Researchers, including a Rutgers astronomer, reveal new insights into a star's death in its final moments
A team of scientists, including Rutgers-New Brunswick's Distinguished Professor Jack Hughes, has made a crucial discovery about the final hours of a massive star before it exploded.
Using data from NASA's space telescope, the Chandra X-ray Observatory, the researchers found that the star's insides violently shifted just hours before it blew apart, creating the famous Cassiopeia A supernova remnant. The observations are reported in the Astrophysical Journal.
"Chandra observed the Cassiopeia A supernova remnant soon after launch in July 1999 and several remarkable findings resulted from those early data," said Hughes, chair of the Department of Physics and Astronomy in the School of Arts and Sciences at Rutgers, and an author of the study. "Now more than 25 years later, after more than a million seconds of Chandra observations, we have made a new fundamental discovery about the stellar explosion that produced this remnant."
Cassiopeia A, or Cas A, was one of the first targets observed by Chandra after its launch in 1999, and an international team of scientists has played a key role in analyzing its data ever since.
"It seems like each time we closely look at Chandra data of Cas A, we learn something new and exciting," said Toshiki Sato of Meiji University in Japan, who led the study. "Now we've taken that invaluable X-ray data, combined it with powerful computer models, and found something extraordinary."
Chandra is a space telescope that looks for X-rays throughout the universe. Much the way regular telescopes allow the observation of stars and planets using visible light, Chandra helps scientists see high-temperature and high-energy objects in space by detecting X-rays those objects emit. Chandra has revealed the existence of exploding stars, black holes and giant clouds of gas to researchers since its launch.
Massive stars such as Cas A form onion-like layers of different elements in their interiors through nuclear reactions as they age.
These layers include hydrogen, helium, carbon and heavier elements such as silicon and iron. When the iron core becomes too heavy, the star collapses and explodes in a supernova.
But the study shows something surprising happened just before Cas A exploded.
"Only several hours before its demise, the progenitor star underwent a violent process of shell mergers within its interior that changed the composition of the star in ways that we can now clearly see in the X-ray emitting debris," Hughes said. "This shell merger process occurs in only certain stars, allowing us to further refine our understanding of the long-dead star whose explosion produced the Cassiopeia A supernova remnant."
The upheaval not only caused material rich in silicon to travel outwards; it also forced material rich in neon to travel inwards. In the Chandra X-ray data, the team found clear traces of these outward silicon flows and inward neon flows in the remains of Cas A's supernova remnants. Small regions rich in silicon but poor in neon are located near regions rich in neon and poor in silicon.
Scientists said this discovery helps explain why Cas A looks lopsided instead of symmetrical and why the leftover core, now a neutron star, is moving so fast.
"Perhaps the most important effect of this change in the star's structure is that it may have helped trigger the explosion itself," said Hiroyuki Uchida, also of Kyoto University and a coauthor of the study. "Such final internal activity of a star may change its fate-whether it will shine as a supernova or not."
