N.C. State astrophysicist studies secrets of dying stars

Stephen Reynolds talks about his research in this video by Brantley Atkinson.

An advanced satellite telescope launched by NASA is giving N.C. State astrophysicist Stephen Reynolds and other researchers their first opportunity to examine interiors of massive exploding stars, known as supernovae.

Until now, scientists have known little about the inner dynamics of supernovae because earlier telescopes were unable to track the projection of radioactive elements after the star’s initial explosion, Reynolds said.

“Traditional satellites have been making beautiful images with low-energy X-rays, but the NuSTAR satellite is the first with focusing optics that can tell us exactly where the high-energy rays are coming from,” said Reynolds, co-author of a paper describing the research.

Materials emitting high-energy X-rays, such as titanium, were undetectable by earlier telescopes as the materials spun away from the center and began to cool, he said.

But NASA’s Nuclear Spectroscopic Telescope Array satellite, developed by researchers from the California Institute of Technology, is helping Reynolds and others “bring the high-energy universe into focus.”

“I work on supernova explosions and their aftermath,” Reynolds added. “We are learning more about what happens inside, as they produce all kinds of radiation, and seeking answers to exactly how a star comes to explode in this way.”

An article by the NuSTAR team in the February issue of the journal Nature included a map following the trail of titanium emitted by a distant star that exploded, and the light of which reached Earth more than 300 years ago. That explosion, which resulted in a remnant known as Cassiopeia A, is a major focus of the NuSTAR project.

Information gleaned from the remnant may help scientists create more accurate computer models of exploding stars, Reynolds said.

What they’re learning

So far, evidence from NuSTAR has led to the discovery that supernovae don’t explode evenly across the circumference.

“Stars are pretty round, but the material we are now seeing, although it is extremely far from the center, is not perfectly round,” he said.

Reynolds and other scientists believe that this may mean that the supernova is slightly asymmetrical in shape, contrary to theories based on existing computer models.

NuSTAR was launched June 13, 2012, and is on a mission to map living stars, collapsed stars and a massive black hole near the center of the Milky Way galaxy.

While small stars may fade away quietly, massive stars – those at least eight times the size of the Earth’s sun – often explode as they expire. These blasts create and disperse elements across the universe. Some of the materials emitted are radioactive, such as titanium, and others are nonradioactive, such as calcium, gold and iron.

Fiona Harrison, the principal investigator of NuSTAR data at the California Institute of Technology in Pasadena, has been studying the supernova Cassiopeia A remnant to learn what a star, which she describes as a spherical ball of gas, looks like as it explodes.

“The fun thing for me is that in almost everything we look at with this telescope, we find something new,” Harrison added. “With Cassiopeia A, people had predicted what we would see, and we saw something different. That’s exciting.”

She said the uneven dispersal of matter may be due to “sloshing” of the star’s internal material before the explosion.

Cassiopeia A is in the Milky Way galaxy and came from a star that exploded about 11,000 light-years from Earth. Light from Cassiopeia A first reached Earth sometime in the 17th century, but there are no firm historical reports of sightings from that period, Reynolds said.

Cassiopeia A produces the strongest radio signal that reaches Earth from outside of Earth’s own solar system. Other supernova remnants are often detected, Reynolds said, but they are from distant galaxies.

While studying supernovae explosions and their aftermath with the NuSTAR telescope, Reynolds and other scientists expect to learn more about the origins of the universe.

NuSTAR is expected to continue sending information to scientists around the world for about 10 years. At about that point, it will to drop back into Earth’s atmosphere, where it will burn up on re-entry, Harrison said.

While NuSTAR remains aloft, Harrison and Reynolds both say they hope to witness another nearby supernova explosion closer to home.

“Once in about every 100 years, a new one occurs in the Milky Way, so we’d have to get pretty lucky to see it,” Harrison said.