Search for the stellar smoking gun

Search for the stellar smoking gun

A forensic space scientist at the University of Manitoba is attempting to solve a stellar mystery
March 16, 2011
The remnant of a supernova explosion, the central
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The remnant of a supernova explosion, the central bright cloud of high-energy electrons is surrounded by a shell of hot gas and high-energy particles.
NASA/CXC/U. Manitoba/H. Matheson & S. Safi-Harb

When big stars go boom, they are like really, really big guns — they leave behind a telltale smoke signal and a path of destruction. Understanding the forensic evidence from just a puff of smoke might sound near impossible, but for Samar Safi-Harb, who holds the Canada Research Chair in Supernova Astrophysics, it’s just another day at the office.

Originally from Beirut, Lebanon, Safi-Harb now studies the remnants of exploding stars, or supernovas, at the University of Manitoba in Winnipeg. The clues she uncovers might unlock the mysteries of why and when a star blew up, how massive it was, how it manifests itself once exploded and how fast the explosive cloud of material is expanding and energizing its surroundings.

One of Safi-Harb’s current investigations focuses on neutron stars (also known as pulsars), which are some of the weirdest objects in the universe. These superhot, superdense, supermagnetic bodies are the leftover cores of stars that exploded in a supernova. Because neutron stars are unlike anything ever seen on Earth, astronomers like Safi-Harb can learn about some of the strangest properties of universal matter.

But one of the most mysterious parts of Safi-Harb’s investigation is that it appears as though many neutron stars were created without an explosion, because no one can find the smoking gun, the remnant shell of the supernova.

What astronomers have found is the oddly formed neutron star wind nebula, created when the neutron star’s extremely powerful magnetic field pushes, pulls and excites the dust and particles in the space far beyond the star. Without the original evidence of the explosion nebula (the supernova remnant), however, astronomers don’t know the original nature of the star or the cause of the explosion.

By using one of the world’s biggest cosmic magnifying glasses — NASA’s Chandra X-ray Observatory — Safi-Harb has recently made a stellar breakthrough.

“For many years, we’ve been studying this one pulsar wind nebula that we used to think didn’t have an [explosion nebula] shell, just like the famous Crab nebula” she says, “but by accumulating a lot of observations with Chandra, we actually did find evidence of a shell surrounding that object for the first time.”

A supernova remnant shell of hot gas surrounds a
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A supernova remnant shell of hot gas surrounds a bright pulsar wind nebula. This image was displayed at the Ontario Science Centre in 2009.
NASA/CXC/U. H. Kumar & S. Safi-Harb

With the first faint evidence of the life of a star before its death, Safi-Harb and other astronomers are hoping they can piece together its final moments and understand why it seemed to explode with such a little puff. Safi-Harb believes that the star may have exploded in an isolated environment where there was nothing to stop the original nebula from scattering evidence into the farthest reaches of space.

For Safi-Harb, the path to forensic space scientist has been unexpected. Originally a pre-med physics student at the American University of Beirut, she never took an astronomy course during her undergrad years, but when she travelled to the United States in 1991 to do her graduate studies in high-energy physics, a chance encounter with astrophysics during a summer research project on a pulsar got her hooked on the ultimate in high-energy astrophysics: exploding stars.

“I knew that physics was for me because I preferred to derive more than memorize. In medicine, you memorize many things, but in physics, you get to derive equations that explain the universe,” says Safi-Harb. “Physics satisfies our curiosity and helps us know how things work, and astrophysics grabs the imagination of many people, including my own, because you get to investigate exotic phenomena like supernovas and pulsars.”

Safi-Harb came to the University of Manitoba in 2000 as an NSERC University Faculty Award Fellow with the goal to launch an astronomy program in the physics and astronomy department.

And, like every good Sherlock Holmes, Safi-Harb has a Watson, in the form of Roland Kothes, a research astronomer at the Dominion Radio Astrophysical Observatory in Penticton, B.C., and an expert in radio astronomy. He is helping Safi-Harb find valuable — and totally different — clues to the same mystery with his expertise as a radio astronomer.

“Samar realized that you need to include other wavelengths, so she approached me to collaborate with my radio observations,” explains Kothes. “And this multi-wavelength view of objects gives you more insight into what these things are about.” Kothes says that radio astronomers can see the history of the object, while X-ray astronomers, like Safi-Harb, can see what is happening right now. By comparing the observations, the duo can learn more about how pulsar wind nebulae evolve.

With this kind of collaboration, it is just a matter of time until the mysterious case of the explosionless pulsars is solved.