The Fates of Universes
Astrophysicist Michael Shara studies star explosions and the evolution of the universe
On a clear autumn night in Montreal many years ago, a 12-year-old Mike Shara rushed to his backyard with a new telescope under one arm. He set up the tripod in the fallen leaves and pointed the lens straight at the Big Dipper. And—yes—it was true! The middle star of the handle was not one, but two stars circling each other. And from The Golden Book of Astronomy, which he had read cover-to-cover, Shara knew the brighter of those two stars was itself two stars, meaning the middle “star” is actually a trio of stars.
“I was seven when I decided to be an astronomer,” says Shara. “I didn’t have a cowboy phase, I didn’t want to be a paleontologist or a business man or anything else. I only ever wanted to be an astronomer.”
That revelation came when he read about a strange cloud in the Orion constellation where new stars were formed every day. “The idea that stars hadn’t existed forever, or had finite lifetimes, or were still being born was something that I found captivating,” says Shara.
Decades later, Shara is still pondering the evolution of the universe and pointing telescopes at binary stars—stars that come in pairs, like in the Big Dipper. He received a doctoral degree in astrophysics from Tel Aviv University in 1977, and now works as curator of astrophysics at the American Museum of Natural History in New York City. Shara calls himself a stellar astronomer, a designation with which his colleagues readily agree.
“He just keeps coming up with these wondrous results, time after time,” says Brad Schaeffer, an astronomer and astrophysicist at Louisiana State University. Schaeffer’s research examines Shara’s most important contribution to science, the “hibernation model” he proposed in the 1980s.
The hibernation model explains the evolution of binary star systems. When two stars orbit each other and one dies, the dead star becomes a white dwarf. That stellar corpse cannibalizes its companion star, zombie-style, by sucking out its hydrogen. When that hydrogen plunges into the white dwarf, it ignites like an H-bomb. The resulting blast is called a nova.
There are several kinds of novae, which vary in brightness, magnetism, and frequency. In his hibernation model, Shara suggested that these different types of novae may actually be stages in the binary star’s life-cycle, similar to the way an egg, larva, and caterpillar are stages in a butterfly’s life-cycle.
He also guessed, based on how common novae are in other galaxies, that our galaxy must contain 100 times more of these explosive star systems than previously estimated. And if there are so many undiscovered binary systems out there, Shara reasoned, then they must not all be exploding or else they’d be visible already. To explain these missing binary cannibals, he proposed that the white dwarf periodically stops sucking hydrogen from its companion, thus pausing the explosions and dimming the star.
Tim Naylor, an astrophysicist at Exeter University in the United Kingdom, vigorously contested the hibernation model when Shara first proposed it. He doesn’t agree with Shara’s estimate of the density of binary star systems in the galaxy; instead, he calculates that the white dwarfs constantly accumulate hydrogen until they explode.
The question still hasn’t been resolved. Since the 1980s, the estimated number of novae in the galaxy has shot up drastically, but not yet as high as Shara estimated. The problem with the hibernation model is that it’s hard to test, since the timescales involved cover at least hundreds of years, says Schaefer. Nevertheless, he says there is some support for the theory, from Shara’s work as well as his own.
True or not, Shara’s theory has generated much discussion and research. Naylor and Shara may disagree on the details, but the dispute is purely academic and both see it as useful and fun. “I thoroughly enjoyed arguing with Mike,” says Naylor. “He’s a good scientist.”
More recently, Shara has begun scanning the skies for huge stellar explosions called supernovae. In particular he’s looking for pre-supernova Wolf-Rayets, monstrous stars that are 20 times the mass of the sun. Shara estimates that up to 10,000 of these stellar heavyweights are scattered throughout the galaxy, and he wants to locate and map them before they explode. But since they’re mixed in with the other 200 billion stars of the Milky Way, searching for one is like looking for a needle in a few thousand haystacks.
Luckily, he doesn’t have to sort through each galactic haystack star by star. Unlike other types of stars and supernovae, Wolf-Rayet pre-supernovae don’t give off hydrogen. This means that Shara’s team can take sweeping photos of the galaxy, and by simply ignoring the stars that give off hydrogen, they can locate one pre-supernova Wolf-Rayet hidden amongst millions of other stars.
By knowing the location of each pre-supernova Wolf-Rayet star, astronomers can watch for one to erupt—and for once, they’ll actually know what kind of star it is that’s blowing up. When one does go supernova, says Shara, it could test an important prediction of what is called stellar evolution theory. That theory describes a star’s life-cycle from birth to death, and predicts that stars begin dying when they run out of hydrogen, the stellar fuel. Scientists think that after a star’s hydrogen is all used up, the dying star will burn its helium supply instead. Shara hopes his pre-supernovae will help astronomers confirm or disprove whether this stage actually exists.
Shara’s childhood heroes—Galileo, Albert Einstein, and Vera Ruben—were famous for their social and political work as well as their discoveries, and Shara doesn’t believe in spending all his time behind a telescope either. That’s why his public outreach through the Museum of Natural History is so important to him.
Most days, Shara works among the moon rocks, meteorites, and giant “death star” of the Hayden Planetarium. As curator of astrophysics, he coordinates the creation of the planetarium’s 3-D space shows. He also lends his expertise toward creating accurate museum displays and arranging exhibitions, like the museum’s 2002 Einstein exhibit, which discussed the physicist’s impact on science and society.
“I love doing it,” says Shara. “It really is the chance to tell a story and to interact with hundreds of thousands of visitors.”
Through these interactions, Shara is able to share his intense curiosity about the universe—a curiosity that is just as strong as the day he looked eagerly up through his first telescope.
“I want to understand how the universe works,” he says. “I’m interested in where it came from. I want to know how it’s going to evolve into the future…What is its ultimate fate?”