Black holes are unimaginably dense objects in space from which nothing – even light can escape. The more a black hole eats, the more it grows in both mass and size. But intriguingly, the most elusive class of black holes is not the largest or the smallest, but the intermediate category and NASA now intends to learn more.
1-100 solar masses = stellar mass
100 to 100,000 solar masses = intermediate-mass black hole
100,000 to billions of solar masses = supermassive black hole
The smallest black holes are called “stellar mass,” boasting between one and 100 times the mass of the Sun.
Stellar mass form when stars explode in violent processes called supernovae.
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Supermassive black holes, on the other hand, are the central anchors of large galaxies
For example, our Sun and all other stars in the Milky Way orbit a black hole called Sagittarius A*, weighing about 4.1 million solar masses.
An even heavier black hole — at a whopping 6.5 billion solar masses — serves as the centre-piece for the galaxy Messier 87 (M87).
M87’s supermassive black hole appears in the famous image from the Event Horizon Telescope, showing a black hole and its “shadow” for the very first time.
Space scientists, know surprisingly little about the intermediate range of black hole, spurring US space agency NASA to search for them more intently.
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What about intermediate black holes?
Scientists are now scrambling to use NASA’s powerful space telescopes to track down far-flung objects fitting the description of these intermediate black holes.
They have found dozens of possible candidates, and are working toward confirming them as black holes.
But once they are found, this opens a whole new mystery: How do intermediate-mass black holes form?
Fiona Harrison, Caltech professor of physics and NASA’s NuSTAR mission. principal investigator, said: “What is fascinating, and why people have spent so much time trying to find these intermediate-mass black holes, is because it sheds light on processes that happened in the early Universe – what were the masses of relic black holes, or new formation mechanisms for black holes that we haven’t thought of yet.”
Searching for black holes of intermediate-mass is difficult because they emit no light.
However, scientists can look for associated signs using sophisticated telescopes and cutting-edge instruments.
For example, because the flow of matter onto a black hole is not constant, the manner in which material is consumed causes certain variations in light output in the environment.
These changes takes place quicker in smaller black holes than larger versions.
Igor Chilingarian, an astrophysicist at the Smithsonian Astrophysical Observatory and Moscow State University, said: “On a timescale of hours, you can do the observational campaign that for classical active galactic nuclei takes months.
The most promising intermediate-mass black hole candidate is called HLX-1, with a mass of about 20,000 times the Sun’s.