The most metal-poor star ever found is helping astronomers learn about the early universe in which it formed.
Just last month, astronomers announced that the star SDSS J001820.5-093939.2 showed signs of being spawned by early supernovae. As evidence, they noted that it has only 1/300th of the sun's iron, and a distinctive pattern of deficient odd-numbered elements compared to those next to them on the periodic table.
While that research was taking place, a star with orders of magnitude less iron was found, bolstering the study of the first stars—a field that is sometimes refered to as “galactic archeology.”
The early universe lacked elements heavier than lithium. Most elements further along the periodic table have been produced in supernovae and then mixed into subsequent generations of stars. The concentrations of these elements reveal a lot about the explosions that preceded them. Iron is commonly used as a shorthand measure for heavier elements in general.
So to discover a star that has no measurable iron spectral lines at all was truly a surprise. The iron of SMSS J031300.36-670839.3 cannot be more than one ten-millionth of that of the sun. This is a hundredth the concentration of the previous record-breaking star. "We received the news of the most iron-poor star with a great excitement," says Ken'ichi Nomoto of the University of Tokyo, "since this star may be the oldest fossil record and may elucidate the unknown nature of the first stars."
"The impact of stars on the surrounding environment depends critically on their masses when they were born," says team leader Miho N. Ishigaki, also of Tokyo University. "However, direct observational constraints of the first stars' masses are not available since most of them likely died out a long, long time ago."
Although SMSS J031300.36-670839.3 has almost no iron or calcium, it does have quite a lot of carbon. Ishigaki and Nomoto modeled the sort of supernova explosion required to leave the surrounding region with plenty of carbon but little else.
In The Astrophysical Journal, they propose that a star with between 25 and 40 times the mass of the sun exploded in a highly asymmetric fashion, with a huge jet expelling the contents of the star's core in such a way that these elements fell back into the remnant black hole. On the other hand, the outer layers of the star—which would have been rich in carbon—were pushed out in all directions, enriching the surrounding gasses and becoming incorporated into stars such as SMSS J031300.36-670839.3.
"If such supernovae are actually possible," Nomoto says, "the result supports the theoretical prediction that the first stars could be typical massive stars rather than monster-like objects with masses more than several hundred times that of the sun."
There is great debate as to whether or not the first stars were much larger than anything that can exist in the universe today. While his work suggests that may not have always been the case, Ishigaki acknowledges there may have been a range of star sizes at the time.
