Outer space is not really a total vacuum. In fact, it is made up of gas and dust particles known as the Interstellar Medium. When a gas in the Interstellar Medium undergoes gravitational collapse, the gas particles have their own gravitational attraction and clump together in a cloud, or a nebula, if you speak fluent latin. When talking in the context of astronomy, nebulae aren't normal clouds - they are massive interstellar clouds made up of plasma, hydrogen, helium and dust. They are also known as 'star nurseries', because they are often where stars are 'born', or formed. An example of some famous star nurseries are the Pillars of Creation, in the Eagle Nebula. Lots of people, me included, are interested in nebulae because of their looks. They can be many shapes and sizes (although all of them are hundreds or millions of light years across) as well as a vast variety of colours. Most people recognise nebulae as being pink or red, but only one type of nebula actually is: an emission nebula. When a star forms inside a nebula, gas and dust squash together under their own gravitational pull. The clouds get denser, and the denser they become, the hotter they get. Eventually they become so hot that hydrogen in them gets ignited and new stars come to life. This is when ultraviolet rays are emitted and the entire nebula is lit up, resulting in a pink or red emission nebula. Orion's nebula is one of these. Apart from emission nebulae we have other types of nebulae known as the reflection nebulae; they are named like this because they do not emit their own light. Reflection nebulae only reflect the light from the nearby stars. These appear blue in colour. The interesting thing about this is that presence of reflection nebula means the presence of an emission nebula somewhere close. There is also another type of nebula known as planetary nebula. Despite the name, planetary nebulae have nothing to do with planets. A planetary nebula forms when a star similar to the size of our Sun starts to expand and becomes a red giant. The core of the red giant is heated so much that the star becomes very unstable, and the outer layer of the star is ejected leaving behind the core. This is known as a white dwarf, and it emits radiations which ionise the gas atoms surrounding it; this leads to spectacularly colourful displays known as a planetary nebula. Eleanor, Year 10
When a huge star runs out of gas and reaches the end of its lifetime, it burns through all the gases (hydrogen, helium, neon, silicon, oxygen and carbon) until it reaches iron. The fusion process that creates iron does not create any energy. This means that the star becomes unstable as the radiation is no longer able to halt gravitational collapse. The core collapses into itself when the iron builds up to a certain mass because it can no longer support itself. This creates a supernova as well as a very dense black hole if the star is big enough. The huge amount of mass is concentrated into a very small area which is why they are so dense and their gravitational pull is so powerful. Some black holes are also thought to be created during the big bang. There are three types of black holes: stellar-mass black holes, supermassive black holes and intermediate-mass black holes. Everything near a black hole gets sucked in and stretched to its breaking point, even light, which is why they are black. A black hole doesn’t have a surface, instead it is just emptiness. Black holes are invisible. We know that black holes exist because of how material such as plasmas, stars and dust that surround them are affected. Black holes are usually surrounded by a spirally disc of material that get so hot that they give off x-rays. The first black hole was discovered by John Wheeler, an American astronomer, in 1971. The concept of black holes where first thought of by John Mitchell, a British philosopher and astronomer. He called them “dark stars”. The French mathematician, Pierre-Simon Laplace, also came up with the idea of black holes in 1796. But they were proven to exist later by Einstein’s theory of General Relativity. His theory showed that light does get affected by gravitational pull, which is what happens in black holes. Most galaxies have a few black holes in their centre. Our galaxy has around 100 million and they vary in sizes, the biggest has a mass of 4 million Suns. But that’s not that big considering the biggest black holes that scientists have found in our Universe can hold up to 5000 times more massive. The biggest black hole that scientists have discovered yet is in the constellation Perseus and has a mass of 17 billion Suns. It is known as object S50014+81. There are three measurable properties, called parameters, of a black hole: it's spin, mass and overall charge. These are the only ones that an outside observer is able to know. This idea is called the ‘no hair theorem’ because however ‘hairy’ or complex something that gets sucked into a black hole is, it will get reduced down to its spin, mass and charge. One of the effects a stellar-sized black hole has on nearby objects is called ‘spaghettification’. It is, quite obviously, when an object becomes stretched out and a compression occurs in the centre, until it reaches its limit and rips apart. This happens due to a gravitational gradient. For example, if you were falling feet first into a black hole (highly unlikely), your feet are physically closer to the black hole and therefore have a stronger gravitational pull towards it than your head. This starts to stretch you out further and further. Also, as your arms are not directly in the centre of your body, they will be pulled in a different vector than your feet or head which causes the edge of your body to be brought inwards, creating a compression in the middle. Overall, it’s not good news for you if you are falling feet first into a black hole!
The edge of a black hole is called the ‘event horizon’. Professor Lawrence Krauss, a theoretical physicist, said, ‘We call the event horizon an event horizon quite simply because it separates space into two regions.’ Gravity near a black hole is so strong that it slows down time. Inside a black hole, time stops. ‘You can never observe an object fall all the way through an event horizon,’ Professor Lawrence Krauss said. As an object gets closer to the black hole, it would seem to fall ever more slowly until, just before it falls through completely, it would freeze and stop moving. This is because its clock is moving infinitely more slowly compared to ours. But it would have actually carried on moving and would have fallen through the event horizon into the black hole. Then it, along with everything else that gets sucked into the black hole, would be pulled towards the centre, which is called the singularity. If you were to fall through the event horizon, the horizon splits into two: the horizon and the anti-horizon. You would fall through the horizon and so it says behind you, while the anti-horizon continues to remain ahead of you and you would never fall through it. A German physicist and astronomer, Karl Schwarzschild, found an equation to solve the size of a non-rotating black hole’s event horizon. This was called the Schwarzschild Radius. When black holes reach the end of their life, they evaporate through the ‘Hawking radiation’ process. This is when, at the edge of a black hole, by the event horizon, one of the virtual particles is pulled into the black hole while the other escapes and becomes a real particle. This means that over an extremely long period of time, black holes lose energy. They become very small and evaporate. Black holes do not obey the laws of physics that we know of. Instead they must follow bigger and more complex laws that scientists have not discovered yet. Selina, Year 10 |
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