Mysteries of the North Star

star trails facing north
Stars in the northern sky appear to circle around Polaris as Earth turns, in this three-hour time exposure made with a simple fixed camera on a tripod. Although it has been much studied, Polaris hides its secrets well.
© 2001 Edwin L. Aguirre and Imelda B. Joson.
William Shakespeare couldn’t have been more wrong. In his 1599 play Julius Caesar, the English bard had the Roman ruler declaim, "I am as constant as the northern star." Since then astronomers have learned not only that Polaris is a variable star (in fact it’s the closest and brightest Cepheid variable), but also that its period and amplitude are changing. Now, astronomers at Villanova University in Pennsylvania say that even the variation in Polaris's variability is varying — and that, moreover, Polaris's average brightness is on a slow increase.

The pulsations of Polaris are hardly conspicuous. The star varies in brightness by just a few percent in a 3.97-day cycle. Surprisingly, in the mid-1980s astronomers found that the amplitude of the pulsations had been diminishing for decades, while the period of the pulsations was increasing by some 3 seconds per year. This led astronomers to believe that Polaris might actually stop varying altogether early in the 21st century, vindicating Shakespeare after all (Sky & Telescope, January 1999, page 18).

But according to Edward F. Guinan and his colleagues, the amplitude of Polaris may now be increasing again: in the mid-1990s it had fallen to just 0.020 magnitude, but new measurements carried out this year indicate a range of 0.038 magnitude per cycle. What’s more, an analysis of all available photoelectric brightness measurements from the last century suggests that the average luminosity of this "constant" star has increased by some 7 percent, from magnitude 2.12 in the early 1900s to 1.95 in 2004. Guinan's group presented these results yesterday at the 204th meeting of the American Astronomical Society in Denver, Colorado.

There are even hints that Polaris's slow brightening trend may date back many centuries. Both Claudius Ptolemy (in the 1st century AD) and Al-Sûfi (10th century) listed Polaris as a 3rd-magnitude star. William Herschel (late 18th century) measured it to be magnitude 2.4, and around 1900 it was cataloged at 2.2. Such a large, long-term brightness increase would be hard to explain theoretically, as Guinan observes; "It should not be getting that bright that quickly." But astronomy historians have long pointed out that old magnitude estimates are notoriously unreliable.

According to stellar evolution models, Polaris is evolving from a blue supergiant into a bloated red supergiant. (It's currently midway along — a yellow-white supergiant of spectral type F.) During this multimillion-year process a star wanders around the Hertzsprung-Russell diagram (a plot of stellar temperature versus light output), repeatedly crossing an area called the instability strip. Stars in this region of the diagram undergo Cepheid-like pulsations. Some of these stars pulse in the "fundamental" mode; others pulse in a more complex "overtone" mode, comparable to the vibration modes of a violin string. However, there’s little agreement on the precise evolutionary stage that Polaris is currently in, or on the pulsation mode that it exhibits.

According to David G. Turner (Saint Mary’s University, Halifax, Canada), Polaris is making its first cross of the instability strip and exhibiting fundamental-mode pulsations. But Guinan and his Villanova colleague Scott G. Engle think the star is in overtone mode and in its second crossing, temporarily getting bluer and hotter. Indeed, data from the International Ultraviolet Explorer suggests that the North Star's average surface temperature has increased some 25° in the past three decades. "My guess is that Polaris is getting close to the left [hot] edge of the instability strip," says Guinan. This close to the border between stability and instability, a star’s behavior might become quite erratic.

Better knowledge of Polaris's luminosity, mass, size, and age would certainly help. These properties would be easier to determine if Polaris were part of a star cluster, in which all stars are at about the same distance and have the same age. At the AAS meeting Turner presented circumstantial evidence that the Pole Star is indeed a member of a very sparse moving group of main-sequence stars in the same part of the sky. Moreover, both Polaris and these other stars exhibit the same motion through space as the Pleiades, suggesting that they are all part of the extended Pleiades Moving Cluster.

But this is inconclusive. If Turner is right, Polaris should be closer to us than astronomers think: his sparse association of stars has an average distance of 306 light-years, while the European satellite Hipparcos determined Polaris's distance as 430 light-years with not nearly so large a margin of error. Still, Turner says "the smaller distance is probably correct. The Hipparcos error may have been caused by Polaris being a multiple system." If Polaris is indeed closer than
originally thought, it’s also less luminous. That would of course change ideas about its evolutionary stage and pulsation mode.

Guinan warns that the mass of Polaris is also poorly constrained (but is probably between 5 and 6 solar masses). "It’s all kind of fuzzy," he says. "I don’t know what the story is." One thing is for sure, though: few stars in the sky are less constant than the northern star. Sorry, Shakespeare.

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Govert Schilling

About Govert Schilling

Contributing Editor Govert Schilling lives in Amersfoort, The Netherlands, but frequently travels the world in search of great astronomy stories.
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