In the news this month: a primitive star in our own backyard
The distribution of the light of different colours coming from the remarkable star SDSS J102915+172927 after it has been split up by the X-Shooter instrument on the ESO VLT. CREDIT: ESO/E. Caffau
Our current model of the early universe says that, as it expanded and cooled after the Big Bang, quarks began to coalesce to form protons and neutrons which, when the temperature dropped far enough, began to form simple nuclei. Eventually this material, mainly hydrogen with some helium and trace amounts of lithium, began to clump together, forming the stars and galaxies that we see today. Heavier elements such as carbon, nitrogen and oxygen, in fact pretty much everything that makes up this planet and all the life on it, were created later by processing of this primitive material in stars and supernova explosions. This processing in nuclear fusion reactions produces all the heavier elements that make up the universe. Since less massive stars last longer before running out of fuel, there should be a population of very low mass stars which have been around since the early days of the universe. Such stars would be small, dim, and have an extremely low proportion of elements heavier than hydrogen and helium and, in a paper published in the journal Nature on September 1st, a team led by Elisabetta Caffau at the University of Heidelberg in Germany have found just such a star in the halo of the Milky Way, but with an unusual chemical make-up.
The star, located in the constellation of Leo and known as SDSS J102915+172927, has been found to have the lowest amount of elements heavier than helium of all stars yet studied, a quantity known as metalicity. While a few other primitive stars with very low metalicities have been found, the others all have carbon, nitrogen and oxygen in far greater quantities than would be expected for stars from the very first population. It is thought that low mass stars such as these could only form after the interstellar gas had been enriched by supernova explosions with elements such as carbon and oxygen, since these elements act as a vital cooling agent, reducing the temperature of the gas cloud to the point where gravity can begin to overcome pressure and cause the clumping which eventually leads to stars. This conclusion means that the low abundance of elements including carbon, nitrogen and oxygen in the newly discovered star does not fit with current models of star formation in the early universe.
A further puzzle with this star is the amount of lithium it contains; it's lithium abundance is at least 50 times smaller than that predicted by big bang nucleosynthesis. The likely explanation is that the stellar material must have experienced temperatures >2 million K, the temperature required to destroy lithium. While the chemical composition of this star is something of a challenge to current models of early star formation, along with other examples that should be unearthed in planned surveys, it should provide clues which will help in our understanding of the very first stellar population.
This blog post is a news story from the Jodcast, aired in the October 2011 edition.
Caffau, E., Bonifacio, P., François, P., Sbordone, L., Monaco, L., Spite, M., Spite, F., Ludwig, H., Cayrel, R., Zaggia, S., Hammer, F., Randich, S., Molaro, P., & Hill, V. (2011). An extremely primitive star in the Galactic halo Nature, 477 (7362), 67-69 DOI: 10.1038/nature10377