Mysteries about black holes that scientists have not been able to explain

Black holes are a scientific mystery – there is so much still to discover. The first image of a black hole was captured in April 2019. Much of what we know about black holes is theory or based on observations of objects near or behind a black hole. This is just a summary of black holes. It’s a subject too deep to cover in one article!

How are black holes formed?

A black hole comes from the death of a large star (at least 10 times bigger than our Sun) exploding at the end of its life in a supernova. The Sun, being too small, won’t ever become a black hole, it will expand, contract and cool off in its death process.

The constant fusion of hydrogen to helium creates the energy and radiation from a star. A star is in a stable state for most of its life as the energy pushing out from the star balances with the gravitational force pulling in.

At the end of a star’s life, stars like our Sun will continue fusing elements together like helium to carbon, carbon to neon, but not much further. Large stars will continue fusing elements until the star reaches iron. Iron is a very stable element, and gravity alone cannot compress it further. Iron builds up in the core, and the internal pressure of energy radiating outwards becomes out of balance with the pressure of gravity pulling inwards. The outer layers of the star are no longer supported by the radiation pressure of nuclear fusion, and the star’s gravity pulls the outer layers into the core. When the incompressible core connects with the outer layers, a shockwave is sent through the densely packed star, which results in the fusion of other elements on the periodic table after iron.

Now the energy being released overwhelms the pressure of gravity, and the collapsing star explodes in a supernova, the largest explosion known. The lighter outer layers are flung off into space, and the remaining core can create a black hole. A black hole has so much mass tightly packed into a small space that, close up, its gravity is so strong that nothing nearby can escape it. To get away from a black hole, you’d have to travel faster than the speed of light, which isn’t possible.

Cygnus X-1 black hole pulling matter from the nearby blue star.
Rights: NASA/CXC/M.Weiss

Cygnus X-1

The Cygnus X-1 black hole formed when a large star caved in. It is pulling matter from the nearby blue star.

How do you observe black holes?

Astronomers observe black holes by watching the light from stars in the background warp as the gravity of the black hole pulls on the light. They also observe stars as they cross the ‘event horizon’ (the point of no return) and the radiation emitting from the black hole. But not everything gets pulled into the black hole. There is an orbital pattern to objects near some black holes. They get close to the black hole and then are ‘flung’ out again – as shown in this YouTube video.

The ‘black’ part of the black hole is the event horizon. If an object breaches the event horizon and approaches the singularity it will become ‘spaghettified’ – stretched and pulled apart by the black hole’s gravitational forces. Scientists think that in the middle of the black hole is a ‘singularity’. It’s at this point in the black hole discussion that classical physics principles can no longer be applied (it stops making sense in this context) and quantum mechanics takes over. The theory is that the singularity is an infinitely small point where gravity and density are also infinite. The black hole is packed with all the heavy elements from the star but in a much smaller space. Imagine the mass of a star 10 times the size of our Sun compressed into something the size of a city.

Creating the first photograph of a black hole

Black holes are fascinating because there is so much we don’t know. It’s an area ripe for investigation, and NASA is doing just that. There is a NASA campaign under way that aims to understand black holes further. From 5–14 April, astronomers used a network of radio telescopes to look at the gigantic Sagittarius A* black hole located at the centre of our galaxy. These telescopes were all pointing towards Sagittarius A* and worked together to create the first photo of a black hole. The data from the radio telescopes will be converted into an image. At the time of writing this article, the photo had not been released.

Sagittarius A* is the black hole at the centre of the Milky Way/
Rights: NASA/UMass/ D.Wang et all., IR: NASA/STScl

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