The stereotypical black hole is a monster, millions to billions of times the mass of our sun, swallowing anything that comes near it.
However, black holes come in three main sizes - those supermassive black holes that we picture as monsters, the baby stellar mass black holes, and the intermediate black holes - the black hole teenagers.
And now, we have some of the best evidence to-date of the intermediate black holes - and it is relatively close to earth.
A star smashes elements together (nuclear fusion) to create heat and energy. These fused atoms add heavier elements in its core. To support this mass, it burns more fuel, creating more mass.
Eventually, the star can't fuse elements fast enough and it collapses. In doing so, part of the star explodes as a supernova. However, most of the mass of the star comes crashing down, being squeezed into a tiny object, creating a black hole.
These stellar mass black holes are the baby black holes about 10 to 60 times the mass of our sun. There could be tens of millions of these small black holes in our Milky Way galaxy.
For the past decade, gravitational wave experiments use super-sensitive lasers and mirrors, have been searching for gravitational waves - the result of when these stellar mass black holes collide, producing ripples or waves.
There are also the supermassive black holes. They reside in the centres of big galaxies, and can weigh millions to billions of times the mass of the Sun. In our Milky Way, we have Sagittarius A*, weighing about 4 million times the mass of the sun.
We can see these by the stars and gas they swallow and affect, the radiation they emit, and even now directly by imaging their silhouette with the Event Horizon Telescope. An outstanding question though has been how we go from stellar mass black holes to the supermassive ones.
There are intermediate mass black holes, which weigh between 100 and 100,000 solar masses. There are less of these than the stellar mass black holes, and most likely more often further way from earth, so we cannot pick them up via gravitational waves. They are too small to see their effect in the centre of a galaxy like the big ones.
Now, using the Hubble Space Telescope, astronomers have spotted the clearest evidence of these teenager, intermediate mass black holes, in Omega Centauri - a globular cluster in the Milky Way.
They found some stars that were moving so fast they should have been ejected or left the globular cluster - an object containing tens of millions of stars. However, they did not, and came back.
For these stars to move fast and come back, there must be a concentrated amount of mass, like a black hole, causing this, weighing about 8,200 times the mass of our sun.
Being in Omega Centauri, it is relatively close - only 17,000 light years away. You can see it as a faint dot in the sky - forming a triangle from the left part of the southern cross and the second of the two pointer stars.
Only a few potential intermediate mass black holes have been found thus far. Just like teenagers, they are elusive and hard to find, but an important step in understanding how black holes grow.
- Brad Tucker is an astrophysicist and cosmologist at Mt Stromlo Observatory and the NationalCentre for the Public Awareness of Science at the ANU.
