Levels of phosphorus 100 times higher than in other parts of the Milky Way have been found in the remains of a supernova called Casseiopeia A, providing strong evidence that the phosphorus was created by the stellar explosion, reports a new study published Thursday online in Science.
Phosphorus is one of the six essential elements for life. The others are hydrogen, carbon, nitrogen, oxygen and sulphur. Hydrogen is the most abundant element in the universe and makes up most of the gas in young stars like our sun. The other essential elements, other than phosphorus, had been measured in and were confirmed to originate from nuclear processes inside stars, including supernovas.
Theoretically, phosphorus should also come from nuclear reactions inside a star, said Dae-Sik Moon, a professor of astronomy and astrophysics at the University of Toronto who co-authored the paper.
"But the abundance of phosphorus was never actually measured … inside a star," he added.
Stars such as the sun are powered by the the nuclear fusion of hydrogen into helium. Once the star starts to run out of hydrogen fuel, its core collapses and it expands into a red giant. Red giants are fuelled by the fusion of helium into carbon. At that point, if the star is massive enough, it goes through more collapses and rounds of fusion until almost everything has fused into iron. At that point, it explodes in a supernova.
Phosphorus had previously been detected in supernovas, but scientists didn't know for sure if it had been made there because they weren't able to tell how much there was relative to the rest of space.
One difficulty was that levels of phosphorus in the Milky Way are generally a lot lower than that of the other essential elements — 50 to 1,900 times lower — making them hard to measure.
However, the international team that Moon was a part of was able to make a measurement using an instrument that he developed called the TripleSpec near-infrared spectrograph.
The device was attached to the Palomar 5-meter Hale telescope in California. The telescope was pointed at Casseiopeia A, which was thought to have exploded about 300 years ago, feeding light from the supernova remnant into the spectrograph.
Each element in an object such as a supernova remnant gives off light with a fingerprint of characteristic colours. Moon's spectrograph is able to detect a wide range of colours, allowing it to compare a wide range of elements.
Compared to iron
The device was able to measure levels of phosphorus by comparing them to levels of iron. Iron is the main product of nuclear reactions in the supernova, and its abundance is well known, allowing researchers to figure out levels of other elements by comparing them to iron. Using it for comparison with phosphorus was particularly convenient because their fingerprints have colours that are close to one another, Moon said.
The study found the ratio of phosphorus to iron was 100 times higher in the remains of the supernova than in the rest of the galaxy.
"So it's a lot," Moon said.
That confirms that phosphorus is made in the supernova, the paper said. It is also consistent with computer models generated by co-authors of the paper at Harvard University predicting what the levels should be.
The team was led by Bon-Chul Koo, a researcher at Seoul National University in Korea and Moon's former master's degree supervisor.
The research was funded in part by the Natural Sciences and Engineering Research Council of Canada.