1.04 Nuclear fusion
In this section we look in more detail at how nuclear fusion occurs to turn the protostar into
what is known as a main sequence star. Note it is only in the central core of a star that the
conditions become extreme enough for nuclear fusion to occur.
Conditions within the protostar
The material inside the protostar is in an extreme condition called a plasma. Atoms no longer
exists in their natural state, the electrons are stripped away and the nuclei and electrons move
as independent particles. If two nuclei collide with sufficient velocity they will fuse together
to form a new nucleus (nuclear fusion). The chances of such collisions occurring increase with
increased density and temperature. The temperature of the gas reflects the typical velocity with
which the nuclei move in the core, so the hotter the core the faster the nuclei move. The denser the
material is the closer together the nuclei are and therefore the greater the chance of collisions
occurring. After about 50 million years as the core slowly contracts it reaches a temperature of
about 10 million Kelvin and nuclear fusion begins. The conditions within the core of the star at
this stage are such that the chances of fusion occurring for an individual nucleus are still
extremely small. However there are such a vast number of nuclei present in the core that the rate
at which reactions occur is extremely high (very roughly about 9 x1037 (9 followed by
37 zeros!) complete reactions per second for our sun).
Nuclear Fusion
The nuclear reactions which occur within the core of a new star involve several stages but the net
result is that hydrogen nuclei are converted into helium. This process is called hydrogen
burning (this term refers to nuclear fusion rather than the chemical burning we are more
familiar with). The mass of the helium atom is slightly less than the mass of the products
that fused to form it. This "lost" mass is converted into energy during the fusion process. Mass
can be thought of as frozen energy and in nuclear reactions mass can be converted into energy. The
amount of energy produced can be calculated from Einstein's famous equation E=mc2. To
put this into perspective our own sun converts about 6 million tons of its own mass into energy
each second! Although this seems an alarmingly high figure the total mass of the sun is so great
that although it has been burning at this rate for about 4.5 billion years it has still only used
about 4% of its total mass!
Stars which are "burning" hydrogen are called main sequence stars this comes from there
position on a HR diagram (a diagram used to plot different types of stars by their luminosity and
temperature). The time during which hydrogen burning occurs in the core of the star is called its
main sequence lifetime
