Winnipeg, June 14, 2004 – David Turner, an astronomer from Saint-Mary`s University (Halifax, Nova Scotia), has discovered that Polaris - the North Star - is a probable member of a sparse cluster of stars never noticed before. Measurement of the cluster`s distance puts Polaris 33% closer than previously thought. Furthermore, the unusual pulsating properties of this star suggest that it may stop pulsating soon. This work is presented at the Canadian Astronomical Society's annual conference held this week in Winnipeg.
The average person is probably unaware that the North Star, Polaris, pulsates over a four-night time span, producing small variations in its brightness. Polaris is the brightest and nearest member of a class of pulsating stars known as Cepheids, all of which obey a relationship between period of pulsation and intrinsic luminosity that makes them valuable gauges of distance to nearby galaxies.
But Polaris is a peculiar member of the class. Its light variations have always been feeble relative to those of other Cepheids. In the early 1900s they were about a tenth the range of most other Cepheids. Over the past half century they dropped precipitously to about 2% a decade ago, prompting a warning from some astronomers that the star might cease to pulsate prior to the turn of the millennium. That did not happen.
The manner of the star's pulsation is also mysterious. Most Cepheids undergo fundamental mode pulsation --- the natural frequency of vibration in an object --- that produces skewed light curves. But the light changes in Polaris are smoothly sinusoidal, which many astronomers ascribe to pulsation in a higher order harmonic. The star's luminosity inferred from its distance established by the Hipparcos mission permits such a possibility. Yet several questions remain.
Recent monitoring of the star's brightness variations confirms that it continues to pulsate, still at a very low level, but also indicate that its pulsation period is increasing at a rate of about 8 seconds per year. Although small, that is a 100 times larger than what is observed in other Cepheids of the same pulsation period.
Such a rapid period change is explained by stellar evolution. The star is changing from a hot, blue, main-sequence star into a cool, red, supergiant star, a process that takes several hundred thousand years to complete, less than 1% of a star's lifetime. All massive stars do it. Most Cepheids have already done it, and are presently in a slower stage of evolution following the red supergiant phase. Very few stars are ever "caught in the act". Polaris is one of at most five such Cepheids, or "first crossers", in a sample of over 200 belonging to our Galaxy.
The poorly populated star cluster around Polaris allows us to derive an independent estimate of the distance to Polaris, since the luminosities of main-sequence companions are well established from stellar evolutionary models as well as from empirical studies of many other nearby star clusters belonging to our Galaxy's disk.
The distance estimated for the cluster (276 light years) is 33 % smaller than the distance to Polaris obtained by the Hipparcos satellite from its parallax (431 light years), although it is tied to the photometric data from the Hipparcos mission. The cluster distance may be the more correct value, in which case it implies that Polaris pulsates in the fundamental mode, not in an overtone.
Further implications are mere speculation. For example, all recognized "first crossing" Cepheids share similar properties: sinusoidal light variations, rapidly increasing periods relative to other Cepheids, and small pulsation amplitudes. They also seem to possess a more restricted set of luminosities and surface temperatures than other Cepheids. If so, perhaps Polaris is heading towards eventual stability as its pulsations continue to die away.
Professor David Turner
Department of Astronomy and Physics
Phone: (902) 420-5635