Hold onto your hats! Is the 'blaze star' T Corona Borealis about to go boom?

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27-03-2025

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A new set of predictions for the so-called "blaze star," T Corona Borealis suggests the star might go nova on either March 27, November 10, or June 25, 2026. However, other astronomers are skeptical about these predictions, which are based on an implied pattern in the explosive system's orbital configuration,
"T Corona Borealis [T CrB] is a unique object that has fascinated amateur and professional astronomers for more than a century," Léa Planquart of the Institut d'Astronomie et d'Astrophysique at the Université Libre de Bruxelles in Belgium, told Space.com.
T CrB is a symbiotic binary, a vampire system in which a white dwarf is siphoning material from a red giant star. A white dwarf is the dense, compact core remnant of a once sun-like star, packing a mass equivalent to that of a star into a volume about the size of Earth. A red giant represents an earlier stage in a star's evolution, when a sun-like star starts to run out of its hydrogen fuel supply and begins to swell. Its distended atmosphere then becomes easy prey to the gravity of the far smaller, but denser, white dwarf.
Material captured from the red giant forms a spiraling disk around the white dwarf, ultimately depositing that material onto the white dwarf's surface. Once enough material has built up, a thermonuclear explosion ignites. It does not destroy the white dwarf, but we can see the light of the explosion across thousands of light-years.
We call this a nova, after the Latin for "new star."
Typically, T CrB languishes at about magnitude +10, which means it is so faint that it can only be seen through moderate-aperture telescopes or large binoculars. However, when it goes nova, it brightens to naked-eye visibility, and hence briefly becomes seen as a "new star" in the night sky.
T CrB is actually even more special than that, because it is one of only 11 known "recurrent" novas, which are seen to go nova repeatedly, with gaps of less than 100 years between explosions. Previously, on February 9, 1946 and May 12, 1866, the white dwarf in the T CrB system went nova. It also went nova around Christmastime in 1787, although the exact date isn't known, and there is also a suggestion that a nova connected to this star was seen sometime in the autumn night sky of 1217.
Prior to the 1946 nova, T CrB brightened slightly in 1938, before dimming again just before going nova. The same pattern has also been seen in T CrB this time around, with it brightening by 0.7 magnitudes in 2015 before dimming again in 2023. This is why astronomers are anticipating a new nova.
Jean Schneider of Paris Observatory, has also noticed what he believes to be a pattern between the timing of the T CrB nova events. The red giant and white dwarf take 227.5687 days to orbit one another, and Schneider believes that each nova takes place after a time equal to an exact whole number of orbits. In other words, something about the position of the white dwarf and the red giant is triggering the nova outbursts, he says.
Yet, because their orbits are circular, no single position should have an effect. So, Schneider proposes the presence of a third object in the T CrB system on a wider, elliptical orbit. Every 79–80 years, he says the third object is close to the white dwarf, meaning that the white dwarf can feed off both the red giant and this hypothetical third object at the same time. This would enhance the rate of matter falling onto the white dwarf, creating the conditions for a nova.
So far, this third object, if it exists, has remained undetected, but Schneider tells Space.com that "it could be detected by astrometry, radial velocity, direct imaging, a transit or microlensing."
Indeed, Schneider wonders whether it hasn't already been detected but just not recognized. On April 21, 2016, the T CrB system suddenly increased in visual brightness by 0.5 magnitudes.
"I have the following, qualitative interpretation, which is that before then, the third body was outside the pixel corresponding to the visual measurements," he said. In other words, the third object moved close enough to the other two components of the T CrB system that from our point of view it was sharing a pixel with them in images, adding its brightness to the combined light of the red giant and white dwarf.
However, other astronomers are not yet convinced. Léa Planquart has studied T CrB and other recurrent novas, and in January published a paper describing the mass transfer between the red giant and the white dwarf based on radial velocity observations with the HERMES spectrograph on the 1.2-meter Mercator telescope at La Palma in Chile. Radial velocity here, for context, refers to the Doppler shifted motions of the individual stars and the matter being transferred between the red giant, what's known as the "accretion disk" and the white dwarf.
"Jean Schneider has suggested the presence of a third companion in an eccentric orbit with a period of 80 years," Planquart told Space.com. "Such additional orbital motion is, however, not detected in our decade-long radial-velocity monitoring."
In other words, radial velocity measurements show no evidence for a third star, although Planquart cannot rule out a low-mass body such as a large exoplanet.
Jeremy Shears, who is the Director of the British Astronomical Association's Variable Star Section, also has doubts. "Most astronomers are skeptical about this prediction, as am I," he told Space.com. "The best thing to do is to keep watching every clear night."
Should there be no third object, and if the pattern seen by Schneider in the dates of previous novas is just a coincidence, then what is happening to T CrB?
Planquart's observations shed some light on the matter, particularly the brightening seen in 1938 and 2015, followed by a dimming, most recently seen in 2023.
"We realized that from 2015 to 2023, the accretion disk around the white dwarf had reached its maximum extension and became hotter and more luminous, leading to increased brightness," Planquart said. This enhanced what Planquart calls "the vampirization effect," increasing the transfer of matter to the white dwarf in a "super-active phase." Then, in 2023, the accretion disk cooled back down again, resulting in the dimming, although matter continues to flow from the disk to the white dwarf at a slower rate.
"It is likely that this enhanced activity is necessary to trigger the nova explosion, as it allows the material to accumulate more rapidly," said Planquart.
Then, in 2023, the accretion disk cooled back down again, resulting in the dimming, although matter continues to flow from the disk to the white dwarf at a slower rate. However, the details are still somewhat unclear — what causes the state change in the accretion disk that leads to the super-active phase, and exactly what is happening on the surface of the white dwarf between the disk cooling again and the nova explosion?
Although Schneider's exact date predictions may or may not come to pass, the pattern of the super-active phase followed by quiescence and dimming suggests that the nova is just around the corner. "We may expect to see the explosion in the coming months — or possibly next year," said Planquart.
When that happens, what can we expect to see in the night sky? In 1946, T CrB reached magnitude +2, meaning it was easily visible to the naked eye, similar in brightness to the stars of the Big Dipper. Shears expects it to be just as bright this time around.
T CrB is located in the constellation of Corona Borealis, the Northern Crown, which is currently visible in the night sky across the whole of the Northern Hemisphere and from as far south as South Africa and Australia (albeit low down in the sky from southerly locations).
"At present T CrB is tenth magnitude, so it is only visible in giant binoculars," said Shears. "But when it rises [in brightness] it will become visible in standard binoculars and then the naked eye."
And the rise in brightness will be rapid. "It's only a matter of a few hours for the rise to occur — precisely how many is not known as the rise has never been caught before," said Shears. "That's why it is so exciting. We hope that with so many observers this time around, we may indeed catch it as it awakes from its slumber."
Indeed there will be many observers, as astronomers wait and watch to catch a glimpse of this rare nova and learn more about what is happening on the surface of this white dwarf when it hosts a giant thermonuclear explosion. "When it explodes, it will be one of the most extensively observed objects, targeted by telescopes worldwide," said Planquart.
As for what the future holds for T CrB, an even larger explosion is on the horizon. The mass of the white dwarf in the T CrB system is 1.37 times the mass of our sun. This is very close to the Chandrasekhar limit, which is 1.44 solar masses, and is the point at which the thermonuclear detonation overcomes the white dwarf and blows it to smithereens as a Type Ia supernova. As it steadily steals mass from its companion red giant and grows in the process, it accelerates its own demise.
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"As white dwarfs approach the Chandrasekhar limit their radius shrinks and their surface gravity is increased," Ken Hinkle, an astronomer at NOIRLab in Tucson, Arizona, told Space.com. "This results in the short time between eruptions."
As the white dwarf inches closer to the Chandrasekhar limit, the nova events will become more frequent, until one day … boom! But it will take hundreds of thousands, if not millions, of years for the white dwarf to get to that stage, so there's no rush to add it to your calendar. In the meantime, we shall keep watching the sky for its latest nova.
Jean Schneider's paper was published in Research Notes of the AAS.