What Colors are Black Holes?

Black holes. From the name itself, we assume that these are black circles in space that are able to suck in light due to their extremely strong gravity. But because of this, it is extremely hard to capture direct images of black holes, but new research has allowed for new methods to detect black holes.

While they differ in size, the largest black holes are often at the center of our galaxy. There are even some supermassive black holes that have a mass of up to a billion suns. Like at the center of the Milky Way galaxy called Sagittarius A*, which isn’t very active, but is still massive. And what is even crazier is that these supermassive black holes are still expanding.

And not all black holes are black. As more material such as gas, dust, stars, or other celestial objects get sucked in by the extreme gravitational force, the black hole heats up and becomes incredibly bright. Bright black holes have the ability to span the entire electromagnetic spectrum, from X-rays to radio waves, and can shine trillions of times brighter than our Sun.

Matter Swirling into
Supermassive Black Hole M87

Some bright black holes, also known as active galactic nuclei (AGN), spew out matter within a jet-like fashion that has the ability to travel millions of miles through space. The ejected matter can be viewed through radio telescopes. Astronomers worry about the impact that AGNs have on their respective galaxies because of their pure power. Many speculate that more galaxies are not able to produce large enough stars due to AGNs.

AGNs can be very hard to detect. Factors that affect its difficulty to detect black holes include what the black hole is “consuming”, which galaxy it is in, and even the angle we view the black hole at. Even using different telescopes produce different results, with X-ray telescopes often being better at detecting AGNs than radio telescopes. Radio telescopes are not able to detect AGN jets because they are not visible in the radio spectrum.

Because of this, astronomers look at spectral energy distribution, which is the relative brightness of a galaxy across different parts of the electromagnetic spectrum. They can be used to measure how many stars are in a certain galaxy, how old they are, and even what they are made out of. This technique is also used to spot AGNs, which allows us to not only measure the properties of black holes but the brightness of them as well.

But it is not the easiest thing to do. Starlight and light from AGNs are very similar and very hard to tell apart, so it is easy for astronomers to confuse the two. And many are still divided about the nature of black holes, so we need to shed some more light to learn about them before the black holes suck that light up.

Black Hole Appearance in Different Wavelengths

Sources:

Bellstedt, Sabine, and Jessica Thorne published. “Some Black Holes Are Anything but Black – and We’ve Found More than 75,000 of the Brightest Ones.” Space.Com, 4 Jan. 2022, https://www.space.com/black-holes-anything-but-black-75000-brightest-ones.