The current Sun from YOHKOH |
Where did the world come from? And what is its ultimate fate? Philosophers
and scientists have struggled with these questions for millennia, most
recently arriving at the Big Bang Theory, which explains how the Universe
might have been made from nothing. The problem is that a Big Bang would
only create hydrogen and helium, the simplest and lightest elements. What
could have produced carbon, oxygen and everything else so essential to
life?
Hydrogen can be converted into other elements by thermonuclear fusion,
the process that powers stars and keeps the Sun shining, so it has long
been suggested that we were formed from elements made inside stars.
Unfortunately, the only element produced by the Sun so far is helium ---
largely irrelevant to the Earth --- and most of that is still trapped in
the Solar core. The only explanation is that we live in a second generation
solar system, the elements now on Earth scattered by prehistoric explosions.
The Hertzsprung-Russell diagrams, shown here, help us to understand how
this happened. |
In the diagram opposite, Hertzsprung and Russell have plotted many
stars' brightness against temperature, which we know from the star's colour:
"white hot" means a higher temperature than "red hot", for example. Most
seem to lie on a diagonal line across the diagram, which they called the
"main sequence", but there are also two distinct groups towards the corners,
descriptively named "red giants" and "white dwarfs". There seem to be more
red giants, but this is only because they are so bright and thus easier
for us to see.
The main sequence represents stars such as the Sun, which are fusing
hydrogen into helium: most stars sit here for most of their active lives.
A star's position on the line is related to its size: the bigger and heavier
the star, the greater the pressure at its core and the more violent its
nuclear reactions. |
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The two corner clusters can be explained by charting the life of a star,
for example the Sun, as plotted below. After billions of years on the main
sequence, the Sun will have converted most of its core hydrogen into helium,
and in turn start fusing helium into elements such as carbon and oxygen.
The star expands, so it gets brighter but colder: a red giant. When the
Sun reaches this stage it will grow so big that it engulfs all the inner
planets, including Earth.
