If you are distressed by the death of stars like Prince or Bowie taken from us too young, I have some bad news for you. The same thing is happening to a different sort of star 7,200 light years from us in the globular cluster M4. It even seems the causes might have a little in common.
Stars of the astronomical type spend most of their lives fueling themselves by turning hydrogen to helium. Eventually, the hydrogen stocks run low and the star contracts until it is so dense that helium atoms can fuse together to become carbon. This causes the star to expand outwards again until it far exceeds its original size, becoming known as a red giant.
However, Ben Maclean, a PhD student at Monash University, Melbourne, told IFLScience, this is only the first red giant phase. “In the second stars burn hydrogen and helium in shells around a carbon/oxygen core,” Maclean said.
At least that is what is supposed to happen. In a paper in the Monthly Notices of the Royal Astronomical Society Maclean reports that roughly half the stars in M4 simply skip the second red giant phase shrinking away to become white dwarfs. Unless they have a companion that can turn them into supernovae, this makes them effectively dead.
“The good thing about globular clusters can get a large proportion of stars in every phase,” Maclean told IFLScience. Maclean counted the number of red giants in each phase in M4 and observed that the ratio between types was skewed. When he investigated further he found all the second phase red giants had low to middling concentrations of sodium. Somehow the high sodium stars are missing out on the last phase of life – a discovery that will no doubt be familiar to doctors warning patients about too much salt in the diet.
One of Maclean's co-authors, Dr Simon Campbell of the Max Plank Institute, previously observed sodium enriched stars in the cluster NGC 6752 also skip this phase, but Maclean told IFLScience the two observations are very different. At this point in stars' lives sodium and helium enrichment go together. In NGC 6752 Campbell found that stars with more than 50 percent helium enrichment, relative to the average, were skipping the last stage of life. In M4 “even stars that were hardly enriched at all, just 1 percent” did the same thing.
The finding was so unexpected Maclean and his co-authors have not explained the cause.
The high efficiency and resolution multi-element spectrograph (HERMES) instrument newly attached to the Anglo Australian Telescope has made it possible to make observations of chemical abundance of thousands of stars at once. AAO/AAT
New instruments are making observations like these easier. Consequently, opportunities to study other globular clusters in the same way are opening up. Maclean has started observing other clusters seeking second stage red giants.
“We don't yet have enough of a sample size to see why this is happening,” he told IFLScience. If it is seen in other clusters something universal is going on we need to explain. If it is not, then M4 is not the typical cluster we thought it was.”