This story caught my attention recently, it pertains to the earliest stars in the Universe. These earliest stars have not yet been directly observed, this story is about observations of unusual stars in our own Galaxy which are believed to be formed from the first generation of stars.
In the early Universe, the only elements created were hydrogen and helium. All the elements heavier than hydrogen and helium have been formed in the interior of stars, and more recent generations of stars contain these heavier elements along with hydrogen and helium formed in the early Universe. As the earliest stars would have been formed from only hydrogen and helium, and because of the absence of an effect called “line blanketing”, this earliest generation of stars could form with masses much greater than subsequent generations. Theoreticians believe that masses beyond 100 solar masses were possible in the first generation of stars.
Theoretical models also suggest that such super-massive stars would have ended their lives in something called a pair-instabiliuty supernova, which is different from the supernova which signals the end of the high mass stars we see around us today. In a pair-instabiliuty supernova, the models suggest that no neutron star or black hole is left at the centre, instead all of the material in the exploding star is sent back into the interstellar medium.
Researchers using the Japanese Subaru telescope in Hawaii have been taking spectra of stars in our Galaxy which show a particularly low level of iron, a level which is about 1,000 times less than in our Sun. Such a low iron level suggests that the stars belong to an earlier generation than our Sun, which is believed to be a third-generation star. The stars they have been observing are not first generation stars, but probably second generation. They have found one star, named SDSS J0018-0939 (it was found to be low in iron by the Sloan Digital Sky Survey) to have a very unusual spectrum. The ratio of the abundance of various elements in the star’s spectrum suggests that it could have been formed from a pair-instabiliuty supernova, and thus be the direct descendent of a first generation star.
The original paper, entitled “A chemical signature of first-generation very massive stars” can be found here.