What are black holes?
Every time someone mentions black holes, we’re left stunned by how little we know about them. Are they dangerous? How does one look like? Is it big enough to wipe out our planet?
In science terms, a black hole is a region of spacetime where gravity is enormously strong. How strong? Well, literally nothing can escape from it – not even particles or electromagnetic radiation, also known as light. The theory of general relativity somehow predicts that a sufficiently compact mass can deform spacetime in order to form a black hole.
So basically, a black hole is an area of space with a gravitational field that nothing can escape from it. Scary? A lot!
And since no light can pass through it, the black holes appear to be black. In some strange cases, black holes are former massive stars that have been crushed down to an extreme density during a particular supernova explosion.
Fun fact – Albert Einstein was the very first to suggest that our never-ending universe contains such dense, massive objects. This theory emerges from Einstein’s equation of general relativity, as a natural consequence of the death of a massive star.
What happens when two black holes collide?
When two black holes collide (usually from spiraling around each other) they send out gravitational waves. These ripples in space and time can be detected only via extremely sensitive instruments used on our planet Earth. Since black holes and their mergers are completely dark, these events are pretty invisible to telescopes and other similar light-detecting instruments that are used by astronomers.
However, theorists did come up with ideas about how a black hole merger produces light signals by causing nearby material to radiate.
Scientists have been using Caltech’s Zwicky Transient Facility (ZTF), located near San Diego, and they have spotted what might be a similar scenario described above. If it’s confirmed, it might just be the first known light flare from a pair of colliding black holes.
The merger was identified on the 21st of May of last year, using two gravitational wave detectors the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO) and the European Virgo detector – in an event called GW190521g.
That detection enabled the ZTF scientist to look for light signals from the location where the signal originated from. These wave detectors also spotted mergers between the very dense cosmic objects called neutron stars, and the astronomers identified light emissions from those collisions.
“This detection is extremely exciting,” said Daniel Stern, coauthor of the new study and an astrophysicist at NASA’s Jet Propulsion Laboratory in Southern California, which is a member of Caltech. “There’s a lot we can learn about these two merging black holes and the environment they were in based on this signal that they sort of inadvertently created. So the detection by ZTF, coupled with what we can learn from the gravitational waves, opens up a new avenue to study both black hole mergers and these disks around supermassive black holes.”
To learn more, read the press release from Caltech.
How are black holes created?
Black holes are created when a massive star reaches its end and implodes – collapsing in on itself.
Interestingly enough, a black hole takes up zero space but has mass – most of it from the fact that it used to be a star. Black holes are always expanding, always getting bigger (more massive, so to speak) as they consumers more and more matter near them. The bigger they get, the larger the zone of “no return” they have. Basically, everything that enters their territory is irrevocably lost to the black hole.
The point of no return on black holes is also known as the event horizon.
Eventually, by expanding and consuming (planets, stars, spaceships, other black holes – you name it) it’s believed that they evolve into the supermassive black holes that they detect at the very center of most of the major galaxies.
But there’s a twist. Two twists, to be precise.
The first twist is that it would actually take longer than the universe’s current age for a black hole to grow to a galaxy-center-sized black hole. So, it’s believed that the universe might have just jumpstarted the whole process by creating a giant primordial black holes in the moment of the Big Bang. But, it’s not as simple as we’ve put it.
The second twist is that there’s almost 1% evidence of so-called intermediate-mass black holes (the ones in between star-sized and galaxy-sized). it’s expected to see some black holes in this middle phase, which is their journey to becoming supermassive but not quite there yet.
How big are they?
About 100 to 100,00 solar masses. Too complicated? 1 solar mass is equal to 4.385e+30. So, pretty big.
There are three types of known black holes:
- Small black holes
- Intermediate-mass black holes
- Supermassive black holes
It’s extremely difficult for scientists to discover black holes – really really difficult. This is due to the fact that, unlike stars, the light that falls into the black hole will never be seen again. However, sometimes black holes have an accretion disk, which is a halo of material around the black hole that glows. The light emitted from this halo makes it possible for scientists to find this, otherwise invisible, objects.
The smallest black hole is called GRO J1655-40. The visible companion star in this system is constantly dumping gas onto the black hole, which generates enough energy to power a microquasar. To give a better view, Quasars develop an extremely luminous active galactic nuclei, which are the centers of galaxies hosting supermassive black holes.
Just between small and supermassive black holes are the intermediate-mass black holes. They’re a long-sought “missing-ling” in the black hole evolution. Only a handful of these black holes candidates have been found to this date. One of them was found by the Hubble Space Telescope just earlier this year. These objects are even more difficult to find than the usual ones. That’s because they tend to be less active due to the fact that they lack the nearby “fuel” to gobble up. The recent intermediate-mass black hole is nearly 50,000 times the mass of the Sun – that’s how big intermediate-mass black holes are.
Sagittarius A*, located 26,000 light-years from the Sun (our galaxy’s central black hole) has a radius of some 17 times that of the Sun. It could very well sit within Mercury’s orbit. It weighs about 4 million solar masses. Despite its mass, the Milky Way’s black hole is very small compared to the other supermassive black holes lurking at the center of our galaxies. The most massive supermassive black hole to date is the Abell 85 galaxy cluster. It’s estimated to be 2 trillion solar masses. The very center of this galaxy is almost as large as the Large Magellanic Cloud – a radius of 7,000 light-years.
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