A star is essentially a huge sphere of hot gas. At its centre, nuclear fusion is going on - this is the process that powers thermonuclear weapons, and it releases a vast amount of energy.
We can divide a star's existence into 5 periods:
| 1 | Formation |
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Stars
form out of gas clouds in space, like the Orion Nebula (left).
These clouds consist mainly of hydrogen, with a few traces of helium and other heavier elements.
The process begins when the cloud begins to collapse. |
A few denser knots will form in the gas. Then, these come together under the influence of gravity to form larger lumps. Eventually, the whole cloud begins to collapse into a sphere (It is thought that such collapses are usually triggered by an outside influence, such as the shockwave from a supernova - these are described below).
This sphere of gas will contract under its own gravity. As it does so, the temperature inside it rises.
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The atoms or molecules in a gas are constantly moving, and the higher the temperature, the faster they move.
They move in random directions, and when they collide they normally bounce off each other.
At extremely high temperatures, however, atoms can collide with such force that they fuse into a single entity. This is called nuclear fusion. |
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Now,
atoms consist of a small, dense nucleus, orbited by a number of negatively charged particles
called electrons. The nucleus itself consists of particles called protons and neutrons.
Protons have a positive electric charge, but neutrons are - as their name suggests - neutral, with no electric charge. |
It is the number of protons in the nucleus that defines an atom as being a particular element - for example, hydrogen has 1, helium 2, Iron 26. In nuclear fusion, the nuclei of several atoms collide and combine together, to form an atom of a different element.
As the new star collapses under its own gravity, the temperature at the centre rises until hydrogen begins to undergo nuclear fusion into helium. However, this process also releases a huge amount of energy, in the form of heat and light, so the star begins to shine.
It also stops collapsing at this point. Although its own gravity is still pulling it inward, the heat from the core makes the gas expand, thus countering the effect. As long as fusion continues, the star remains stable.
| 2 | The Main Sequence |
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How long a star can remain in this state depends on how massive it is. The bigger it is, the higher the temperatures at its core, and the fiercer the fusion reaction. Hence, the more massive the star, the more quickly it will burn out. You can tell roughly how massive a star is by its colour. A low-mass star will only glow red, while a middling star like the Sun will look orange or yellow. Very massive stars will appear white or blue.
| 3 | The Red Giant Stage |
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A star like the Sun will turn into a red giant star. As the star begins to contract, the temperature rises. Although the core of the star will have been converted into helium, the outer regions will still consist of hydrogen, since they were never hot enough for fusion to start. Now, however, the rising temperature allows fusion to begin in a new zone, above the core. Because this new shell of fusion is nearer the surface, it temporarily gains the upper hand over gravity, and the star expands to giant size. However, because the outer layers are now spread over such a huge volume, they fade to red. Thus, we have a red giant star. The Sun will reach this stage in about 4 billion years, and any life remaining on Earth will then be extinguished.
A small, faint star will not get such a second chance. As it runs out of hydrogen fuel it will begin to contract, but the temperature will not rise enough to start a new round of fusion so the star will simply fade away into darkness.
A massive white or blue star will not become a red giant either. When the initial phase of hydrogen burning ends, it will contract. However, the temperature in the core will rise until the helium starts to undergo fusion itself, to form carbon. The temperature around the core will also rise enough for the unburnt hydrogen there to begin fusion. So, although the star may expand to giant size, it will certainly not fade to red.
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Indeed,
a massive star can undergo this process several times. When helium in the core has all fused into carbon,
the carbon will then start to undergo fusion.
LEFT: A star with many concentric layers of fusion. |
The temperature will then rise enough to start fusing helium in the area around the core, and the hydrogen around that will start...and so on, until the star has a structure like an onion, with many layers of fusion, each burning a different element.
| 4 | Explosion and Collapse |
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For a Sun-like star, the end comes after its red giant phase. It will again run out of fuel, and this time will not be able to halt its collapse. The core of the star will collapse so rapidly, and so completely, that the atoms of which it is composed will break down. They are crushed together so violently that their shells of electrons will break open, leaving a chaotic soup of sub-atomic particles. This is known as degenerate matter.
Because the electrons and atomic nuclei are squeezed so closely together, degenerate matter is very dense - a handful of it would weigh many tons. Therefore the collapsed core of the star is very small, and is referred to as a white dwarf star. The Sun will end up as a white dwarf, no bigger than the Earth.
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The outer layers
of the red giant will simply drift off into space, forming what astronomers call a
planetary nebula. These spherical gas clouds have nothing to do with planets, but early
telescopes could only show them as a small disk, that somewhat resembled a planet.
LEFT: The planetary nebula MyCn18. |
For a large star, the end will be more violent. As mentioned above, it will continue to fuse heavier and heavier elements, until it has a structure like an onion. However, this cannot go on indefinitely. Eventually, the core will have been fused into iron. Again, when fusion stops in the core, the star will begin to contract. But although iron can undergo fusion, to make it do so requires far more energy than it gives out, so this time there is nothing to stop the core collapsing. Because of the star's great mass, the core collapses far more rapidly and violently than in a smaller star. The atoms will be crushed even beyond the stage of degenerate matter. The positively charged protons (in the atomic nuclei) and the negatively charged electrons will be so compressed that they merge, to form neutrons.
All that you are left with is a super-dense ball of neutrons, a few miles in diameter. For obvious reasons, this is called a neutron star. However, although the collapse of the core comes to an abrupt halt at this point, all the rest of the star is still falling in on top of it at great speed. The result is a massive shockwave that blows the star to pieces. This gigantic explosion is called a supernova, and is one of the most violent events in the universe.
| 5 | The End: Stellar Remnants |
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The
neutron stars produced by supernovae may still be visible to us as pulsars.
Some neutron stars give out jets of radio energy, which are swept round like the beam of a lighthouse as they rotate. A radio telescope can pick this up as a regular pulse of energy, hence the name pulsar, which is short for pulsating star. |
However, it is believed that the most massive stars do not even leave neutron stars behind them. When the largest stars collapse, even the neutrons are crushed out of existence. Theory predicts that nothing can stop the collapse, leaving an object that is infinitely dense and infinitely small. This is a black hole. It is so named because the more dense an object is, the higher its escape velocity (the velocity you would need to go at to avoid being pulled back by its gravity). A black hole has an escape velocity higher than the speed of light. Since light is the fastest thing in the universe, you are left with a small region of space from which nothing can escape. Anything falling into it disappears for ever.
Finally, the outer layers of the supernova star will be hurled off into space. However, space is not entirely empty, and even between the stars there is a little extremely tenuous gas. There are also large gas clouds, as mentioned at the top of this page. As the debris from the supernova smashes through this cold, stationary material, it may trigger the collapse of the gas clouds, to form the next generation of stars.
URL: http://www.randomnotes.co.uk/Astronomy/Starsprint.htm