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When Shakespeare wrote that Julius Caesar was ‘constant as the northern
star’, he was mistaken: the North Star is variable, changing its brightness
by a few per cent every four days. But Shakespeare’s words will soon ring
true, according to astronomers in Canada. They predict that next year the
North Star will stop being variable.

At 330 light years away, Polaris, the North Star, is the nearest and
brightest of a class of stars known as Cepheids – yellow supergiants that
brighten and dim as they expand and contract periodically (see ‘Starry
signposts to the Universe’, New Å®ÉúСÊÓƵ, 6 June 1992). For most of this
century, the star’s yellow light fluctuated by about 10 per cent, but in
the early 1980s Armando Arellano Ferro in Canada discovered that this
variation had dropped to 5 per cent (Astrophysical Journal, vol 274, p
755).

Now Don Fernie, Karl Kamper and Sara Seager of the University of Toronto
have found that Polaris’s variability has plummeted even further: today the
star’s light varies by only 1 per cent. The astronomers made their
observations of Polaris last year with telescopes at the David Dunlap
Observatory.

Fernie and his colleagues also observed the red shift and blue shift of
light in the star’s spectrum. When a Cepheid shrinks, its light is
red-shifted a little because its surface moves away from us; similarly, the
light is blue-shifted when the star expands and its surface moves towards
us. Fernie and his colleagues found that Polaris’s red and blue shifts have
weakened, showing that the star’s pulsations are decreasing. ‘The plunge
towards zero seems irrevocable,’ say the astronomers. ‘The star seems
destined to stop pulsating in 1994.’

If Polaris does stop varying, it will be the first Cepheid ever observed to
leave the class. This will be a remarkable event, because if Polaris is a
typical Cepheid it has spent tens of thousands of years in this stage of
evolution.

Fernie and his colleagues believe that Polaris is getting bigger and cooler,
because its pulsation period is getting longer. The longer a Cepheid’s
period, the larger the star. Polaris’s pulsation period is currently 3.97
days, but it is growing by 3 seconds each year.

However, the increase in Polaris’s size cannot explain why its pulsations
are declining. So the astronomers admit that Polaris poses a mystery.
Another puzzle is that RT Aurigae, a Cepheid of similar colour and period,
behaves quite differently from Polaris, varying its brightness by more than
a factor of two. The astronomers’ paper, en-titled ‘Goodbye to Polaris the
Cepheid’, will appear in the 20 October issue of The Astrophysical Journal.