Talk Astronomy

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• Comment by Stuart Lowe on Apr 21 2009:

  This four minute video was the introduction to our part of the 100 Hours of Astronomy 24-hour observatory webcast (Around the World in 80 Telescopes). We have had a video about Jodrell before, but we thought we'd share this new version with you in a better quality than was seen on the Ustream feed.

  For those that are wondering, the -250 degrees below zero that Tim mentions was on the Celcius temperature scale.

• Comment by EarthUnit on Apr 21 2009:

  Thanks for putting the video up Stuart, and thank Tim for answering a question I was just going to ask an astronomer, (after spending the last few hours trying to find the answer).

  Last night I listened to the Jocelyn Bell interview again, in it she said that the distance to the pulsar could be approximately worked out by the fact that the higher frequencies reached us before for the lower ones.

  She explained that this was due to “dispersion” of the frequencies by magnetic fields. I could not think of what could affect the pulse in between the Earth and the neutron star.
  I suppose if the galactic magnetic field can move electrons it will affect a EM pulse, nice one Tim

• Comment by Roy Smits on Apr 25 2009:

  Hi EarthUnit,

  About dispersion. When electromagnetic waves interact with electrons, it causes a delay that depends on the frequency of the electromagnetic wave. High frequency waves are barely affected, whereas low frequency waves (say below 1 GHz) are delayed significantly. Over a distance of several hundreds of lightyears, the delay can become several seconds. By measuring the delay between two frequencies you can calculate how many electrons the radio wave passed. If you also have an idea of the electron density between you and the source (in this case a pulsar), you can derive how far the source is away from you.

  So it is just the electrons in the interstellar medium that cause the delay. No magnetic field is required.

  Cheers,
  Roy

• Comment by EarthUnit on Apr 25 2009:

  Thanks for putting me right about that Roy, its appreciated.

  In the Jocelyn Bell, she talked about “whistlers” that is hearing lightning strikes on the radio from the other side of the world, and I put 2 and 2 together and thought bouncing off the magnetic fields :-~ If I had bothered to think about it I’m sure some pretty strange effects would happen to the light if it travelled parallel or across the magnetic field.

  Thanks again Roy for taking the time to correct me on that

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