February 9 marked the 79th anniversary of T Coronae Borealis’ most recent outburst in 1946. Earlier predictions have come and gone—will 2025 be the year the “Blaze Star” finally erupts. | By Tim Lyster
T Coronae Borealis (T CrB), a dim binary star in a diminutive constellation, remains popular fodder for science journalists around the world. The recurrent nova continues to tease the astronomical community with its flickerings and spectroscopic shenanigans. Typically glowing dimly around 10th magnitude, T CrB’s last two outbursts—observed on May 12, 1866, when it climbed to second magnitude; and February 9, 1946, when it reached third—were separated by 80 years, so it seemed reasonable to expect the next episode would occur sometime in 2026.
But when it entered its pre-eruption dip two years ago, Brad Schaefer, Professor Emeritus of Physics and Astronomy at LSU, suggested that the timing for the next explosion be moved up to early to mid-2024:
“If the Dip in 2023 is similar in timing to that in 1945, then the primary eruption should occur roughly 1.1±0.3 years later, or in 2024.4±0.3. This prediction is substantially improved over the prior predictions based only on the 2015 rise to the high-state. Still, possible deviations from the behavior in 1946 could create an early or a late eruption.”
Here we are approaching March 2025, and the so-called “Blaze Star” isn’t blazing. Those “possible deviations” seem to have pushed the eruption date out to . . . when? When will it next erupt?
New Year, New Predictions
Recent observations described below might indicate that T CrB is on the verge of another brightening episode, and so it remains critically important for observers—both professional and amateur—to continue monitoring the star.
In early 2025, significant spectral changes were detected in T CrB. Researchers at the Thueringer Landessternwarte Tautenburg observed a sharp increase in the emission lines within the star’s spectrum. Over approximately two-and-a-half weeks leading up to February 9, the equivalent width of hydrogen lines doubled. Additionally, helium emission lines, which had previously vanished in December, reappeared with intensities increasing more than fourfold since January 21. These changes suggest a substantial rise in the accretion rate onto the white dwarf component of the system, a potential precursor to a nova eruption.
In a provocatively titled paper “T CrB on the Verge of an Outburst: H alpha Profile Evolution and Accretion Activity,” complementary findings were reported by a team analyzing Hα profiles of T CrB. Between January 15 and February 7, there was a notable enhancement in both the intensity and width of the Hα emission line. The flux increased by a factor of approximately 2.3, and the Full Width at Zero Intensity (FWZI) expanded from 668 km/s to 755 km/s. These observations further supported the hypothesis of an escalating accretion rate, indicating that T CrB may be on the brink of an outburst.
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But not so fast! Schaefer noted that simply because there are signs of enhanced accretion does not mean the conditions on the white dwarf are right for the eruption to occur:
“What I found most interesting was [the authors’] pointer to a recently published paper (Habtie & Das 2025, MNRAS, 537, 2046) pointing to their measures leading up to the recent eruption of RS Oph. In the year or so in advance of eruption, the emission line strength increased by over a factor of ten times. This precedent circumvents the lack of any theoretical understanding about how the accretion disk (?) can know in advance that the temperature and pressure deep on the white dwarf is approaching the critical trigger conditions.”
Responding on the same forum thread, AAVSO executive director Dr. Brian Kloppenborg took issue with the second paper’s conclusion that the “sudden rapid increase in emission line strength indicates an imminent eruption.” He pointed out that “photometrically . . . T CrB is highly variable” and that earlier instances of H-alpha emission fluctuation were not predictive of a nova event:
“If the authors were to investigate the high resolution spectroscopy around the events mentioned above, they would find that H-alpha was at a significantly greater intensity and greater width than normal. Yet in all of these situations, the eruption didn’t happen. Thus, I think we need to treat these papers with a high degree of skepticism.”
Continued Monitoring is Critical!
Clearly, the exact timing of T CrB's next eruption remains uncertain. Continuous monitoring is essential to capture the eruption's onset and evolution, providing valuable data to enhance our understanding of recurrent novae. AAVSO observers play a pivotal role in this endeavor, contributing to a global effort to document this rare and dynamic astronomical phenomenon.
And if a weary observer decides that one night—perhaps tonight!—they will relax their usual vigilance, and forgo their observational duties of T CrB? Let me remind you of the words of Leslie Peltier, who tracked the star for years, setting an alarm for late-night sorties, only to miss the 1946 eruption when he awoke with a cold and decided to remain in bed:
“We had been friends for many years; on thousands of nights I had watched over it as it slept and then, it arose in my hour of weakness as I nodded at my post. I still am watching it but now it is with wary eye. There is no warmth between us any more.”
T CrB stands on the cusp of a remarkable transformation. The collaborative efforts of the astronomical community, bolstered by vigilant monitoring and data collection, will ensure that we are prepared to witness and study this extraordinary celestial phenomenon. 