How do you figure out the mass of something that you can't even see? That's the problem astronomers are faced with when calculating the mass of a black hole. Black holes have such an intense gravitational pull that not even light can escape them. That means astronomers have to use indirect ways to observe and measure them.
Now a team of astronomers has figured out the most precise way yet to calculate the mass of a supermassive black hole. Their results are published in the Astrophysical Journal Letters.
The black hole they studied sits at the center of a galaxy about 73 million light-years away from us. The team used the ALMA telescope in Chile to figure out the black hole is 660 million times the mass of our sun. To put that in perspective, the sun's mass is about 1,989,100,000,000,000,000,000 billion kilograms, or about 333,000 times the mass of Earth. The team's calculation has a measurement uncertainty of just 10%.
They got such an accurate reading by measuring the speed of gas and dust swirling around inside the black hole's gravitational pull.
"For a precise measurement, we need to zoom in to the very center of a galaxy where the black hole's gravitational pull is the dominant force," Aaron Barth, lead author on the new study, said in a statement.
The cold, gas and dust the astronomers measured was swirling around at a blistering 500 kilometers per second.
Usually astronomers calculate the mass of a black hole by measuring hotter, ionized gas that glows in visible light. Ionized gas is much more turbulent and harder to measure, which leads to a less precise black hole mass. For this study, astronomers used colder, more stable gas that can only be seen with a radio telescope like ALMA.
The team is planning to try out the technique on more black holes soon. More accurate mass measurements of black holes might help us figure out how they form at the center of galaxies.