Stellar Distances

Vekkypoo's bud writes: "Researchers at the Palomar Observatory have measured the most accurate value of a stellar distance to date. Using the Palomar Testbed Interferometer, the distance to the star Zeta Geminorum in the constellation Gemini is 1100 lightyears. That's accurate to about 13%. While that seems like a large error margin, it's actually three times more accurate than the best previous measurement. Check it out here."

Comments

Not quite that simple...

[tesserae]

From the article:

Cepheid variables are stars that have very predictable relationships between their absolute brightness and the frequency with which they brighten up. A Cepheid is useful for measuring distances because, if it is known how bright the star really is, then it is a simple task to measure how bright it appears on Earth and then calculate the distance.

While this is mostly true, there's not simply a "very predictable relationship" between luminosity and distance -- as demonstrated by new information on Polaris (the North Star, a Ceppheid variable). In particular, check this graphic, which shows two distinct period-luminosity relationships (differing in the zero-point offset), depending on whether the Cepheid is a fundamental-mode or first overtone pulsator (and note that there's arguably some evidence of other pulsation modes in the clustering of points off the main trends, in the lower right-hand corner of the plot).

Just knowing the brightness of the Cepheid variable won't tell you how far away it is; you also have to know the pulsation mode, which isn't always easy. But then, astronomy hasn't ever been really easy... :) This data on Polaris comes from ground-based interferometry, by the way -- just a different instrument than that mentioned in the article -- as well as measurements from the Hipparcos satellite.

[I hope everyone can get to those URLs without having to pay -- I can't check that from here, 'cause I'm already logged into Science Online by default. Some info's available to everyone, some requires a subscription.]

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