Black Holes – 27th August 2009

This evening’s talk on Black Holes was given by Phil Uttley of Southampton University. He gave us an interesting and well-illustrated talk which covered the subject of black holes thoroughly, beginning with the first suggestion of “dark stars” by Rev. John Mitchell in 1784. In the 19th century the idea was set aside when light was shown to behave as a wave, but the work of Einstein at the beginning of the 20th century brought the concept back into consideration. Schwarzchild’s solution of Einstein’s field equations for a spherical non-rotating body showed that in certain situations the gradient of space-time became infinite, so that nothing could escape. Eddington and Finkelstein refined this, and showed that material could, however, cross the “event horizon” and enter the zone so delimited. The term “black hole” was coined by Wheeler in 1967.

Phil went on to describe the ways black holes might be observed, first pointing out that, as they emit no light, it is very difficult to observe them directly. However, if they’re part of a binary system then they’ll develop a hot accretion disk. Cygnus X1 is though to be coming from an accretion disk round a black hole accompanying an observable blue super-giant star. Lone black holes might be detected because they could cause gravitational micro-lensing. Quasars and active galaxies have something very energetic at their hearts, and super-massive black holes are considered the most likely explanation. Stars at the very centre of the Milky Way have been observed to be orbiting a point at which nothing can be observed, but their behaviour implies that there must be a very massive black hole at that point. Observations of other galaxies have also led to the conclusion that there are super-massive black holes at the centres of most (and maybe, all) galaxies.It is even possible that these black holes may control the size of their galaxies through a process of active galaxy feedback, where excess material is thrown out along massive jets from the black hole’s poles.

Black holes are clearly going to be a subject for research for some time to come.

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