Remember that star that was supposed to explode in 2024? It didn't, but it could still happen soon

In apr 2024, the to the highest degree awaited(p) skywatching case was, of trend, the total solar eclipse visible across Eastern Canada and the U.S.

But the second most-anticipated story was the possible appearance of a new star in the sky: T Coronae Borealis.

To be clear, the star — often referred to as T Cor Bor — already exists, but isn't visible to the unaided eye. Instead, it was expected to brighten to naked-eye visibility by the end of the year. 

But … whomp, whomp. Nada. Zilch. Bupkus. No big, bright new star in the sky.

So, what's the deal?

"The issue of T Cor Bor is that despite the fact we've studied the system for a very long period of time, there's still a lot of uncertainty regarding the physics of the system," said Brian Kloppenborg, an astrophysicist and executive director of the American Association of Variable Star Observers (AAVSO).

T Cor Bor lies roughly 3,000 light years from us and is part of a binary, or two-star, system in the constellation Corona Borealis. One star is a large red giant; the other is a small but incredibly dense white dwarf.

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As they orbit each another, material from the red giant sloughs off, falls into the white dwarf and heats up in a process known as accretion. But there is a limit as to how much the white dwarf can take. Once that limit is reached, it erupts in a thermonuclear explosion called a nova. That's when the star will brighten to naked-eye visibility.

This happens roughly once every 80 years. The last eruption was in 1946.

"We thought we knew what the accretion rate was. We thought we knew how much material was falling onto the star, but unfortunately, we were off by a little bit," Kloppenborg said.

T Corona Borealis

In the case of T Cor Bor, historically, there has been a "high state" of activity, then a return to a "low state" roughly 10 years before an eruption. That began around 2014, hence the 2024 prediction. 

"[Ever] since then, there have been a number of predictions of imminent eruptions, based on quivers in the light curve … but [this] never convinced anyone," Brad Schaefer, an astronomer and professor emeritus at Louisiana State University who has been studying T Cor Bor for decades, said in an email. "Still, everyone is certain that T CrB will erupt soon."

One paper even forecasts that it could happen as soon as this week.

The research note, published by the American Astronomical Society in October 2024, looked at eruption dates and came up with several: August 12, 2024, March 27, 2025, November 10, 2025 and June 25, 2026.

Conversely, a paper published in the journal Astronomy & Astrophysics in March suggested the 80 year cycle could be off by as much as 10 years.

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Even though T Cor Bor hasn't done its thing yet, Kloppenborg said interest hasn't waned among amateur astronomers, who have been watching it closely and providing thousands of observations.

"We receive a data point once every six minutes. And this is people out there doing this. This isn't machines," he said. "This is all 100 per cent humans, which is absolutely wonderful to see."

Go, citizen science.

When T Cor Bor erupts, it will brighten from a dim 10th-magnitude star to second magnitude — from impossible to see without binoculars to naked-eye visibility. It will remain that bright for roughly two days before dimming again to sixth magnitude, which is just at the limit of what our eyes can see from a dark-sky location. But it will still be easy to spot with binoculars.

So, don't give up on T Cor Bor just yet.

"We have been keeping track of the amount of accreted gas since 1946, and we gotta be close to the trigger level," Schaefer said in an email. "So [T Cor Bor] could go off tonight, and it will erupt sometime in the upcoming months. Fortunately, the minute-by-minute monitoring … and the instant observing plans are all in place, just awaiting the blow-up."

Senior Science Reporter

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