
Courtesy of Chandra X-ray Observatory Center/NASA
Nearly 35 years ago, Museum curator Michael Shara made a prediction that became of one of his most cited—and most contested—studies to date: that colossal hydrogen bombs called novae go through a very long-term life cycle after erupting, fading to obscurity for hundreds of thousands of years and then building back up to become full-fledged novae once again.
Now, with computing power unheard of when Shara first proposed what is known as the “hibernation scenario,” he and colleagues are the first to fully model the work and incorporate the feedback factors now known to control these systems. The new study, published in the journal Nature Astronomy, supports the original prediction while illuminating new details.
“We’ve now quantified the suggestion from decades ago that most of these systems are deeply hibernating, waiting to wake up, and we haven’t yet identified them,” Shara said.
Novae, or cataclysmic binary systems, occur when a star like our Sun—a red dwarf—is cannibalized by a white dwarf, a dead star. The white dwarf builds up a critical layer of hydrogen that it steals from the red dwarf, and that hydrogen explodes as a gigantic bomb, producing a burst of light up to 1 million times brighter than the Sun.
ASHLEY PAGNOTTA (Davis Postdoctoral Fellow, Department of Astrophysics): In astronomy, things either happen almost instantaneously, or over tens of thousands, millions of years. But one of the things that we’re starting to learn is that the things that we previously thought were constant over a hundred years, we’re seeing changes over decades—essentially the course of a human lifetime.
My name is Ashley Pagnotta. I am a Kathryn Davis postdoctoral fellow at the Museum of Natural History. I research stars that explode and I teach future teachers.
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PAGNOTTA: One of the differences between astro collections and natural history collections is that because ours live on hard drives, instead of in cabinets, they take up a whole lot less room. Theoretical and computational astrophysicists run these enormous simulations. So their collections are the outputs of those simulations.
For me, because I’m an observational astronomer, my collections are images.
Now, when we take pictures of the sky, we use basically digital cameras. Before we had that technology, we used glass plates that were coated with a photo emulsion. They would be put onto the end of the telescope, exposed like film, and then developed. And the advantage of this was that they, for the first time, had an objective record of what the sky looked like.
The Harvard Plate Collection is the largest in the world—about half a million glass plates. They date all the way back to 1890. Harvard had telescopes all over the world, and they would do what we call sky patrol—basically, take pictures every night in an organized fashion so they could cover the whole sky. And then they would regularly repeat that, so they could look for things that were changing.
MIKE SHARA (Curator, Department of Astrophysics): Once we had a permanent record on photographic plates, we knew that we could go back a week later, a year later, a century later, and in some sense, recover what galaxies, stars, and so on were like during those observations.
My name is Mike Shara. I’m a curator of astrophysics here at the American Museum of Natural History. What I do is exploding stars.
I can’t send a time machine into the past. We simply have one record of the universe streaming by us, and because the astronomers a century and more ago were snapping pictures, we have a continuous record over an enormously long period of time.
It’s a completely unique history and digitizing their collections is very, very high on the wish list of most of the world’s astronomers.
PAGNOTTA: Harvard has started a scanning program and here at the Museum, we’re also working to help bring the plates into the modern era and to make them more available to us, for the science that we want to do, but also to the broader astronomical community.
So, one type of star that I’m interested in is called a Cepheid variable, and these were discovered by Henrietta Leavitt who worked at the Harvard Observatory in the early 1900s.
She realized that you can use these Cepheid variables to measure distances. Which is really useful because we can’t just stretch a yardstick halfway across the galaxy.
Leavitt in the early 1900s compiled a catalog of all these variable stars in the Large and the Small Magellanic Clouds—mini-galaxies that are actually gravitationally bound to our galaxy, the Milky Way. Later in the 1950s and ‘60s, Cecilia Payne-Gaposchkin updated Henrietta Leavitt’s catalog. So, we have these two fantastic catalogs that are crucial to measure distances.
But we went to look for them in our modern digital catalogs of stars and they weren’t there. So, what we decided to do with a couple of groups of high school students who work here through what we call the SRMP program—the Science Research Mentorship Program—was to update those catalogs.
JULIA KRUK (Science Research Mentoring Program Student): My name is Julia Kruk.
ZACHARY MURRAY (Science Research Mentoring Program Student): I am Zachary Murray. We participated in the SRMP program.
Essentially, they were taking pictures of the sky and printing them on photographic plates, and then Henrietta Leavitt used an X-Y coordinate system to chart the locations of the stars.
What we start with is a sheet with columns of numbers on it. But these positions were only valid for the era in which she did her observations.
KRUK: We had to make sure that it was accurate so we could find these stars today.
MURRAY: So, in order to correct that, we converted those X-Y coordinate positions to modern spherical coordinates.
PAGNOTTA: But they weren’t quite as modern as we use today. Because the Earth kind of wobbles on its axis, that means where we see the stars changes very slightly over time. So, the next step was they had to what we call precess them all the way forward. So, they actually wrote a computer program to get modern coordinates.
KRUK: We’re publishing this data and making it available to the astronomical community— a century’s worth of data in one location.
SHARA: It’s one thing to collect and to generate data. It’s entirely another thing to make sense of it. And that’s what makes it more and more scientifically valuable. By involving a team of students who are willing to get their hands dirty and look at the data, we begin to train the next generation of scientists and at the same time, selfishly, we get something out of it.
PAGNOTTA: Once this catalog is complete—and it’s almost finished—we will have a digital, fully accessible catalog that anyone in the world can use. And then from there, you can start to do science—see how do these stars change over time. We think that they probably do change over 100 years, but we don’t really know what they do. Nobody’s ever looked before.
Shara’s 1986 study proposed that, after an eruption, a nova enters a cycle and becomes “nova-like,” then a dwarf nova, and then, after a hibernation as a so-called detached binary, it returns to a dwarf nova, then nova-like, and finally a nova. This cycle repeats up to 100,000 times over billions of years. “In the same way that an egg, a caterpillar, a pupa, and a butterfly are all life stages of the same organism, these binaries are all the same objects seen at different phases of their lives,” Shara said.
For the new study, Shara and his colleagues at Ariel University and Tel-Aviv University in Israel built a set of simulations to follow thousands of novae eruptions and their effects on their red dwarf companions. They found that cataclysmic binaries do not simply alternate through each of the four states their whole lives.
Newborn binaries, considered as the first few percent of a system’s life, only alternate between nova and nova-like states. Then, for the next 10 percent of their lifetimes, the binaries alternate through three states: nova, nova-like, and dwarf nova. For the remaining nearly 90 percent of their lifetimes, they continuously cycle through all four states.
The study also showed that almost all of the novae we observe today occur near the beginning of a binary system’s life as opposed to the end.
“Statistically, that means that the systems we observe—the ones that are popping off all of the time—are the newborn ones,” Shara said. “And that’s just about 5 percent of the total binaries out there. The vast majority are in the detached state, and we’ve been ignoring them because they’re so faint and common. We know that they’re there. Now we just have to work hard to find them and connect them to novae.”