Friday, June 18, 2004

How Far the Pleiades?

Tom's view of Saturn from Phoebe (thanks, Starry Night Pro) is gorgeous! It raises the question: Just how far away are the Pleiades, the most famous, and almost the nearest, cluster of stars.

The most direct way of measuring the distance to a star is the so-called method of parallax: the tiny shift in the apparent position of a star (against the background of more distant stars) when seen from different places in the Earth's orbit. Hold up you finger in front of your nose and look first with one eye, then the other. Notice the shift. The nearer your finger, the greater the shift. Ditto for stars.

However, stars are so far away compared to the size of Earth's orbit that this method traditionally worked only for some thousands of the nearest stars. At greater distances the parallax is too small to measure even with the best Earth-based telescopes, due to atmospheric blurring.

Enter the Hipparcos satellite telescope, launched in 1989 to measure stellar parallaxes from above the atmosphere. Hipparcos has obtained the distances of more than a million stars with unprecedented precision, including a distance for the Pleiades -- 385 plus or minus 13 light-years.

Alas, this is in conflict with another standard method for measuring distances to stellar clusters, called main-sequence fitting. This method uses the well-established relation between the color of a star in the prime of its life and the star's absolute or true brightness. A comparison of the absolute brightness of a star and its apparent brightness yields distance.

For the Pleiades, main-sequence fitting gives a distance of 430 plus or minus 13 light-years. Uh, oh!

If Hipparcos is right, and main-sequence fitting wrong, then our estimate for the size of the universe (which depends on main-sequence fitting for distant clusters) is in error, perhaps by as much as ten percent.

If Hipparcos suffers from some sort of systematic error, then its wealth of data is called into question.

Sorting out this discrepancy involves -- you got it! -- devising ever more exact and clever ways to measure the distance to the Pleiades. That little group of stars, which has fascinated stargazers from time immemorial, has a yet bigger role to play in our quest to measure the universe.

See Bohdan Paczynski, A Problem of Distance: Nature, 22 January, 2004.