Edinburgh, July 21 : The North Star has thrown astronomers into confusion with some unexpected activity that indicates it is not as constant as previously thought.
Though Polaris had long been known to be a Cepheid variable star, changing in brightness about every four days, in recent decades, astronomers have noticed the star's vibrations were dying away.
Now, according to a report in the Scotsman, astronomers have been stunned to discover the star seems to have come back to life again.
Astronomers 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.
But, 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.
According to Dr Alan Penny from the School of Physics and Astronomy at the University of St Andrews, "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," he added.
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 that 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," he added.
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.