Posted: July 21, 2008
The Northern Star, whose vibrations were thought to be dying away, appears to have come to life again, and the vibrations are on the rise, say astronomers presenting their research at the ‘Cool Stars 15’ conference at the University of St Andrews today.
Measurements of the amplitude of oscillation of the Pole Star go back more than a century and show a progressive decrease, originally thought to be associated with the aging nature of the star. But after four and a half years of nearly continuous observations, an international team of astronomers have shown that these stellar vibrations are not only stabilising, but are on the rise, with a 30 percent increase observed between 2003 and 2006.
"It was thought the structure of the star was changing to switch off the vibration,” says Dr Alan Penny of the University of St Andrews. “Yet the team has found that about ten years ago the vibrations started picking up and are now back up at the four percent level."
The amplitude of the Pole Star’s four-day light variation has been steadily decreasing over the last 100 years, but recent observations show this amplitude is now on the rise. Image: Bruntt et al, 2008.
Polaris is a Cepheid variable star, getting brighter and fainter every four days. Because of the strict correlation between their period of variability and luminosity, Cepheids play a vital role as 'standard candles' in determining stellar distances, and thus the size of the Universe. The precise details of their variations, however, are not well understood. The slow decline in variability of Polaris’ oscillation was in itself unusual, as no other Cepheid is known to do this, and the revelation of Polaris’ revival suggests there may be a complex process controlling the star’s behaviour. One possibility, say the astronomers, is that there could be more than one mode of variability in the outer layers of the star.
The discovery resulted from a number of separate measurements made by different researchers around the world.
"It was only through an innovative use of two small relatively unknown telescopes in space and a telescope in Arizona that we were able to discover and follow this star's recovery so accurately," says Penny.
Professor Joel Eaton of Tennessee State University was already making long-term spectroscopic observations of the Pole Star using the ground-based Automated Spectroscopic Telescope (AST) located in Arizona, while Dr Hans Bruntt of the University of Sydney had made photometric measurements from the WIRE star tracker, a now defunct NASA infrared space telescope. Additional photometric measurements were made by Dr Alan Penny of the University of St Andrews using the SMEI instrument on the CORIOLIS satellite, a device predominately employed to watch matter being ejected from the Sun. The astronomers combined their observational efforts, and when the SMEI data were analysed the recovery of Polaris’ oscillations was seen, and could also be traced in the WIRE data. The AST observations was able to confirm the observations of Polaris by watching the change in velocity of the surface of the star as the variation caused it to expand and contract.
But whatever is going on inside Polaris it doesn’t appear to be a simple progression through ‘normal’ stellar evolution, unless this stage of development is more complex than previously thought, conclude the researchers in their paper, which will be published in the August 10 edition of the 'Astrophysical Journal'. A pre-publication paper is available from astro-ph at: http://arxiv.org/abs/0804.3593