NASA/ESA/The Hubble Heritage Team (STScI/AURA)
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The red super-giant star Betelgeuse has dramatically dimmed, leading to speculation of an imminent supernova. But that may not happen for 1 million years.
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A more common but less widely known way for stars to die is a red nova: When a bigger star rips a smaller one to shreds and gobbles it up.
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Astronomer Larry Molnar and his colleagues have worked to define the stages that lead to a red nova and identified seven candidates in a near-final stage.
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Ongoing surveys of the night sky may soon turn up a candidate that’s in a final 10-year phase of becoming a red nova.
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Supernovas, the cataclysmic explosions of giant stars, are one of nature’s most awesome spectacles.
That’s why many people are excited that Betelgeuse, a bright red super giant star in the constellation Orion, has suddenly and dramatically dimmed. This rapid fading could mean the star is about to blast its guts into deep space, turn half as bright as a full moon, and shine for months (even during the day).
Or maybe not.
Astronomers think Betelgeuse’s dimming is most likely just another short phase in the life of a giant star that’s burning through the last dregs of its fuel.
“If we look at the past brightness of Betelgeuse, it has gone up and down,” Larry Molnar, an astronomer at Calvin University, told Business Insider. “It could still be a million years before it explodes.”
But if seeing a star explode before you die is on your to-do list, don’t lose hope yet.
For years, Molnar and his colleagues have studied little-known yet extremely bright stellar mergers called “red novas,” so named for the dominant color of light they produce. Such events occur when two roughly sun-size stars get locked in a deadly orbital dance. At some point they violently merge, blast millions of trillions of tons of superheated plasma into space, and briefly outshine themselves thousands of times over.
Molnar and his colleagues have identified seven candidate double-star systems in a “penultimate” stage of becoming red novas. What’s more, the group is proposing a new way to locate and predict more in the future.
If they’re correct, astronomers and night-sky watchers alike will have a chance of knowing about a catastrophe (or two) years ahead of time, then watching it from a comfortable number of light-years away within the decade.
Red novas: A bright alternative to a supernova
Catching a supernova would be a rare and amazing sight: such explosions can shine more brightly than an entire galaxy.
But out of the hundreds of billions of stars in our Milky Way galaxy, which stretches about 100,000 light-years wide, only one explodes roughly every 50 years, according to NASA. What’s more, there’s no guarantee the next one will be as close as Betelgeuse — about 643 light-years from Earth — and easy to spot with the naked eye.
That’s why red novas are a next-best spectacle to try and witness, as violent and visible stellar deaths go. Based on previous occurrences, they are visible to Earth once every 10 years.
The latest known red nova was V1307 Scorpii, a double-star system that unexpectedly merged and detonated in 2008. Another possible red nova was V838 Monocerotis (shown at the top of this story); the star system suddenly flashed in 2002, temporarily becoming 600,000 times brighter than the sun and the brightest object in the Milky Way galaxy.
Though astronomers aren’t totally certain V838 Monocerotis is a red nova, the Hubble telescope has kept its eye on the expanding gas and dust of the object. Researchers in 2006 morphed eight of those Hubble photos to show off the red nova’s evolution in an animation that compresses four years into six seconds.
ESA/Hubble (M. Kornmesser & L. L. Christensen)
Molnar thought he’d found two stars about to explode as a red nova in 2017 — the system KIC 9832227 — which he predicted would happen around 2022. But that prediction was canceled after another scientist discovered a subtle yet critical error in 1990s data that Molnar had relied on.
“We now have strong reason to believe these guys shouldn’t be merging anytime soon,” he said.
That hasn’t dissuaded Molnar from looking for more, though.
How a red nova works
Calvin College/Cara Alexander, Daniel Van Noord, Chris Spedden, and Larry Molnar
Red novas can be seen about once a decade because one in roughly every 500 sun-like stars is part of a system called a contact binary: two stars that orbit so close together, they share superheated gases. If you could visit such a system, it’d look like a giant glowing peanut or bowling pin (shown above).
But this closer relationship is also a dance of death.
No one is sure of the stages that lead to a red nova, which is what motivated Molnar and his colleagues to formulate them — then test those ideas by finding stars that may fit the bill.
Molnar thinks two stars, one larger than the other, rotate at a speed that matches the other’s orbit, much like the Earth and its moon (which is why we only see one face). This is called tidal locking.
But the bigger star ages more rapidly over billions of years than its smaller companion because its stronger gravity fuses hydrogen fuel into helium at a faster rate. Once the star’s core runs low on hydrogen, Molnar says things decidedly take a turn for the worse, according to his recent predictive work.
The larger star begins fusing its helium into carbon and oxygen, causing it to start expanding outward. To counteract this imbalance, the smaller star begins drifting away and losing material to the bigger star.
“There’s this stage that’s very long and slow, where gradually the little guy loses material to the big guy,” Molnar said.
But this transfer can’t last forever, Molnar said. After the smaller star shrinks to about 10-15% of the bigger star’s mass, it begins to fall back toward the bigger star and spin faster. This “penultimate” stage that lasts perhaps 1,000-10,000 years.
“The final stage is when they’ve moved in enough, their common atmosphere is so big, that they begin to lose material out the back end,” Molnar said. “It looks like a garden hose spinning around your head.”
Losing its gas to space causes the smaller star to spin even faster than before. After about 10 years, Molnar thinks, this runaway scenario ends catastrophically: The smaller star shreds to pieces, falls deep into the larger star, and triggers a massive explosion.
“By falling deep into a star, you release a tremendous amount of energy,” Molnar said. “It pushes out the outer parts of the star into a giant expanding explosion, which we see as a red nova.”
Depending on the size of the two stars, among other factors, the merger can liberate as much energy as the sun may generate during its entire multi-billion-year-lifespan over a very short period of time.
Seven new red nova candidates
NASA/JPL-Caltech
After predicting the stages leading up to red novas, Molnar and his colleagues tapped vast surveys of stars in the night sky to find candidates for them.
In a 25-year-long star survey called OGLE, the team found 184,000 contact binary systems. After running the data through a supercomputer, seven of these systems emerged as candidates in the near-final 1,000- to 10,000-year-long stage.
Molnar plans to present the candidates and the work that led to them next week at the 235th meeting of the American Astronomical Society in Hawaii.
“These seven guys? I don’t expect any of them to explode in the next decade. But I expect all of them to merge within the next 1,000 years or 10,000 years,” Molnar said. “It’s pretty darn short compared to Betelgeuse or anything else we know, but it’s still too long to wait up for. You’d like to have it happen within your lifetime.”
Molnar and his team are working toward finding a candidate in that final, roughly 10-year long stage. One issue is that OGLE only looks at stars within a narrow slice — toward the Milky Way’s dense core.
So the team now trying to use data from several other long-term surveys of the night sky, such as those seeking near-Earth objects that may pose a threat to Earth, to find more contact binaries and red nova candidates.
Statistically, if they can find about 100 or more “penultimate” candidates, they may find one system that’s going to merge and explode within 10 years.
“There must be one in one of these surveys,” Molnar said. “We must look proactively, and promptly. The last one that showed up in 2008? It was sitting in the survey data and nobody was studying it.”
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