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Thursday, January 26, 2006

Henrietta Swan Leavitt and the size of the universe

Talk about a big subject!

How do we know how large the universe is, or how far away a particular extra-terrestrial body, such as another star, is? The short answer is, we have to make some assumptions, and do some measurements. (The same is true for learning about the past, or predicting the future.)

The first basic tool for determining the distance of objects was triangulation. In other words, measure the apparent angles of the direction of an object from two points, as far apart as reasonably possible, and use trigonometric functions, or some such mathematics, to determine the distance. The problem with triangulation is that most objects outside of our solar system are so far away that the apparent angles will be the same, or, to put it another way, the triangle used will be too skinny to use. About the longest possible distance between observation points is the distance between the position of the earth, six months apart, when we have traveled half way around the earth's orbit. Even that distance is a tiny fraction of most distances to other stars and galaxies.

Well, can't you determine how far away something is by its brightness? Brighter objects should be closer, shouldn't they? Well, yes, all other things being equal, but they aren't. Some objects are much brighter than others. We can, for example, triangulate the distances to the moon and the sun, and the moon is closer, even though it appears less bright than the sun. A distant galaxy would not seem very bright, but a nearby meteor, quite small, could appear as quite bright.

Enter Henrietta Leavitt. Leavitt, who, like most female English-speaking scientists, wasn't given the status or position that her good work deserved, discovered that there was a type of variable star, and that a few dozen of these were found in the Magellanic Clouds, the two galaxies closest to ours. (These two are not visible from the Northern Hemisphere.) Her assumption was that all the stars in these two galaxies were essentially the same distance from us, so that the brightest-appearing of them should actually be the brightest, and largest of stars of that type. She found a relationship between the period of the variation and the brightness. This enabled astronomers who had found such variable stars elsewhere to measure the period, and, from that, infer the brightness, hence the distance, of such stars.

The Wikipedia article, which I linked to in the previous paragraph, has, somewhat to my surprise, a link to a web page entitled World's Greatest Creation Scientists from Y1K to Y2K, where she is listed as one of a few such. Based on the best biography of Leavitt, Miss Leavitt'sStars: The Untold Story of the Woman Who Discovered How to Measure the Universe, by George Johnson (New York: Norton, 2005) it seems that Leavitt was, indeed, a believer. There doesn't seem to be enough information to infer any particular belief about origins, such as young-earth creationism or Intelligent Design. (The web page does not do so for Leavitt.)

I am not an astronomer. I was amazed to read in Johnson's book that there was still some controversy, as late as 1996 (there probably still is) about what assumptions to make about measuring the distance of distant objects.

1 comment:

Kelly said...

I really enjoy looking at your flickr photos! I love looking at nature photography & you have some beautiful photos!!