P1bL11 Birth of a Star

Key Words

Chain reaction - the splitting of a nucleus sets off further splitting of other nuclei.

Gravity - a weak attractive force between all objects that have mass.

Kelvin - a unit of temperature.  0 K = -273 ^oC.

Nebula - a cloud of gas and dust in space.

Nuclear fusion reaction - nuclei of smaller atoms join together to form a bigger atom.

Protostar - a star in its early stages of development.

Radiation pressure - an force acting outwards due to the continuous nuclear explosion.

Star - an object fuelled by fusion reactions, giving out light and other EM radiations.

Grade E

Space is not empty.  There is a lot of material out there, often in the form of large clouds called nebulae (singular - nebula).  Gravity pulls the particles together.  The process is painfully slow, taking thousands of millions of years.

As more material comes together, it starts to get hot, about 15 million Kelvin.  This is more than hot enough to fry bacon.  

At this temperature the nuclei of small atoms can join together in a nuclear fusion reaction, which gives out lots more energy.  This in turn forms a chain reaction, and the star lights up. The Sun fuses 4 million tonnes of hydrogen every second to from helium.  There is a huge amount of hydrogen in the Sun.  It has been burning for 4500 million years, and has enough for another 4500 million years.  It will not go out any time soon.                                        

Grade C

The reason that the mass of gas and dust gets hot is that the particles gain speed as they are pulled together.  As more join the ball, they get squashed together, and get even hotter.  While most energy is kept inside, some is released as infra red radiation.  The dull red mass is called a protostar.  It is not a full star, because the self-sustaining fusion reactions have not started.  Eventually the reactions do start.  The star glows brighter.

If there is not sufficient material, the protostar will not light.  It will continue to give out energy, but not very much.  If the failed protostar ends up orbiting another star, it becomes a planet referred to as a gas giant.  It is thought that Jupiter is a failed star.

The fusion reaction is summed up as:

hydrogen ® helium + energy

Grade A

Nuclear fusion in a star is uncontrolled.  On Earth we can recreate the process in a hydrogen bomb.  The amount of hydrogen used would fill a party balloon, yet has the destructive power of many millions of tonnes of TNT.  While a hydrogen bomb contains just a few grams of hydrogen (strictly speaking, deuterium, an isotope of hydrogen), the Sun fuses 4 million tonnes a second.  Why does it not explode?

The answer is that gravity holds it together.  The outward force of the explosion, the radiation pressure, is balanced by the inward force of gravity.

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