Twinkle twinkle Northern Star

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SHAKESPEARE'S Julius Caesar once described himself as "constant as the Northern Star/ Of whose true-fix'd and resting quality/ There is no fellow in the firmament".

But far from staying firm and fixed, the North Star has thrown astronomers into confusion with some unexpected activity.

The star is not, it seems, quite as constant as was previously thought.

Polaris had long been known to be a Cepheid variable star, changing in brightness about every four days. But in recent decades astronomers have noticed the star's vibrations were dying away.

Now they have been stunned to discover the star seems to have come back to life again.

The discovery will be announced to 350 international delegates today on the first day of the Cool Stars 15 conference at the University of St Andrews.

Dr Alan Penny from the School of Physics and Astronomy at the University of St Andrews will tell the conference that he was part of an international team of astronomers who observed that vibrations in the Pole star, which had been fading away to almost nothing over the past hundred years, have recovered and are now increasing.

The scientists are at a loss to explain the sudden change.

The astronomers were watching Polaris in the expectation that they would catch the star switching off its vibrations completely when they made the surprising observation of their revival.

Dr Penny said: "One hundred years ago Polaris varied by 10 per cent, but over the last century the variations became smaller and smaller until ten years ago it only varied by 2 per cent.

"It was thought the structure of the star was changing to switch off the vibration. Yet the team has found that about ten years ago the vibrations started picking up and are now back up at the 4 per cent level."

Cepheid stars are generally known to get brighter and fainter every four days, but the details of their variations are not well understood.

Dr Penny said he was pleased the existing theory had been proved wrong, because it would further astronomical understanding.

"Now we know it's doing this we will watch it for another 100 years and see what it does," he said. "We have found something new that we need to understand. That means we can make progress. We are very excited when we are proved wrong."

The slow decline in the vibrations in the star was in itself unusual, as no other Cepheid is known to have done this.

Astronomers thought Polaris was ageing and its structure was changing so that it was no longer unstable. The scientists were following its progress to learn about how stars age.

But now Polaris has started vibrating again, this explanation of the ageing process seems unlikely.

The scientists think a more complex process may be at work, which will require extra studying before it is understood.


&#149 The North, or Northern Star, is the prominent pole star that lies closest in the sky to the north celestial pole, and which appears directly overhead to an observer at the Earth's North Pole.

&#149 The current North Star is Polaris, which is 430 light-years from Earth.

&#149 A common method for locating Polaris in the sky is to use the "pointer stars" which are the two stars furthest from the "handle" of the Big Dipper.

&#149 The North Star has historically been used for navigation, both to find north and to determine latitude.

&149 Currently, there is no "South Star" as useful as Polaris; the faint star Octantis is closest to the south celestial pole. However, the constellation Crux, the Southern Cross, points towards the pole.

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