Open Writing

We have all heard of Black Holes. But what are they?

Think of them as the final chapter in the life story of massive stars.

When we look at the night sky, we imagine that the stars stay the same for ever—that they have always been there and always will be. But in fact, stars are being born and are dying all the time. In our Milky Way galaxy there are about 400 billion stars of all ages and sizes. Our own star, the Sun, is very average. It is medium sized and is in its most stable phase, about half-way through its expected life of 10 billion years.

So, before we talk about stars dying, let's talk about how they are born. In addition to solid bodies, stars and planets, there are also huge clouds of dust and gas, called nebulae floating freely through space. Over time, these clouds accumulate through gravitational attraction into denser patches, which begin to spin and then clump together more and more.

Eventually, enough matter has been drawn in to reach a critical point, when nuclear reactions start at the core, and a star has been born. This can take anything from ten thousand to a million years. Hydrogen is converted to helium, then later in the star's life, to heavier elements, such as iron, carbon and silicon. The star will continue to burn for anything from a few million years to many billion, depending on its size. But sooner or later, all its fuel will be exhausted and it will then die. The way it dies depends on its size.

Small stars just shrink and cool until they no longer shine. Medium stars, like our sun, have a more exciting end. As the fuel runs out, the heavier elements at the centre contract, while the outer shell expands to become a red giant. Unfortunately, this will completely engulf the earth, along with most of the rest of the solar system. The core finally contracts to become a white dwarf.

But stars which are about 10 times more massive than our sun, or larger, have a truly spectacular end. They become supernovas. Once their fuel is exhausted, they die in a huge explosion, where all the outer layers are blown off into space, to provide the raw material for the next generation of new stars. A supernova occurs about once every 50 years in our own galaxy, and is an amazing sight. Only iron and heavier elements are left in the core. Due to its mass, this core continues to contract, leaving first a neutron star. Then, if it is massive enough, contraction continues until the atoms themselves are crushed and gravity is so great that nothing can escape, not even light. Hence, we have a Black Hole. Of course we can never see a black hole, but we know they are there, because of the effect they have on nearby stars. They distort the light coming to us from other stars that pass behind and around them. And as they continue to suck in other debris that comes close to them, they emit strong X- and gamma-rays, which can be detected by astronomical equipment.

We know that Black Holes are out there, and we know where many of them are. There is an enormous one at the very centre of our Milky Way galaxy, which continues to draw in surrounding debris and even small stars, planets and asteroids. Hence it continues to grow bigger and stronger. But what happens to the matter that is sucked into it, we don't know. The everyday physics that we understand and which rules the rest of the universe breaks down in Black Holes, and we can never go there to find out. Some astronomers believe that when a Black Hole becomes big enough, it will spontaneously explode to create a new universe. Maybe that is the explanation of the Big Bang that created our universe in the first place. We just don't know. But astronomers and physicists, such as Stephen Hawking continue to work on these mysteries. Meanwhile, we can only marvel at the workings of the cosmos, which we are only just beginning to understand.

© Wendy Ogbourne

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Wendy writes for Bonzer! magazine. Please visit www.bonzer.org.au