By Christine Sutton
The Sun is an unremarkable star: "In the community of stars ... a respectable middle-class citizen", according to Arthur Eddington. Yet it is vastly interesting. As the nearest star by far, it is the one we understand the best; moreover, it has the fundamental attraction of being the dynamo that drives life on Earth. The sunlight we see comes only from its surface layers, but the energy that fuels life itself originates far deeper, in a hot, dense thermonuclear furnace buried at the Sun's core.
Sunlight takes eight minutes to travel the 150 million kilometres to Earth from the Sun's surface, yet the energy it brings with it, which started out only 700 000 kilometres further away, in the Sun's centre, was released a million years previously. On its way to the solar surface, the energy has been absorbed and reemitted in a myriad interactions in the Sun's hot, gaseous interior. The light that reaches us is only remotely connected with the central processes that keep the Sun burning.
Were this the whole story, the centre of the Sun would be forever hidden from view, but fortunately it is not. A small fraction of the Sun's energy, about 2 per cent, exists in a different form, which can travel through the gassy mass as if it were not there at all, taking only three seconds to reach the surface from the centre. This energy is in the form of neutrinos. Largely unaffected by the Earth, the solar neutrinos stream through us constantly, raining down on us by day and up through the Earth by night. Most exciting of all, they bear the imprint of their birth at the heart of the Sun. They provide our only mirror on the Sun's core, if only we could see into it!
Can we detect these neutrinos? Can we use them to "see" directly to the Sun's core? Yes - in principle. But there is a problem. The very reason that the neutrinos can arrive at Earth as messengers straight from the centre of the Sun makes them almost impossible to stop. The weakness of their interactions with matter allows them to escape rapidly from the Sun, but also provides an unrelieved headache for experimental physicists. You have to be particularly stubborn to dedicate your life to detecting neutrinos from the Sun.