September 18, 2025
5 Min read
How do you weigh a black hole?
Presembllate the mass of a black hole is delicate, but astronomers have designed several methods to measure the weight of these galactic glutons
This junk of shredded pixels can look like a thunderbolt of rainbow color, but it is actually a spectrum of light collected by the Hubble space telescope which shows the swirling movement of gas and stars in the heart of the neighboring Galaxy M84. Blue (LEFT) and red (RIGHT) The parts of the spectrum show movements towards and far from an observer respectively; Carefully measure these movements allowed astronomers to weigh the central supermassif black hole of M84.
NASA, Gary Bower, Richard Green (Noao), the Stis Instruments Definition team
In a recent room for my chronicle The Universe, I wrote on the biggest black holes in the cosmos. These can tip the balance to several billion times the mass of the sun, even exceeding whole galaxies.
But how do we know that? Black holes are rather famous for being, well, black Because they can even encompass the light itself. So how can we understand how massive they are?
There are several ways, in fact, mainly depending on the type of black hole that we examine.
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The most common black hole type we know is a black hole of stellar mass; One with a few to a few dozen times the mass of the sun. This type is generally formed when a massive star explodes at the ends of its short lifespan and its nucleus collapses. The infallible material becomes so dense that gravity rises from the uphill, becoming so strong that nothing, not even the photons, can escape its reach after being too close. No light comes out of this object, so it is black, And all that falls cannot come out, like an infinitely deep hole.
Named such an object is not very difficult.
Because they are forged with stellar nuclei, black holes must have a mass close to that of a star. The theoretical calculations support it, giving a minimum mass of about three times that of the sun. There is no clear upper limit, but astronomers consider that black holes of less than 100 solar masses are in the category of black holes of stellar mass.
Of course, measuring this mass in fact is difficult for an object that you cannot 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, it can remain linked to her stellar companion, which can betray his presence. We could, for example, seek stars that seem to be in orbit a massive but invisible object. This method is difficult but in fact managed to find several black holes.
Imagine seeing one of these essentially “before” systems with regard to their co-organ movement. As the two turn, the visible star spends half of this orbital period moving towards the earth and the other half moving away. This induces a doppler offset in its light, shortening the wavelengths when it goes to us – a so -called blueshift – and lengthening them as a “red to the red” when it moves away.
This is the key to finding the mass of the black hole! Using the laws of movement derived by the German astronomer Johannes Kepler at the beginning of the 17th century, the total mass of the system can be calculated by simply knowing the orbital period and the stellar speed. We can estimate the mass of the visible star using our understanding of stellar physics and subtract this from the total to weigh the black hole.
This method works even if the system is too far from us to see the star move physically. Binaries like this can also be found in other respects: for example, if the black hole steals the material of its stellar companion, this material heats up as much as it falls into the mouth of the black hole that it emits high energy x rays. If we see hearty x -rays from what seems to be a normal star, we can be suspect that a black hole works there. The very first confirmed black hole, Cygnus X-1, was found exactly in this way, and the Doppler method revealed that its mass was about 21 times that of the sun.
A variation in this method can also be used on supermassive black holes. These objects are absolute beasts, millions or billions of times the mass of the sun, and are in the cores of all the large galaxies. They are far too strong to orbit a single star, but in fact, many stars can orbit them. The closer these stars of this monster in the middle, the more they have it surrounded quickly. Each of these stars will have a large change of doppler in their light, with about half moving towards us and moving away halfway. Measuring them en masse, we will see this characteristic duality between Blueshift and Redshift in their combined light. Again, the speeds of these stars depend on the mass of the object they orbit, so we can use it to weigh the black hole.
On the basis of almost 20 years of observations by the very large telescope of the Southern European Observatory in Chile, this accelerated video shows stars in orbit around Sagittarius A *, the black supermassive hole in the heart of the Milky Way.
Astronomers have done so with many galaxies, thanks to instruments such as the imaging spectrograph of the space telescope (STI), a camera on which I worked which is on board the Hubble space telescope. STI has been partly designed to make this kind of observation. Shortly after the astronauts installed it on Hubble, the STIs watched the Galaxy M84 nearby and easily detected a huge doppler offset around the galactic nucleus, the Santant to a central black hole with at least 300 million solar masses – a giant indeed.
The current models of formation of the galaxy indicate that the mass of a galaxy is linked to the mass of its central black hole, with larger galaxies that tend to have a larger central black hole. It is not a hard and fast rule – our own Milky Way extended to a relatively undersized black hole, for example – but if you measure enough galaxies, the pattern becomes clear. Although this trend does not give you an extremely precise measurement, it can be used to assess the central mass of the central black hole of a galaxy. More than a dozen black mass holes elusive – from 100 to 100,000 solar masses – may have been found in dwarf galaxies in this way.
There are also many more indirect methods. The X -rays emitted by the material because it falls into a black hole can be used to estimate its mass, for example. Sometimes, in chaos crowded in a galactic center, a star can wander too close to the central supermassive black hole and shred itself with its powerful tide force. This tide disturbance event creates a really immense explosion, and the time it takes to brighten up and fade is linked to the mass of the black hole, which in turn can be used to estimate the tonnage of the monster.
In addition, when the black holes collide and merge, they give off an astounding amount of energy in the form of gravitational waves – strikes them in the fabric of space -time. Ecoded in these undulations is the mass of the two convergent black holes, as well as that of the last final black hole, a little larger. The first gravitational waves were detected a decade ago and to date, about 300 others have been found. Current detectors can only detect black stellar holes mergers, but future space observatories such as Lisa (Laser interferometer space antenna) should also be able to “hear” the waves of black holes in Supermassive collision.
The black holes themselves do not emit light, but that does not mean that they are entirely invisible. They prove to be multitude of ways, and if we are intelligent – and we are – we can use it by taking the measurement.
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