Anna O’Grady remembers the day she decided to make a career out of stargazing.
“When I was in Grade 8, my mother bought a book for me called ‘The Atlas of the Universe,’ which was sort of like a coffee-table, pop science book on astronomy,” the Kilbride native said in a recent phone interview.
“I think I read the entire book in a day, and so that was when I kind of knew that was what I wanted to do.”
O’Grady, who attended Bishops College high school in St. John’s, says she was always interested in science, about the way things work.
“I always had an interest in the night sky, and the planets in particular.”
At Memorial University, she took advantage of a handful of astronomy courses offered through the geology and physics departments.
“Those were really, really excellent and I learned a lot from those.”
That’s also where she met professors Ivan Booth and Hari Kunduri, who specialize in black holes.
After graduating in 2016, O’Grady went to the University of Toronto to study astronomy at the David A. Dunlap Department of Astronomy & Astrophysics and the Dunlap Institute for Astronomy & Astrophysics.
In October 2020, still a PhD student, she was lead author of a paper in The Astrophysical Journal (https://ui.adsabs.harvard.edu/abs/2020ApJ...901..135O/abstract) that may well shake up what’s known about the nature of exploding stars.
“This was extremely careful and clever detective work on Anna’s part,” Bryan Gaensler, one of her supervisors and a contributor to the paper, told the Dunlop Institutes newsletter. “She started by studying 1.5 million stars, and was able to identify 12 extremely unusual stars that are hard to explain. I’m excited about what more we can learn about this strange population from additional data.”
Saying the stars are hard to explain is an understatement, but O’Grady does her best to put it in layman’s terms.
She and her colleagues were originally looking for something called Thorne–Żytkow objects (TZOs) in two galaxies on the fringes of the Milky Way called the Magellanic Clouds.
A TZO, she explains, is a star within a star — “almost like a turducken of a star.”
The outside part is a massive red star. But the core is a neutron star, which is what’s leftover after a star explodes (a supernova).
If a star is big enough, the explosion creates a black hole — an object so compressed and dense, even light can’t escape. Neutron stars, which are not quite as dense, are the result of a smaller explosion.
Do TZOs actually exist?
“That is something of an open question,” O’Grady says.
The theory’s been around for decades, but the discovery of an object dubbed HV 2112 in the Small Megallenic Cloud in 2014 is considered a very strong candidate of being the real deal.
O’Grady went looking for more. But she found something else.
In theory, TZOs have to have a minimum solar mass. O’Grady found 11 similar objects, but realized they were too small to be TZOs.
Instead, she realized they were super-AGB stars.
Without belabouring the definition, super asymptotical giant branch stars are essentially a missing link between smaller stars that eventually die out (like our own sun) and those that explode into supernovas.
“We were not searching for these stars at all, but sometimes science throws curve balls at you,” O’Grady said.
Before O’Grady’s paper, there was only one known candidate for a super-AGB. Now there are 12.
O’Grady says she’s received a lot of positive feedback from colleagues, but time will tell how her discovery stands up to scrutiny as others follow up on her findings.
“There’s always this kind of passing the ball back and forth, back and forth, between observation and theory in science.”
Either way, her research contributes to a broader understanding of what the universe is made of and how it works
“By finding what we think are these stars, that fit on the tipping point between stars that don’t explode and stars that do explode … it will allow us to get a better understanding about which stars explode.”
When you’re talking about galaxies, you have to think big — and far.
“It’s hard to prove beyond the shadow of a doubt a lot of things in astrophysics, because we can’t go out and check in person,” quips U of T astronomy student Anna O’Grady.
O’Grady has been poking round in the Magellanic Clouds, two galaxies that orbit our own Milky Way. On average, the stars there are about 50,000 parsecs (50 kiloparsecs) away from Earth.
One parsec is about 3.26 light-years — the distance that light travels in a year.
So, the light O’Grady is looking at was generated about 160,000 years ago.
How long ago was that?
Well, Neanderthals still roamed the Earth.
Humans were wearing clothes and had started adding seafood to our diets, but there was no evidence people were using symbols yet, let alone written language.
What about the brightest star in the night sky?
That would be Sirius, the Dog Star. It’s 8.6 light-years away.
When you look at that star today, the light you see left its source when Barack Obama was still U.S. president and Nelson Mandela was still alive.
Peter Jackson, Local Journalism Initiative Reporter, The Telegram