30 May 2013

Astrophysicists have found that contrary to decades of orthodoxy, stars with a high sodium content die before reaching the final, spectacular stages of life.

In a study published today in Nature, an international group of researchers led by Dr Simon Campbell of the Monash Centre for Astrophysics (MoCA), used the European Southern Observatory's 'Very Large Telescope' (VLT) to observe NGC 6752, a globular cluster of stars in our galaxy, 13,000 light years from Earth. 

They found that 70 per cent of stars in the tightly bound group fail to reach the final red giant phase. This phase is the last stage of nuclear burning before stars form a planetary nebula, where the gas and dust emitted through copius stellar winds are colourfully illuminated by radiation from the star's naked core.

Dr Campbell said the results were startling because prior to this, it was thought that all low-mass stars, including our Sun, would progress to this final red giant phase.

"We, and other groups in the world, have modeled the entire life-time of these stars and all the models indicate that they pass through this phase. It turns out that the models are not accurately predicting what we have observed here," Dr Campbell said. 

"If it were just a small number of stars not making it to this stage, we could put it down to uncertainties in the observations or in the models, but it is a huge proportion of stars - 70 per cent, all those with high sodium content - that are following this newly observed pattern. It can not be ignored."

Globular clusters, which contain about a million stars, are some of the oldest structures in the Universe, having formed shortly after the Big Bang around 14 billion years ago. These very old stars are highly homogenous in mass and age, and so are widely used as natural laboratories for constraining the computer modelling of stars.

Co-author on the paper and Director of MoCA, Professor John Lattanzio, said that the sodium was a marker and unlikely to be the cause of the early death of the stars.

"Although, at this stage, we don't know the causes, this finding affects our understanding of some of the oldest stars in the Universe - stars that we routinely use to  compare with our computer models to test how accurate they are," Professor Lattanzio said. 

The MoCA group will now observe some of the other 157 globular clusters in the Milky Way to confirm that the pattern holds. They are also working on new theoretical models to try and understand what is causing so many stars to fail to reach a stellar old age.