics?
Stellar dynamics is a direct method where we physically watch the orbit of individual stars over many years to find the central mass. Reverberation mapping is an indirect method for very distant, active black holes, where we measure the time delay (or echo) of light between the center and gas clouds to find the distance.
What telescope took the picture of the black hole?
The first-ever picture of a black hole’s shadow was taken by the Event Horizon Telescope (EHT). The EHT is not a single telescope but a global network of radio telescopes on different continents, all working together to create one “virtual” telescope the size of Earth.
Can we weigh smaller, stellar-mass black holes the same way?
Yes, but on a smaller scale. Stellar-mass black holes (which are 5 to 100 times the Sun’s mass) are weighed by watching their “companion star.” If a black hole is in orbit with a normal star, we can measure the star’s wobble and orbit to calculate the mass of its invisible black hole partner.