Object lesson No 71

7th September 2001 at 01:00
What's 1.4 million kilometres across, has a surface temperature of 5,500C and is not expected to change much in the next 5 billion years? Need another clue? OK, what's round, yellow and hides behind clouds?

Despite its importance for us Earthlings - particularly those on holiday - we should get the Sun in perspective. It is nothing special. The light of our lives, whose formation gave rise to our solar system, is a mediocre, middle-aged star, a yellow dwarf of spectral class G2. There are billions of such orbs in the Milky Way.

Still, the statistics of this swirl of hydrogen and helium amaze. The Sun is, at heart, a nuclear reactor. The compressed gases at its core are so hot that fusion takes place. When the nuclei of hydrogen atoms collide, they combine to make helium nuclei with a bit left over. This bit is energy. The Sun converts 5 million tonnes of matter into energy in the form of gamma rays every second - and will have done so for nearly 10 billion years before it starts to dim.

Once released, the gamma rays start their long journey to the gaseous surface. Several hundred thousand years later, their degraded remnants shoot through the so-called photosphere causing it to wobble up and down by four kilometres every three hours. After that, it's a quick 150 million kilometre trip to burn bodies on the Costa del Sol.

The Sun rotates once every 25 days at its middle, and once every 35 at its poles. Scientists think that this uneven spin affects its magnetic fields, sometimes twisting them into "ropes" of high strength that burst out in loops reaching the edge of the solar atmosphere or corona. Sunspots, which can be 50,000 kilometres across, occur where these ropes emerge, and appear as dark stains because they are about 1,500C cooler than the surrounding surface. The average number of sunspots seems to vary over an 11-year cycle, the occasional dramatic fall in numbers may have affected the Earth's climate, though no one is quite sure how.

The magnetic ropes may also be responsible for solar flares. When two twists of opposite polarity bend and touch, violent eruptions occur, sometimes creating flaming clouds a million kilometres high and releasing high-speed particles and radiation to travel on the solar wind to Earth.

There, they enliven polar nights with the light show known as an auroraI just another shining example of the power of an ordinary star.

Stephanie Northen

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