Globular clusters are vast cities of tens of thousands of stars, traditionally thought to have been formed from a single interstellar cloud at roughly the same time. But Omega Centauri is different. As viewed by Hubble, this southern cluster (15,000 light years away in the direction of the constellation Centaurus) contains two separate stellar populations. Its blue stars, about one quarter of the total, are well outnumbered by a second hydrogen-burning population of redder stars.
Now the European Southern Observatory’s Very Large Telescope has collected data that show the blue stars, contrary to expectation, are metal-rich when compared to their red counterparts, meaning they include elements heavier than hydrogen and helium. Astronomers call elements heavier than helium ‘metals’ — the Sun, for example, is made up of 70 percent hydrogen and 28 percent helium, with the remaining two percent being classed as metals.
Current theories of star formation suggest that as metallicity increases, stars become redder. According to the ESO team, the only way to explain the oddity of metal-rich blue stars is to assume that they have a far higher helium content than their red counterparts. That would make them the most helium-rich stars ever found, and raises a major question: with the present abundance of helium in the Milky Way at 28 percent, how did this globular cluster produce stars with a 39 percent abundance?
Image: Omega Centauri, largest of the 160 globular clusters in the Mily Way. In the center of such a cluster, stars are packed more than 10,000 times more closely than in the neighborhood of our Sun. Credit: P. Seitzer (U. Michigan).
ESO’s Luigi Bedin has a possible solution:
“The scenario we presently favour is one in which the high helium content originates from material ejected during the supernovae explosions of massive stars. It is possible that the total mass of Omega Centauri was just right to allow the material expelled by high-mass supernovae to escape, while the matter from explosions of stars with about 10-12 times the mass of the Sun was retained.”
Omega Centauri, then, would have seen two generations of stars. The first produced the redder stars, whose most massive members exploded as supernovae within tens of millions of years. The second population of blue stars then formed from this helium-rich environment. But the real issue raised by Omega Centauri is broader: why did this globular cluster produce extremely helium-rich stars, whereas all other known clusters did not?
Centauri Dreams take: another suggestion that globular cluster star formation is poorly understood comes from a Hubble study that scanned the globular star cluster 47 Tucanae (located 15,000 light-years away in the southern constellation Tucana) using the transit method to search for planets. As reported in 2000, the team expected to find about 17 hot Jupiter-class planets, but they found none at all. Is the deficiency of heavier elements found in most globular clusters an indication that planets can only form in a metal-rich universe? And if so, what does this imply about Omega Centauri?
For more on the Omega Centauri conundrum, see Piotto, G., Villanova, S. Begin, L. et al., “Metallicities on the Double Main Sequence of omega Centauri Imply Large Helium Enhancement,” now available on the ArXiv site and also in the March 10 issue of the Astrophysical Journal, Vol. 621, p. 777. ESO’s press release is here.