How old is that star?

We're a funny species.

On one side we try to set up standard ways of referring to things, so that people around the world, working in other disciplines, stand a chance of understanding what is being discussed.

However, there is also this other impulse to develop a jargon that we understand in our group, but outsiders do not. A good example is calculus. To dentists, this is something nasty on your teeth, but to physicists and mathematicians, it is a calculating tool we cannot do without.

A second example is what is regarded as a metal. Physicists, chemists and engineers agree on what is a metal and what is not. Iron, copper, zinc and gold are metals; sulphur, carbon and oxygen are not.

Astronomers have, however, a different idea. They refer to hydrogen and helium as non-metals, but all the other elements, including sulphur, carbon and oxygen are referred to as metals.

There is a clear reason for considering hydrogen and helium as different from the other elements, although the terminology could be a bit less confusing.

Hydrogen and helium are the most common elements in the universe, and are unique in that all the hydrogen and most of the helium were produced right back at the beginning of the universe.

All the other elements were formed as a by-product of energy production in stars. Let's refer to hydrogen and helium as primary elements and the others as secondary ones.

The first stars to form in the universe, around 100 million years after the Big Bang, formed from hydrogen and helium because at the time there was nothing else available.

Hydrogen is a star's primary energy source. Then, when making the energy those stars needed to shine, they produced more helium as a waste product, along with sulphur, carbon, oxygen and the other elements.

Some are made during routine energy production, and the rest when large stars explode at the ends of their lives. The explosion distributes all those secondary, waste elements out into space, where they mix with the gas and dust clouds waiting to form new stars.

One interesting and useful fact about stars is that the energy production takes place in their cores, and the waste products stay there. What we see at the surface is the material from which the stars formed, unchanged.

This means we can identify old stars, because we see no or very weak signatures of secondary elements in their surface layers, or younger stars, because they formed from material containing secondary elements from early generations of stars. Our Sun is a young star, since its surface contains secondary elements.

There is a complication. The length of a star's life depends on its mass. Stars with large masses get through their lives in about ten million years or so, going out with a huge explosion.

Our Sun is a low-mass star, and will last about 10 billion years. A star with a tenth of our Sun's mass will last far longer. This means most of the stars formed in the youth of the universe and still shining are low-mass stars, such as red dwarfs. Young stars might be low mass, or high-mass ones that have not yet reached the blowing up stage.

The short lives of high-mass stars have worked out in our favour. These stars are the biggest producers of secondary elements, and thanks to there having been many generations of them since the beginning of the universe, there are large amounts of secondary elements out there, available to make planets, complex molecules, and of course us.

• After dark, Saturn and brilliant Jupiter lie close together low in the south
• Mars, even brighter, rises in the east.
• Venus, brighter still, rises in the early hours. It is well worth getting out the telescope.
• The Moon will be full on the 31st. 

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.