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Skip to 0 minutes and 7 secondsWhat's inside a black hole? Black holes are like gigantic vacuum cleaners, the end point in the lives of really big stars. Before black holes were ever discovered, people thought about the possibility of very heavy stars squashed into a relatively small volume, such that their gravity so strong, not even light can escape their grip. James Bradley was an Astronomer Royal here at the Observatory in Greenwich. And one of the amazing things he did was to work out an accurate value for the speed of light. This value was needed to calculate how heavy a star must be to pull light in and swallow it. 300 years later, one of the astronomers at the Observatory would like to investigate a real black hole.

Skip to 0 minutes and 53 secondsLiz would like to know what would happen to her if she approached one, and would particularly like to know what could be inside. The trick of it is finding one to begin with. The black hole itself is invisible. Instead astronomers look for the powerful effects they exert on nearby matter. Sometimes this is seen as material swirling around a greedy black hole, but those without a cosmic buffet close by are harder to spot. However, if we look towards the centre of our Milky Way, we see lots of stars circling an invisible object. These stars have similar orbits to the planets in our solar system, which helps us work out that there is something in the middle. A supermassive black hole.

Skip to 1 minute and 34 secondsThe one in our galaxy is nearly a quarter of a million trillion kilometres away, so we are in no danger of being pulled in. So what would happen to Liz if she approached a black hole? Well, near the boundary, called the event horizon, she would feel the effects of the strong gravitational forces from this collapsed star. At the point of the event horizon, Liz would have to travel at the speed of light to escape the gravity of the black hole. And this is impossible for her or any object to achieve. Also, the strong gravitational field would slow down time.

Skip to 2 minutes and 9 secondsIf we could see Liz's watch from our perspective here on Earth, it would appear to us to tick a lot slower than our watches, although time for her would pass by normally. Eventually, after a few years set outside the event horizon of the black hole, Liz would be shocked to know that thousands of years may have passed back here on Earth. It's as if the black hole is a time travelling machine allowing Liz to travel into the future. Also, from our perspective, we would see her become redder and redder until she disappeared. The strong gravity would stretch the light to longer wavelength low energy light that we can't see with our eyes.

Skip to 2 minutes and 47 secondsThis would happen before she crossed the event horizon. But what would happen to Liz if she stepped in? Well, if she fell in feet first, her feet would be pulled further along than her head, so she have to endure spaghettification. She would be ripped apart and pulled into the centre, the singularity, an exotic region of zero volume and infinite gravity. Or is it? Liz wisely decides not to go in, but thinks about the possibilities for what she would find inside. Some physicists believe if Liz jumped in there would be no sign of her presence inside the black hole, apart from a slight increase in its mass.

Skip to 3 minutes and 25 secondsThe cosmologist Stephen Hawking, believes that particles can tumble out, so that eventually, over time black holes evaporate. We may not ever know for sure if we can ever retrieve information from a black hole. Some physicists think that the event horizon acts as a veil blocking our view of the weird physics happening inside. Some theorise that the fabric of space is pictured at the singularity. American physicist Lee Smolin, suggests that time ends inside a black hole and begins with the start of new universe forever hidden from our view. A popular theory for the nature of a black hole brings in the idea of a ball of vibrating strings. These strings would keep information about the contents.

Skip to 4 minutes and 9 secondsLiz's atoms would be shredded into their constituent strings and would merge with the existing orchestra inside. It's Liz, but not as we know her. We might not ever know for sure what's inside a black hole, but it is fun thinking about what could be happening. And who knows, Liz may end up in a whole other universe with its very own Greenwich Observatory.

The big unknowns

We get asked about black holes by children on an almost daily basis - they are very aware of their existence and are incredibly curious about these elusive dark objects. Here are some of our answers to the most popular questions:

What are black holes?

Stars much bigger and more massive than the Sun eventually explode when they run out of fuel. This is called a supernova and it’s brighter than a whole galaxy of hundreds of billions of stars! The leftover star is very small and compact - it is still heavier than the Sun but could fit into the UK (imagine that - one million Earths could fit inside the Sun and this dead star could fit into our country!).

Some continue to glow and give off a beam of particles, similar to a lighthouse and its spinning beams of light. They are so dense that if you took a sugarcube of material from these stars it would weigh the same as Mount Everest!

Other dead stars become so dense that their gravity is strong enough to swallow up light and anything else that falls in. These are black holes.

What would happen if I got close to one?

If you approached a black hole you would feel the incredibly strong gravitational pull - your feet going in would be pulled more than your head and you would be stretched, scientists call this effect spaghettification!

Then you would be pulled in to the centre and squashed into little particles to join everything else that fell in to this cosmic death machine.

However if you sat outside the boundary of the black hole (called the event horizon) and somehow protected yourself from spaghettification, time would tick slower for you. You wouldn’t notice this, for you time would tick at its normal rate. However if astronomers on Earth could see your watch they would see that it was ticking slower than their watches. Strong gravity slows down time!

Let’s say you spend a year very close to the black hole and then you decide to return home. How old would you be? Well many years may have passed on Earth, tens, hundreds, thousands or maybe even hundreds of thousands of years will have gone by without you realising!

Where is the nearest black hole to us?

The closest to Earth is called V616 Monocerotis - it is 3000 light-years away. This means that if we sent a text message to this black hole it would take 3000 years to get there, travelling at the speed of light (300 000 kilometres per second). This black hole is around 10 times heavier than the Sun.

How many of them are there in the Milky Way?

It is thought there are 100 million black holes in our galaxy. There are at least a thousand times as many stars as there are black holes.

The biggest black hole in the Milky Way is called Sagittarius A* and it lives in the centre of our galaxy. It is 4 million times heavier than our Sun! Don’t worry though - a text message to Sagittarius A* would take 26 000 years to get there.

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This video is from the free online course:

Teaching Primary Science: the Solar System and Beyond

Royal Observatory Greenwich