SPP Stars - Polaris

Polaris - An Enigmatic Cepheid

Polaris is probably the best-known star in the sky, despite being only a moderately bright star. Called the "North Star" by many people it owes its popularity to its position in the sky: Polaris lies very close to the north celestial pole, and therefore hangs almost stationary in the sky, marking the north direction every night.

Polaris is the brightest star in the constellation Ursa Minor (the "Little Bear"), it is known to astronomers by its official name alpha Ursa Minoris. The bright stars in each constellation where assigned a Greek letter starting at alpha, roughly in descending order by brightness. "Alpha" therefore usually signifies the brightest star in a constellation, and that is true in the case of Polaris. However, Ursa Minor is a small constellation composed of obscure stars, so being the brightest of the lot doesn't mean that much; Polaris is only the 48th brightest star in the sky.

In contrast to its seemingly unwavering position in the sky, astronomers in the 19th Century discovered that the brightness of Polaris varies in a 3.97 day cycle. Further visual and spectroscopic observations confirmed that it was a classical Cepheid - the brightest in the sky. delta Cephei, the star that gives its named to this class of stars, was discovered 70 years earlier but it is more than a magnitude fainter than Polaris.

Observations of Polaris showed that it was apparently an ordinary classical Cepheid with a brightness variation of close to 15%, but by the 1980s it was clear that the amplitude was shrinking at an ever-increasing rate. By the early 1990s it had shrunk to less than 1% and it appeared that it would soon cease to vary, at least at a detectable level. Unexpectedly, however, the amplitude began to increase again after 2000. As yet there are no good explanations for the drop and subsequent slow recovery in Polaris' amplitude.

Adding to the mystery is the possibility that Polaris has gradually been increasing in brightness over the past few millenia. A study by Engle, et.al in 2004 presented evidence that Polaris has it had more than doubled in brightness. Interpreting ancient records is not straightforward, but more accurate data taken over the course of the 20th Century is consistent with a one magnitude increase in brightness. Again, astronomers are at a loss to explain this additional enigmatic behavior by the brightest Cepheid variable in the sky.

Figure: Change in amplitude of Polaris. Credit: H. Bruntt et al. 2008

Further Reading

Mysteries of the North Star (Sky and Telescope)

The North Star Mysteries (Engle, et.al)

Slacker Astronomy podcast - Polaris