The Pole Star Grows Up--Perhaps
On any list of useful stars, Polaris would have to rank second only to our sun. North Star, Pole Star, by whatever name, it can be found easily by a sky-watching novice seeking direction. Alaskans have even claimed its image on the state flag and in our official song: "The great North Star, with its steady light..." Ah, but its light hasn't been so steady.
Supergiant Polaris lies far from Earth, so distant that its light takes from 500 to 800 years to reach us. The glittering point we could see this spring, marked by the Big Dipper's pointer stars, showed light that perhaps set off on its journey while the first telescopes were being crafted in European workshops.
That light varies in intensity, partly because Polaris is not a solitary star. It's what astronomers call a binary, a stellar duet. Polaris the bright has a small, invisible companion. The twinned swings of these two bodies orbiting about one another puts some shifts in the starlight received here from the Pole Star.
Even more alteration in its light comes from the nature of the star itself. It's classified as a Cepheid variable, a pulsating star. Named after the constellation Cepheus, where such stars were first labeled, these stars pulse with a frequency related to their intrinsic brightness.
Polaris pulses on a four-day cycle. At least, its pulses came regularly every four days from 1896 to at least 1956; faithful star-watchers recorded its steady beat over that period. In 1987, when astronomers at the University of British Columbia again took up the watch, they found that things had changed with Polaris.
Then-student Nadine Dinshaw used a comparatively small (16-inch) telescope, but she had a sophisticated electronic spectrometer that could make precise records of the star's emitted light. Analysis of the 237 spectra captured over an eight-month span indicated that the four-day pulsation period is slowly lengthening.
Dinshaw and others at the university checked the spectral records carefully and hunted through data on Polaris gathered at other times and places. They confirmed that not only is the pulsation slowing, its amplitude is shrinking, and has been since at least 1980. The pulsation-related change in brightness is now barely discernible through a small telescope.
This is exactly what theory had predicted for this class of variable stars. The UBC scientists make an analogy with bells: a bell's pitch gets lower as the bell gets larger. The "ringing," or vibrations, of a pulsating star should gradually slow as the star ages, cools, and expands. Once whatever internal activity driving the pulsation cycle has died away, the vibration should slow, diminish, and eventually stop; the bell is no longer being rung.
One problem with this tidy theory is that it also predicts that a Cepheid variable spends at least 40,000 years as an unstable, pulsating star before undergoing the 10- to 20-year gradual change to steadiness. The odds against catching any variable star actually outgrowing its moody adolescence are truly astronomical, and it seems more improbable still that it would be well-known Polaris that provides the opportunity. Yet that seems to be just what's happening. The pulsation amplitude continues to shrink, and the UBC astronomers believe it will reach zero within the next few years at most.
Apparently the star that has guided so many travelers has granted one more favor to humankind, by showing that an astrophysical theory does work in nature. Then too, before long, Alaska's state song will be more accurate: the great North Star will indeed shine with a steady light.
