Reading the Sun's biography

The window in my den faces east. On the windowsill, I have a prism, a piece of glass around eight centimetres long and triangular in cross-section.

It is mounted on one of those wood and brass things in which people used to keep business cards, with one of the flat faces of the prism at the top. This means that when the morning sun shines through the window, a rainbow, or spectrum is cast on the ceiling.

I do this not only because the colours are beautiful, but also because, at a more deep level, in that band of colours, the Sun is telling us all about itself.

It is telling us:

• Its temperature
• What it's made of
• What type of star it is
• How old it is
• The strength of its magnetic fields
• Its rotation
• Waves on its surface
• Its atmosphere.

This ability to analyze the light from even the most distant stars, and find out about them, is one of the most wonderful things in astronomy.

The tool we use to do this, the spectroscope, is probably just as important as the telescope in our efforts to understand a little about the universe we live in.

It all started with Isaac Newton, who must be one of the most important scientists in our history.

He made a hole in a window blind, allowing a narrow beam of sunlight to shine in. He then passed it through a glass prism and broke that white light into the rainbow of colours of which it is composed.

Of course, with just a prism, we cannot get much further, but modern spectroscopes can spread out the light much more, making it possible to seek fine detail.

When this was first done for sunlight, the resulting spectrum was found to be crossed by many dark lines, some very narrow, some broad and dark, and some in groups.

The explanation for these lines lies in the basic structure of atoms.

An atom consists of a nucleus, with a positive electrical charge, surrounded by one or more electrons, which orbit around it. However, it is easier to understand what is going on if we imagine instead that each atom is like a stadium or amphitheatre.

The nucleus is the middle, and the electrons are scattered among the seats.

• To move back one row needs a fixed amount of energy.
• To move forward one row involves the release of a fixed amount of energy.
• Every row has its individual energy fee and each type of atom has its own particular stadium
  and energy pricing.

Light comes in little indivisible packets, or quanta (singular, quantum). The amount of energy in a quantum depends on its wavelength, or colour.

A quantum of blue light has about half the wavelength of red light and contains twice the energy. The light from a star is made up of a mixture of colours, which therefore covers a wide range of energies.

This light radiates out into space, encountering the atoms making up that star's atmosphere. Just as an example, imagine there are atoms with electrons sitting in row six, which have a desire to move into row seven.

There is a prescribed amount of energy needed to make this happen, which corresponds to light of a very particular colour. If the starlight contains this colour, the electron will grab it and use the energy to lift itself into row seven.

This means light of that particular colour is being removed from the spectrum, leaving a narrow gap, a dark line. With lots of different atoms and electrons changing rows, we see lots of little energy grabs scattered across the spectrum as dark lines.

Each type of atom has different numbers of electrons, which are distributed differently among the rows, so as they jump around in the Sun or starlight they make their own unique combination of dark lines.

This means that if a star is at all visible in our telescopes, no matter how far away it is, we can read the message of its light, thanks to Isaac Newton.

• Saturn and Jupiter lie low in the southern sky overnight
• Mars rises around 10 p.m.
• Venus, shining even brighter than Jupiter, rises around 3 a.m. The Moon will be will reach First Quarter on the 25th and be Full on Sept. 1.

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