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Tips & history for new astronomers
Astronomers in the world's oddest observatory cannot see the sky at all, and do not need to. The observatory is about I600m (1 mile) underground in an old mine in the western United States, and its 'telescope' consists only of a huge tank of cleaning fluid. Yet it can 'see' into the core of the Sun better than any visual observatory in existence.
The reason has to do with a sub-atomic particle called a neutrino. Neutrinos have no electrical charge and virtually no mass. As a result. they can pass through most forms of matter unchecked - a property which makes them extremely difficultto detect. Most ofthe radiation generated at the Sun's core is absorbed by the Sun's surface layers.
Neutrinos, however, once created, are unaffected by the outer layers and so provide a direct indication of the activity in the core. Neutrinos are also largely unaffected by the Earth, so most pass straight through without even slowing down. But they can sometimes react with atoms of chlorine-hence the tank of cleaning fluid which is rich in chlorine.
By monitoring the reactions in the tank, scientists can gauge the number of neutrinos reaching the Earth, and thus the number being produced by the Sun. The tank has to be so far underground in order to block cosmic rays which would otherwise swamp the neutrinos' effects.
The smallest stars known are neutron stars, so called because their electrons and protons have been crushed together to form the sub-atomic particles called neutrons. A typical neutron star is only about 25km (15 miles) across, yet it may contain as much mass as a star the size of the Sun. As a result, a pinhead of neutron star material would weigh about 1 million tonnes-as much as two of the world's largest supertanker ships put together.
Neutron stars are believed to be the remnants of large stars which have erupted as supernovae at the end of their lives. Their intense gravity has the effect of concentrating the powerful radiation they give off into beams. Since many neutron stars also spin, the radiation sweeps across space like the beam from a lighthouse. As the beams flash past Earth, the star seems to pulse with energy-which is why rotating neutron stars are known as pulsars.
The first pulsar was discovered by radio astronomers at Cambridge, England, in 1967. Hundreds of others have been mapped since then. The first pulsar to be identified optically as well as from its radio waves was the small, faint star at the core ofthe Crab Nebula. Spotted in 1968, the star spins 30 times a second. Since then, other pulsars have been spotted which spin thousands of times a second.
One of the largest stars known is Betelgeuse, a red supergiant which marks the right shoulder of Orion. If Betelgeuse were positioned where the Sun is now, it would engulf the orbits of Mercury, Venus, Earth and Mars. If it were possible to make a telephone call from one side to the other, your voice, travelling at the speed of light, would take half an hour to reach the other end of the line-even if the call were routed straight through the star's centre.
If the call were routed around the surface of the star, your voice would take an hour and a half to get through. Some astronomers believe that a mysterious and invisible object in the stellar group known as Epsilon Aurigae may be still larger. The object has not been seen directly but every so often it seems to cause a partial eclipse of its visible neighbour for a staggering two years at a time. By observing these effects, some astronomers calculate that, if the object is a star, it may be more than 3000 million km (2000 million miles) across-large enough, if it were in our solar system. to engulf all the planets out to Saturn.