How do we get from clouds of gas and dust in interstellar space to stars like the Sun? It takes the right triggering event, which can cause such a cloud to collapse under its own gravity, and we’ve generally assumed that the trigger was a supernova. Indeed, one way to check the theory is to look for the radioactive isotope iron 60 (60Fe), which is considered a marker for a supernova as it can only originate in such an event. Early Solar System materials have shown high levels of iron 60, so a supernova has been assumed to have nudged the Solar System into formation.
But Haolan Tang and Nicolas Dauphas, two researchers from the University of Chicago, have produced results than draw this picture into question. Their samples of meteorites were the same materials other researchers had studied, but the Chicago team used different methods to remove impurities from the observation, producing results with, they believe, fewer errors. What they found was that levels of iron 60 were steady — and low — in the early Solar System material. In other words, there is no clear sign of the explosion of a nearby star just before our Sun was born.
These results seem to be backed up by Tang and Dauphas’ study of iron 58, an isotope likewise produced in supernovae. “The two isotopes act like inseparable twins: Once we knew where iron 58 was, we knew iron 60 couldn’t be very far away,” Dauphas explained. And sure enough, the meteorite samples showed little variation in iron 58, leading to the belief that both isotopes were uniformly distributed. In that case, the iron 60 we do find at low levels must have come from iron 60 that had accumulated in the interstellar medium from supernovae in the distant past.
Image: Scientists in the University of Chicago’s Origins Laboratory have published the latest in a series of papers about the origin of the solar system. Infant stars glow reddish-pink in this infrared image of the Serpens star-forming region, captured by NASA’s Spitzer Space Telescope. Four-and-a half billion years ago, the sun may have looked much like one of the baby stars deeply embedded in the cosmic cloud of gas and dust that collapsed to create it. Credit: NASA/JPL-Caltech/L. Cieza (University of Texas at Austin).
Still problematic is the presence of aluminum 26, which implies that a nearby star was a factor. But did it need to be a supernova? The authors explore the alternatives:
…a nearby stellar source for 26Al is still needed. This could have been a passing AGB-star that delivered 26Al and little 60Fe through stellar winds (Wasserburg et al., 2006). However, the probability of encountering an evolved AGB-star in a starforming region is very low (less than 3 in a million; Kastner and Myers, 1994). A massive star is therefore the favored source for 26Al, requiring special circumstances in order to avoid delivering too much 60Fe.
An AGB (asymptotic giant branch) star is a star of intermediate mass (between 0.6 and 10 solar masses) late in its evolution, appearing to the observer as a red giant. The Chicago researchers make the case that a much larger star, perhaps of 30 solar masses or larger) is implicated, one that spreads aluminum 26 as it sheds its outer layers while keeping iron 60 locked up in its interior:
We favor the first scenario of injection by winds from one or several massive stars… because decoupling of 26Al from 60Fe is a natural outcome of the evolution of massive stars and such pollution is expected to occur in star-forming regions. In the parent molecular cloud to the Sun, massive stars evolved rapidly, blew-off 26Al-rich winds, and carved a bubble within the cloud. The Sun formed later and incorporated some of the bubble shell material polluted with 26Al from the first generation of massive stars.
It’s an intriguing notion, though as I read the paper it can only argue that a nearby supernova is not consistent with the iron 60 levels we see when early Solar System materials are properly analyzed. It takes the giant nearby star scenario to explain the aluminum 26 levels, which otherwise imply a supernova, and on that issue I suspect there will be plenty of debate.
I highlight this paper because of the nature of the investigation. The universe carries its history in the materials around us, which here suggest the passage of a giant star through a cloud of gas and dust that, billions of years later, would become a system populated by beings capable of recovering its origins. And if that doesn’t evoke a sense of awe and perspective, it’s hard to see what would. The paper is Tang and Dauphas, “Abundance, distribution, and origin of 60Fe in the solar protoplanetary disk,” Earth and Planetary Science Letters Vol. 359-360, (2012), pp. 248-263 (preprint).