There is a common misconception about black holes. Many are led to believe that black holes are infinitely heavy and therefore have infinite gravity. This is not true. While black holes are extremely heavy, they do not have an infinite mass or infinite gravity.
While their gravity is very strong it is dependent on how massive the black hole is. Like everything else in the Universe, black holes have a range of masses. There are 4 types of black holes: miniature, stellar, intermediate, and supermassive.
Miniature black holes weigh less than 3 times the mass of our Sun. These black holes have the lowest gravitational pull. On the other end of the scale, we have supermassive black holes, weighing anywhere between millions to billions of times the mass of our Sun.
Table of Contents
Can We See Black Holes?
As many of you may know, a black hole gets its name because we can’t see it. The gravitational force caused by a black hole is so strong that not even light can escape its power. So, if we can’t see them, how do we know they are there? Because this gravitational pull is so strong, it can affect stars and planets near the black hole.
By observing how these objects move and with lots of fancy math, astronomers can deduce there is a black hole and where it is. We can never actually see the black hole itself. However, if the black hole has a strong enough gravitational pull it will cause the gas and dust around it to spin so fast that they heat up and begin to glow.
This is what we see, an orange glowing mass with a black hole in the middle. Most of the photos of a black hole that you see are actually artist conceptions of what we think they look like. The world’s first picture of a black hole was captured by the Event Horizon Telescope in 2019. This image is of the supermassive black hole at the center of our own Milky Way galaxy.
Astronomers do not know why these black holes are so massive, but they believe it could be linked to their presence at the center of their host galaxy. It is assumed that in the center of every massive galaxy there resides a supermassive black hole.
What Makes Supermassive Black Holes Special?
Supermassive black holes stand out amongst other black holes because they are so large, they may affect their entire host galaxy. Astronomers see a strong relationship between a host galaxy’s stars and the mass of its central supermassive black hole.
Although, the physical mechanisms as to why there is this relationship are not yet understood. A supermassive black hole that is actively accreting material from its host galaxy is categorized as an active galactic nucleus. Essentially all massive galaxies host a supermassive black hole, but not all are visible as an active galactic nucleus.
Star forming galaxies lack a discernable active galactic nucleus, and while they are presumably still hosting a central supermassive black hole, star formation is the main source of energy being emitted, therefore they are identified as star forming galaxies.
It is still unclear how or when in a galaxy’s evolution these active galactic nuclei become activated. Some active galactic nuclei can be triggered during the merging of two galaxies, but for non-merging galaxies, active galactic nucleus activation is still an open question.
A very powerful active galactic nucleus may kill off star formation in its host galaxy through energetic winds or radiation, blowing the dust used to form stars out of the galaxy entirely. Some of the most luminous active galactic nuclei are found in galaxies with highly depleted star formation rates.
How Do We Detect if A Supermassive Black Hole is Part of An Active Galactic Nucleus?
Active galactic nuclei emission can be detected in a large range of wavelengths of light, from radio to gamma-rays. Different components of the active galactic nucleus release different types of light.
The supermassive black hole itself does not release any light. Around the supermassive black hole there is an accretion disk, we see optical and ultraviolet measurements from here. Around the accretion disk is a corona, this is where we see X-ray emission. Surrounding all of this is a dusty torus.
This torus reemits light in the form of infrared radiation. Occasionally, if the active galactic nucleus is strong enough, it will shoot out jets. We can detect the jets in the ultraviolet or in radio. The two most common wavelengths of light astronomers use to detect active galactic nuclei are X-rays and infrared.
Active galactic nuclei are usually luminous infrared sources. However, so are dusty star forming galaxies. Dust can be made to emit energy by an active galactic nucleus or by star formation. Because of this, special techniques are used to determine if this infrared emission is coming from the active galactic nuclei or from star formation in other parts of the galaxy.
X-rays are great for detecting active galactic nuclei because usually, active galactic nuclei are up to 5 times brighter in the X-ray than star forming galaxies. This makes them easy to distinguish. However, if there is a lot of dust in the galaxy it can absorb the X-rays before they reach our telescope, and we won’t detect them.
A common problem in astronomy is that there are galaxies with infrared detections indicative of an active galactic nuclei, but astronomers do not see a matching X-ray measurement. To determine if there really is an active galactic nucleus present, and it’s just hidden by dust, astronomers must use more complex X-ray detection methods.
Combining these special X-ray methods and more advanced infrared methods, astronomers can find hidden active galactic nuclei. Studying these hidden active galactic nuclei along side very luminous active galactic nuclei can help astronomers build a picture of how active galactic nuclei evolve in and with their host galaxies.
Though astronomers have made great strides in understanding these mysterious objects there is still much work to be done to understand how these black holes evolve and how they affect our Universe.
References
- https://www.scientificamerican.com/gallery/the-smallest-known-black-hole/#:~:text=Link%20copied!-,NASA%20scientists%20have%20identified%20the%20lightest%20black%20hole%20yet%2C%20just,in%20at%206.3%20solar%20masses.
- https://www.nationalgeographic.com/science/article/first-picture-black-hole-revealed-m87-event-horizon-telescope-astrophysics
- https://eventhorizontelescope.org/blog/astronomers-reveal-first-image-black-hole-heart-our-galaxy
- https://iopscience.iop.org/article/10.3847/1538-3881/ac0530/pdf
- Silk, J., & Rees, M. J. 1998, A&A, 331, L1. https://arxiv.org/abs/astro-ph/9801013
- Ferrarese, L., & Merritt, D. 2000, ApJL, 539, L9, doi: 10.1086/312838
- Gebhardt, K., Bender, R., Bower, G., et al. 2000, ApJL, 539, L13, doi: 10.1086/312840
- Treister, E., & Urry, C. M. 2012, Advances in Astronomy, 2012, 516193, doi: 10.1155/2012/516193
- Kormendy, J., & Ho, L. C. 2013, arXiv e-prints, arXiv:1308.6483. https://arxiv.org/abs/1308.6483
- King, A. 2025, ApJL, 596, L27, doi: 10.1086/379143
Sun, M., Trump, J. R., Brandt, W. N., et al. 2015, ApJ, 802, 14, doi: 10.1088/0004-637X/802/1/14
