The structure of the Sun

Rather bizarrely, when you stop and think about it, the Sun is a ball of burning gas just floating in space. And not just floating idly — actually, it's zooming around the centre of the Galaxy at a rate of knots, but that's another story . The gas that burns in the centre of the Sun is hydrogen , the simplest of all elements. It's worth remembering that the Universe likes to keep things simple.

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The Sun is about 74% hydrogen by mass, 92% by volume, and yet it accounts for about 98% of the solar system's mass. The average distance of the Sun from the Earth is about 150 million kilometres, or 93 million miles. If ‘The Stig’ from the BBC's Top Gear team drove at 93mph, since there is no speed limit in space other than the speed of light, even he would take 115 years to get there. And yet on a misty or foggy morning, when you get a chance to look straight at the Sun without squinting, it still looks massive, even at this distance. That's because the Sun is 1.39 million kilometres across, or 865,000 miles.

But given all this, its overwhelming mass, its great magnitude, and how long ‘The Stig’ would take to drive out there, the Sun is only a dwarf. Now one of the best things about dwarf stars like our Sun is that they have long, and hopefully fruitful, lives. Astronomers think the Sun is about 4.5 billion years old, and that it will spend 10 billion years altogether happily burning hydrogen in the way it's been doing so to date. And, the theory goes, the longer living and more stable the star, the longer living and more stable its planets. So if life develops on one of those planets, it has lengthy, relatively stable periods in which to evolve into intelligent beings that can read web pages, such as this one.

It has long been suspected that stars other than the Sun are orbited by their own planetary systems. Indeed, extrasolar planets — those beyond our solar system family of eight planets — became a subject of scientific study in the mid-19th Century, but only 14 years ago did the tipping point occur. For the first time, confirmed findings of extrasolar planets were made. To the naked eye, the extrasolar planets themselves cannot be seen, but their parent stars are clearly visible. Indeed, our night sky will easily reveal its secrets. Stars in the constellations of Ursa Major, Andromeda and Pegasus, for instance, each have stars around which extrasolar planets are known to orbit.

From any garden on Earth, where these constellations are visible, the extrasolar planets orbit unknown to the naked eye. To date over 300 extrasolar planets have been found in the local solar neighbourhood of space. Many believe that around one in ten of all Sun-like stars have planets. The true proportion may be far higher and, if that's true, the Universe may be replete with planets, heightening the possibility that some might support extraterrestrial life.