kcarroll (08/04/82)
A supernova's gravitational effect on nearby stars might not be negligible, for at least two reasons. The first is that the supernova need not produce a spherically-symmetric nebula; hence the center of mass of the supernova remnant would be slightly different from that of the pre-nova star. It might even be significantly different, as I beleive that the amount of the star blown off by the S-N would be on the order of 50%, rather than 0.1% or so.(although this is based on a rather hazy recollection of some Asimov article(how's <that> for an example of pleading higher authority?)). Working out the ncenter of mass of the nebula would be tricky, as not all the gas in it would be radiating (ie. we wouldn't be able to see much of it); hence it would be hard to correlate the mass-center shift with effects on other stars, in any useful manner. The other possible effect is based on the presumed wave nature of gravity (appropriate, as this experiment is designed to measure the gravity-wave propagation speed, which can only be done if gravity <is> wavelike). From what I've read of supernovae, they are quite rapid events, and involve large masses being shuffled about very quickly. This ought to generate a rapid change in the local gravity-wave medium, perhaps analogous to a shock wave in air. The effects of the passage of this wave are what we would look for in nearby stars and nebula. I can imagine it causing a ripple through a nebula, or a sudden shift in the period of a variable star. (of course, as far as astrophysics goes, I have more imagination than real knowledge) The place where this experimaent might really fall down is the use of "known" distances between stars as part of the calculation of the speed of gravity. To my knowledge, it's only for the very near stars that we have any precise idea of distance. The farther stars are too far to use parallax as an accurate distance-measurement, and so relative magnitude of the stars must be used. Except, the ABSOLUTE magnitudes of the stars are not really known, so this isn't all that accurate, either. As a result, I'd be surprised if the distance of a star more than (say) 10 parsecs away was known to better than (say) 5% accuracy. (can anybody out there in net.space-land confirm or refute this?) Kieran A. Carroll ...decvax!utzoo!kcarroll
gdw (08/06/82)
#R:utzoo:-233800:harpo:11700001:000:290 harpo!gdw Aug 6 16:21:00 1982 I thought that distances were measured using "red shift", viz. the Doppler shift (toward longer wavelengths) of it's spectrum due to it's velocity away from Earth caused by the expanding universe. This velocity times the Hubble constant equals the distance of the star with good accuracy.