I’m currently reading up on cosmology, and it seems that much of our knowledge of using Cepheid variable stars as a distance indicator for galaxies comes from the study of the Large Magellanic Cloud. Of course, since the LMC is a reference point for the Cepheids as a distance indicator, our current data on the distances to other galaxies is subject to change with our knowledge of the distance to the LMC. Out of curiousity, what is the currently accepted distance to the LMC? How is this distance determined?
According to a couple of sites on the net the distance is 179000 light years. I’m pretty sure it is measured by parallax. Using the diameter of the earth’s orbit as a baseline means a shift in position of the cloud against the background stars of about 6.5 seconds of arc when measured at 6 month intervals.
Oh my God! Big arithmentic error! Ignore previous post. The distance is correct but forget the parallax number. It’s way off. So far of that I’m ashamed to give the correct number.
So someone else will have to answer the second part of your question.
I’m calculating a parallax of 1.82 * 10 ^ -5 arc seconds. Is that in the ball park of what you’re getting?
Well, actually it’s twice that because 6 months apart means a baseline of twice the distance to the sun. :o
Several steps:
First, the Cepheid scale was originally calibrated using distances to star clusters in our own Milky Way galaxy. The distances to those star clusters were determined using the Moving Cluster Method (really only useful for the Hyades cluster) and main-sequence fitting. The M.C. method is a trignometric method, and does not require a lower rung on the distance ladder. The Main Sequence was calibrated using nearby stars, for which the method of trigonometric parralax can be used, using the Earth-Sun distance as baseline.
Then, using the Cepheid scale, the distances to other nearby galaxies were determined, including the Large Magellanic Cloud. There’s nothing particularly special about the LMC at this point; it’s just another galaxy that happens to be near to us.
Then, in 1987, we got really lucky. We saw a supernova in the LMC (actually discovered by naked eye, which is worthy of note). And shortly after the supernova, we saw a ring expanding around the point of the supernova. Based on our existing value of the distance to the LMC and the rate at which we saw the ring expanding, it was really expanding at darned near the speed of light. Which means that it is, in fact, light. The light from the supernova is expanding out into a cloud of gas which surrounded the original star, and illuminating it. We see the illuminated spherical shell as a ring.
Now comes the good part. We know how long ago the supernova was, and we know the speed of light. So we know exactly how big that illuminated shell should be. Knowing how big it really is, and seeing how big it appears in the sky, we can exactly calculate the distance to the supernova, without recourse to lower rungs on the distance ladder. Since SN1987a is in the LMC, then, this lets us get a very good value for the distance to the LMC. Which in turn makes it a good target for calibrating other methods.
Interesting story Chronos about the 1987 appearance of the supernova.
Just out of curiosity, how close was the original distance calculation done prior to this event, to the calculation done using the supernova?
Since the original distance was used to estimate the expansion of the supernova ring at the speed of light, I’m concluding that the original estimate was pretty solid.
Have you tried MapQuest?
Yes! And it turns out there’s a Comfort Inn right at the exit!