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Polaris: Not So Close After All

18 January 2013 5:35 pm
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NASA/ESA G. Bacon (STScI)

Disputed distance. An artist's concept of Polaris with two companion stars.

Last November, astronomer David Turner made headlines by claiming that one of the sky's best known objects—the North Star, Polaris—was actually 111 light-years closer than thought. If true, the finding might have forced researchers to rethink how they calculate distances in the cosmos as well as what they know about some aspects of stellar physics. But a new study argues that distance measurements of the familiar star made some 2 decades ago by the European Space Agency's venerable Hipparcos satellite are still spot on. Experts appear to agree.

Astronomers arguably made the most accurate measure of the distance to Polaris in the mid-1990s. The star, five times as massive as the sun, is a so-called Classical Cepheid: a rapidly aging giant star that has used up its hydrogen fuel and is now burning helium in its core. In this period of instability, its outer stellar envelope expands and contracts over periods of days to a few months. Scientists working with the Hipparcos satellite measured Polaris's distance by taking its trigonometric parallax; that is, how, over a period of months or years, the star moves across our line of sight in relation to other objects in the sky. Polaris, the team calculated, was 434 light-years away.

But last year, Turner challenged that figure. He and his team broke light from Polaris into its component wavelengths—a technique called spectroscopy—and compared 23 spectra from the star. From the new data and Polaris's known apparent brightness, the astronomers were able to infer its absolute visual brightness by comparing ratios within its spectra. In contrast, the Hipparcos team got a much brighter absolute luminosity for Polaris than did Turner's team. Even so, Turner's team used their own absolute luminosity measurements to, in turn, calculate Polaris's true color and temperature, which ultimately gave them the new distance: 323 light-years.

Now the Hipparcos team is back in the form of one of its former team members, Floor van Leeuwen, a researcher at the University of Cambridge in the United Kingdom. In a paper to be published in an upcoming issue of Astronomy & Astrophysics, van Leeuwen notes that during its 3-year mission, Hipparcos took 127 measurements of Polaris, 124 of which he describes as "robust." Hipparcos's measurements would have to be off by 23 times the spacecraft's margin of error, he calculates, for Turner to be right. "What I did in my letter was to show the huge improbability of Turner's claim from the point of view of the Hipparcos data," he says. "The assumption that [Turner's] models can predict more accurately what the distance of an object is than the actual measurements puts science completely upside down."

Instead, van Leeuwen argues, the discrepancy lies in how Turner and colleagues interpreted Polaris's luminosity and in particular how that relates to what is known about how Polaris pulsates. As a result, as van Leeuwen asserts in his paper, Turner's new brightness calculations are greatly underestimated, which van Leeuwen says caused Turner's team to also underestimate the star's distance.

Astronomers are lining up behind van Leeuwen. "The claimed evidence given by Turner et al. seems to me to be quite uncertain," says Michael Feast of the University of Cape Town in South Africa. In contrast, the Hipparcos distance for Polaris "seems firm," he says. Scott Engle, an astronomer at Villanova University in Pennsylvania, agrees that van Leewen's data are "too strong to ignore." He says he is inclined to trust Hipparcos's trigonometric parallax measurements as a more direct method of measurement over Turner's spectroscopic parallax.

The debate isn't just academic. Astronomers have used stars like Polaris for decades as "standard candles" in calibrating cosmic distances and estimating how fast the universe is expanding. If this very basic measurement of Polaris's distance were to remain in dispute, it would arguably cast doubt on the cosmological distance scale as a whole, and cause astronomers to wonder about the reliability of distance measurements for much of Hipparcos's 120,000 star catalogue.

Astronomers may soon settle the controversy. Barbara McArthur, a research astronomer at the University of Texas, Austin, says she plans to collect recent data and derive a new parallax for Polaris B, a companion star to Polaris thought to be about the same distance from Earth. The result, she says, should go a long way toward determining whether Hipparcos's direct measurements of Polaris are indeed closer to the mark than Turner's indirect ones.

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