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Sunday, March 17, 2013

Table-Top Astrophysics

An article from the Economist on how some astronomers are trying to perform lab experiments to learn more about the cosmos. For instance, the article describes:
Ben Murdin of the University of Surrey, for example, has been making white dwarfs. A white dwarf is the stellar equivalent of a shrunken but feisty old-age pensioner. It has run out of fuel and is contracting and cooling as it heads towards oblivion—but taking its time about it. As they shrink white dwarfs pack a mass up to eight times the sun’s into a volume the size of Earth. A consequence of stuffing so much matter into so little space is that white dwarfs have powerful magnetic fields. Many aspects of a white dwarf’s mechanics, including how long it will last, are thought to depend on its magnetism. But it is hard to measure.
To make estimates, scientists examine the light a white dwarf emits for telltale patterns left by stellar ingredients like hydrogen. They then compare this spectrum with a theory, based on calculations from first principles, of how magnetic fields effect light emitted by hydrogen. The predictions agree with experiments up to the strongest fields mankind can muster—about 1,000 tesla, generated in a thermonuclear detonator. The problem is that the theory puts white dwarfs’ magnetic fields at 100,000 tesla or more, well beyond humanity’s reach.
Dr Murdin built his own little white dwarf to see if the theory looked good. It consists of a silicon crystal sprinkled with phosphorus atoms. A silicon atom has four electrons in its outer shell. In a crystal, all four are used to bind it to neighbouring atoms. Phosphorus has five outer electrons. Insert a phosphorus atom into the silicon lattice and you are left with an unused electron. Since phosphorus also has one more proton in its nucleus than silicon does, taken together the extra particles resemble a hydrogen atom: a single electron tethered to a single proton.
However, the extra electron is much less tightly held by the extra proton in this pseudo-hydrogen than it would be in real hydrogen. This weaker grasp means that it takes much less magnetism to make a given change in the pseudo-hydrogen’s spectrum than it would for real hydrogen. So when Dr Murdin placed the crystal in a 30-tesla magnet at Radboud University in the Netherlands (his lab in Guildford lacks the necessary kit), he was mimicking the conditions in a 100,000-tesla white dwarf. And the spectrum came out looking just the way the theory predicted.
Read the whole thing.

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