2. Our favourite star2. Our favourite star

The Sun photographed in soft X-rays

The Sun’s statsThe Sun’s stats

Distance:Distance: 1 AU=150 1 AU=150 million kmmillion km

Radius: Radius: 700,000 km700,000 km Mass: Mass: 2 x 102 x 103030 kg kg Average density:Average density: 1.4 kg/litre1.4 kg/litre Surface temperature:Surface temperature: 5,800 K5,800 K Luminosity:Luminosity: 3.9 x 103.9 x 102626 watts . watts .

It gives us life and It gives us life and energyenergy

Humans have Humans have always revered the always revered the SunSun

But where did it But where did it get its energy get its energy from?from?

If it were coal it If it were coal it would only last would only last 10,000 years!10,000 years!

Atkinson, 1920: The Sun’s energy Atkinson, 1920: The Sun’s energy comes from the fusion of comes from the fusion of hydrogen nuclei to heliumhydrogen nuclei to helium

The forces between nucleons are about The forces between nucleons are about 100,000,000 times as strong as those between 100,000,000 times as strong as those between atoms.atoms.

This means that the energy involved This means that the energy involved in nuclear reactions will be hundreds in nuclear reactions will be hundreds of millions times greater than that of millions times greater than that involved in chemical reactionsinvolved in chemical reactions

The Sun would last nearly 100 billion The Sun would last nearly 100 billion years on its hydrogen fuel – but other years on its hydrogen fuel – but other processes will cause problems before processes will cause problems before then!then!

We now know a lot about how the Sun We now know a lot about how the Sun works from computer models based on works from computer models based on the laws of physics.the laws of physics.

The corona, the source of the solar wind The corona, the source of the solar wind becomes visible in an eclipse.becomes visible in an eclipse.

The solar wind The solar wind brings us auroras.brings us auroras.

Sunspots are the result of ‘knots’ in the Sunspots are the result of ‘knots’ in the magnetic field of the Sun. They ‘untie’ every 11 magnetic field of the Sun. They ‘untie’ every 11 years.years.

Our insight into the Our insight into the workings of the Sun workings of the Sun

enables us to understand enables us to understand the stars better …the stars better …