The Star That Time Forgot
No supernova explosion answering to this description had ever been witnessed, and the idea remained a mere twinkling in the theorists' eyes. That is, it did until Avishay Gal-Yam, an astrophysicist at the Weizmann Institute of Science in Rehovot, Israel, and his collaborators started looking at SN 2007bi. The more they compared the data with various supernova models, the more they became convinced that the pair-instability model was the answer to the conundrum this explosion posed. "Only a pair-instability supernova can produce that much radioactive nickel," says Gal-Yam. With the model, they could even calculate how big the exploding star had been: a whopping 300 times the mass of the sun (Nature, vol 462, p 624).
Problem solved? Not a bit of it. The finding came with a massive sting in its tail: according to all our theories and all our observations, stars that big simply should not exist.
At least, they should not exist in the kind of universe we see around us today. In the decades since the pair-instability model was born, theory and some comprehensive sweeps of the night sky have combined to show that the composition of the modern cosmos prevents stars reaching such huge sizes. The presence of appreciable quantities of what astronomers call metals - elements heavier than hydrogen and helium - causes gas clouds to collapse speedily into "pocket-sized" stars. That is why most stars today are celestial minnows, containing less mass than our sun. The absolute upper limit on a modern star, theory and observations of our galaxy agree, lies at about 150 solar masses. A monster of 300 solar masses is an implausible interloper into this settled scene.
Things were different in early cosmic times, some 13 billion years ago in the pristine universe immediately after the big bang. Back then, solar giants ruled the roost. Only hydrogen, helium and trace amounts of lithium were floating around the cosmos, and much bigger quantities of these elements had to accumulate before they fell under the spell of gravity and were pulled together to form a star. As a result, the first stars in the universe were humongous, containing anything up to several hundred solar masses.