Authors: Oleksandr Burlaka and Natalia Virnina
In the summer of 2023, scientists made a prediction that the star T Coronae Borealis would erupt as a bright new star in about a year. The opportunity to see such a rare phenomenon for many months became a leading topic of discussion among stargazing enthusiasts. However, it never exploded.
T Coronae Borealis
“A new star is expected to appear in the sky this year!”. Articles with such or similar headlines are published practically throughout 2024. Their primary source was an announcement on the website of the American Association of Variable Star Observers, authored by Louisiana State University astronomer Bradley Schaefer and two colleagues. This announcement is about the intriguingly unique T Coronae Borealis (T CrB), or as it is also called, the Blaze Star.
Bradley Schaefer is widely known as a researcher of novae and supernovae and is considered one of the best experts in the world on T Coronae Borealis. And the object itself is under constant observation by astronomers.
Schaefer’s attention was caught by the fact that the star had become slightly brighter since 2015, and in March-April 2023 this “active state” was replaced by a definite decline. This behavior was precisely the same as observed before the last explosion, which occurred almost 80 years ago. Therefore, it was reasonable to assume that the star was ready to explode again.
Since the announcement of the upcoming T CrB explosion, a massive observing campaign has unfolded across the Northern Hemisphere. Since then, about 500 professionals and enthusiasts have sent their results to the official database of the American Association of Variable Star Observers alone.
T Coronae Borealis is unique for several reasons, but the most important one is that among all the stars that have shown themselves to be novae (so-called recurrent novae) more than once, it is the closest to Earth and also the only one whose explosion can be observed without optical instruments. Based on observations from past centuries, we can expect that at some point a celestial object normally invisible to the naked eye will become a star commensurate in brightness with the Polar Star.
The explosions had been expected since the spring of 2024, but October has already ended, and the colorful show hasn’t happened. So did the scientists deceive the people? In order to answer this question, we need to examine in more detail what we know about T Coronae Borealis in general.
Previous explosions
May 12, 1866, another luminary suddenly appeared in the constellation Corona Borealis, which with its brilliance was not inferior to its brightest star — Alfecca. It is difficult to state who first noticed it, as there are three official discoverers: J. Birmingham, M. Walter and J. Schmidt. The latter was one of the most skillful observers of his time.
At the beginning of that memorable night, he carefully surveyed the sky and noticed nothing unusual about Coronae Borealis. A few hours later, Coronae was adorned with a new “diamond” — a star with a brilliance of 2,0m, that is about as bright as Polar and the most conspicuous stars of Ursa Major. This fact gives an idea of how quickly the explosion developed — literally within a few hours.
After the explosion of 1866, the character of our story entered the catalog of variable stars as T Coronae Borealis, it was classified as a nova and became the object of close attention of astronomers all over the world. The reason for this favorable attitude of scientists is not least because T CrB was at that time the brightest nova of the Northern Hemisphere during the entire observation period.
The explosion of 1866 did not last long. Having reached its peak, the star began to lose brightness quite quickly: after a week it could no longer be seen with the naked eye, and after another 3 weeks it completely returned to its normal state.
However, ~80 days after that, researchers faced a real surprise: the brightness of T Coronae Borealis increased again, but only to 8th stellar magnitude (still beyond the capability of human eyes). The star maintained this brightness for another 80 days, most of the time staying at the same level. Astronomers of the 19th century didn’t even know what the novae were and why they exploded, but they continued to make observations, accumulating precious material for our contemporaries.
In 1920, the famous American amateur astronomer Leslie Peltier joined the observations. T Coronae Borealis truly fascinated him. He observed the star every clear night for years, waiting for a second explosion.
For 60 years the star showed only minor fluctuations in brightness, but in 1936, astronomers noticed that the brightness of T CrB increased slightly, and in 1944-45 began to rapidly decrease. And then Leslie Peltier made the audacious suggestion that the events of 1866 would soon repeat themselves. But no one knew when.
T Coronae Borealis exploded again on the night of February 8-9, 1946. Later, in his book Starlight Nights: The Adventures of a Star-Gazer, Peltier touchingly recalled the explosion:
“For more than twenty- five years I looked in on it from night to night as it tossed and turned in fitful slumber. Then, one night in February 1946 it stirred, slowly opened its eyes, then quickly threw aside the draperies of its couch and rose! Full eighty years had passed since last the star had shattered the symmetry of the Northern Crown. And where was I, its self- appointed guardian on that once-in-a-lifetime night when it awoke? I was asleep!”.
Regular observations on cold winter nights are very draining. Therefore, when Peltier woke up at 2:30 to check a few pre-dawn variable sightings and felt sick, he was forced to cancel the observations and return to bed:
“And thus I missed the night of nights in the life of T Coronae. It was the night the spectroscopists long for. It is in those earliest hours of awakening that the newborn star-with all the exuber- ance of youth, divulges its most intimate secrets.”
This time scientists studied its behavior even more carefully, for the first time made a spectral analysis of the nova and all the time compared the fresh data with previous observations. They found that the star’s light curve (change in brightness) after the explosion is almost identical to the previous time, including a long secondary plateau.
Obviously, whatever happened with T Coronae Borealis, this development is typical for it. Scientists concluded that this star was representative of recurrent novae, and began to wait for the next explosion, which, based on previous experience, was expected to occur somewhere around 2026.
But the “first signals” became visible sooner than astronomers expected. After the 2015-2023 active phase, T Coronae Borealis began to dim, and this was a very strong sign that an explosion was approaching, even though, based on the previous inter-explosion interval, it was a bit early for a new one.
After all, as the study of other repeated novae has shown, the interval between explosions is not something absolutely invariable, and for an average of 80 years, a deviation of 12-20 months is not so large.
What the T Coronae Borealis system is like
Modern scientists already generally understand the structure of T Coronae Borealis. As with all objects where new explosions occur, it is a close binary system, one of the stars in which is a white dwarf. The second component makes it rare, if not unique.
It is not a subgiant (a star just starting to move off the main sequence), but a true red giant. It is indeed a giant star that, with a mass of 1.12 solar masses, has a radius of more than 26 million kilometers — 66 times larger than our luminary. Despite its grandiose size, the giant is slightly inferior in mass to the white dwarf, which is 35% larger than our Sun by this parameter. Compared to the red giant, the more massive white dwarf looks like a small dot — its size is about the same as Earth’s.
If we consider exactly the distance between the centers, it turns out that the center of mass is a little closer to the white dwarf. However, we should keep in mind that the radius of the red giant approaches half of this value.
Therefore, in practice, T Coronae Borealis system is as follows. The huge red giant orbits around a point that is distant from its surface by an amount much smaller than its radius. And on the opposite side, at a distance that is slightly less than the radius of the red giant, a tiny white dwarf point moves around the same point. One complete orbit occurs in 227 Earth days.
Billions of years of evolution have caused the red giant to completely fill its Roche cavity, a pear-shaped region of space around a component of the binary system that is dominated by its gravity. By contrast, the white dwarf sits deep inside its Roche cavity, surrounded by the substance it “steals” from its not-so-fat companion. It is interesting that the Roche cavity of a white dwarf is larger than that of a red giant.
Many years ago (no one knows exactly how long ago it was), a kind of “bridge” stretched between two stars in space. The matter began to flow from the red giant to the white dwarf, while it moved not directly to it, but outlined complex trajectories. Eventually, the process of matter transfer stabilized, and an accretion disk of gas formed around the white dwarf, which it “took away” from its giant neighbor.
Gradually hydrogen from the accretion disk got directly to the surface of the white dwarf, accumulated there, and at some point, the conditions for the beginning of a chain thermonuclear reaction were formed.
We observe it as a flare, which is actually a thermonuclear explosion, and that’s why the flare is so short. The cataclysm destroys the accretion disk and the “bridge” that feeds it, and after some time, the flow of matter is restored, and the whole process is repeated for the next 80 years.
Red giant and blue light
Astronomers, both professional and amateur, make observations in a variety of bands, from infrared to ultraviolet. This approach replaces low-resolution spectra in a certain way. Most often, brilliance measurements are made in two bands, green and blue. This is standard practice in the study of all stars. Scientists do this because the luminaries have different temperatures.
This means that their emission peaks at different wavelengths, so the difference between the brightness level at green and blue wavelengths is the easiest way to characterize the emission of a luminary.
In T Coronae Borealis, most of the time before explosions, this difference is very large. This is not surprising, given that we see mostly light from a cold red giant whose surface temperature is “only” 2,870K.
However, when the luminosity increases decades before the explosion, the “blue” curve almost aligns with the “green” curve. During the explosion, the two curves are almost indistinguishable from each other. This confirms that it is the white dwarf, whose surface is heated to 10,000 K, that plays the main role in all these events.
What is a re-explosion?
The whole scene seems fantastical, but generally understandable. However, it hides several mysteries, actually more interesting than the question of why the explosion hasn’t occurred in 2024. And the first of these is the re-increase in brightness almost 3 months after the main explosion. It has been observed twice already, and both times the brightness changes were the same, but they were quite different from the main explosion — neither in the shape of the curve nor in amplitude.
Scientists can’t say what it is at all, but they have a few assumptions. According to the first one, a dense ring of material forms around the white dwarf soon after the explosion, which falls on the hot star and triggers a new thermonuclear reaction. However, in fact, this is possible only if it is not a white dwarf, but a star of the main sequence, so this version should be discarded.
The second version, or rather, a group of versions, is built around the assumption that the explosion on the white dwarf additionally heats up the hemisphere of the red giant turned to it, and precisely its light we see during the secondary explosion. However, if this were indeed the case, the secondary explosion would come immediately after the first. But there is still an 80-day interval between them, in which T Coronae Borealis is in the passive phase. Therefore, this version is also not suitable.
The next explanation belongs to Schaefer himself, the same one who made the prediction of this year’s T Coronae Borealis explosion. He noticed that during the re-explosion, the brightness in the blue and green wavelengths approach each other, so a white dwarf, not a red giant, must be responsible for it too.
He suggested that it was actually about another thermonuclear explosion. The surface temperature of the white dwarf at this moment is higher than usual, and this leads to a premature start of the new mechanism: hydrogen flares up, although not as strongly as in a conventional explosion, but the burning lasts longer. At the same time, the described theory is not confirmed by any numerical calculations.
Finally, we can recall that the accretion disk around a white dwarf can emit quite a bit of light. During an explosion, it is completely destroyed, and after the thermonuclear reaction stops, its formation begins again. The motion of material inside the Roche cavity of a dead star, as already indicated, is rather chaotic at the beginning. Therefore, it is quite possible that the secondary explosion is actually some process related to the stabilization of the disk.
Mystery of the decreasing rotation period
The great mystery of T Coronae Borealis is related to the rotation period of this amazing pair. The figure of 227 days is actually quite approximate. Centuries of observation of the system have shown that this value has been changing all the time.
Between the first and second explosions, the period slowly increased, by 55 seconds per year. And this is perfectly explained by the nature of the system itself. The red giant is losing some material and with it its moment of inertia. Therefore, it is gradually moving away from the white dwarf.
During the 1946 explosion, the rotation period of the system increased abruptly by quite a significant amount. And this can be explained very easily, too. The thermonuclear explosion pushes the two luminaries away from each other and the distance between them increases.
However, a strange effect was observed after that. As before, the rotation period continued to change. However, it did not increase, as one might expect, but decreased, and quite rapidly.
As with the re-explosion, scientists don’t have a good explanation. The easiest thing to do would be to blame the dust and gas around the stellar pair, or their magnetic fields, for this effect. However, Bradley Schaefer’s calculations show that these effects are too weak to explain the observed phenomenon.
Putting the blame on hypothetical interactions of some of the luminaries with material flowing from one to the other won’t work either. Even in the active state of the system, when its brightness increases before an explosion, its total mass is millionths of a solar mass. And in the passive state, this value is in general billionths of a solar mass.
That’s why decreasing the rotation period remains one big mystery. The same is true for the increase in system brightness decades before the explosion and the deep “hole” just before it. While the first still has some correspondences in other repeat novae, the second is unique to T Coronae Borealis. We can only speculate about the nature of these processes. In particular, they may be related to the accretion disk, but what exactly is going on — there are no assumptions.
When should we expect an explosion?
But back to the speculation about when T Coronae Borealis should erupt again. The question of how reliable any predictions are is quite complicated. We can well trust the observations which indicate that in recent years this system has behavior very similar to the behavior prior to the 1946 explosion. They were carried out by a lot of people all over the world, so there can be no mistake or lie here.
The other point is how accurately we can say that this behavior means there will be an explosion in a certain number of months. Astronomers have seen it exactly once, because no one had made such observations before the 1866 explosion.
The conclusion that it must happen the same way is reached from the great similarity of the main and re-explosion in 1866 and 1946. But that doesn’t mean that the rest of the processes there are the same every time. Observations of other repeated novae indicate that the repeatability of events in such systems is far from hourly accurate.
But even how much the interval between explosions in the case of T Coronae Borealis can vary from one time to the next remains a mystery to scientists. The fact is that it is very different from the other known repeated novae, where the partner of the white dwarf is not a giant, but only a subgiant, and therefore the rotation period of the system there is much smaller. Therefore, the analogies just don’t work.
When exactly T Coronae Borealis exploded before 1866 – no one is 100 percent sure, either. There is evidence that this happened in 1787. At least in Francis Wollaston’s catalog published in 1888, there is a luminary with very close coordinates. In addition, there is evidence of an explosion in 1217 in a medieval chronicle, but everything is misty there in general.
Assuming that the fluctuating period between explosions is no more than a few months, T Coronae Borealis should not become novae until late 2025-early 2026. Schaefer’s conclusions about the explosion contradict this, but they are based on slightly more solid observations. Except that both are conclusions drawn essentially from a single observation, for which there is no confidence about how typical that case was.
From an everyday logic standpoint, both conclusions are better reasoned than most predictions we encounter in life. Astronomy often deals with phenomena that repeat many times, for which variability is known and all observed processes are well explained.
None of this is about T Coronae Borealis, with its many mysteries. Scientists aren’t sure when this nova should explode, but the reasons to expect it in the near future are absolutely solid. And scientists are looking forward to this moment, ready to send the entire diverse arsenal of ground and space telescopes — from radio and infrared, and to those that work in the X-ray and gamma-ray range.
