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Nova in Corona Borealis
Right Ascension=15 59 30.16
Declination= +25 55 12.6 (2000.0)
Quietly out of view for the past 80 years, the faint star T Coronae Borealis is about to "go nova." A seemingly new star perhaps as bright as Polaris (magnitude=2) will appear briefly in the constellation Corona Borealis (CrB), the Northern Crown. Within days it will fade back into obscurity for naked eye observers.
Astronomers have observed this star, dubbed The Blaze Star, in 1217, 1787, 1866, and 1946. Though it's known to be a recurring nova that explodes roughly every 80 years, the specific timing of its periodic recurrence is inexact. And so we wait. In the interim, astronomers are observing T CrB in anticipation of an imminent spectacle.
T CrB is actually a binary star. NASA animations depict a small, dense white dwarf star that is gravitationally sucking material from a nearby red giant star. As the small star cannibalizes the bigger one, stellar material accretes in a disk around the white dwarf star. Eventually, yet with regularity, the cannibal star pukes that accreted disk in a massive hydrogen explosion. Kaboom!
Material pulled from a red giant star accretes in a disk around a dense white dwarf star. When the disk reaches a critical mass, it explodes as a nova. Credit: NASA/CXC/M. Weiss
The outburst of surface material is not enough to blow the white dwarf star apart, but it brightens enough to be visible to the naked eye. The process will resume without interruption until about 2105, when the denser cannibal star will again reach its bellyful and explode anew.
Predicting Nova's Reappearance
Astronomers suggest several recent observations indicate the apparition will occur before its 8-decade bookend in 2026.
Over the last several years, T CrB has been ramping up from a "low state" to a "high state" similar to how it behaved in the mid-1800s and1940s. In 2024 astronomers observed the light curve of T CrB take a "pre-eruption dip" in brightness--again, paralleling the data from 1946. The light curves were lining up, and simple extrapolation suggested a 2024 nova was imminent.
The recent light curve of T CrB parallels the observations from 1855 and 1946.
Image: AAVSO, B. Schaefer
The American Association of Variable Star Observers (AAVSO) launched Observing Campaigns #875: Monitoring T CrB, which rallied amateur and professional astronomers alike to be on the lookout for an impending outburst. Despite some false alarms, nothing happened in 2024.
Additional observations in multiple wavelengths have reinforced expectations. For example, The Astronomer's Telegram lists:
Meanwhile, a paper authored by Jean Schneider, published October 2024 by the American Astronomical Society, concludes:
"By extrapolating the empirical fact that the previous T CrB eruption dates were separated by an integer multiple of the orbital period 228 days, the next eruptions should appear at 2025 March 27, 2025 November 10, 2026 June 25 or 2027 February 8. No physical hypothesis is made behind this extrapolation."
Nova Watch
I respect the cautious advice of astronomer Dr. Jim Kaler, who frequently added the caveat when citing new research that "nobody really knows for sure" why [fill in the blank...]. Nonetheless, in June 2024 I embraced the prediction and started my own campaign to document the rise and fall of T CrB with my cell phone camera.
Every clear night I have attempted to photograph Corona Borealis. Over 9 months through early March 2025 I've succeeded in imaging the constellation on over 100 clear nights. See blog posts tagged "nova."
Finding Corona Borealis
The easiest way to find Corona Borealis is to star hop from the handle of the Big Dipper, following the "arc to Arcturus." Star hop through two second-magnitude stars (Izar and Gemma), as shown. When T CrB explodes, it will appear nearby as a third second-magnitude star.
Coronal Borealis is shaped like a U. Over the hours, days, and months its orientation (and the Big Dipper's orientation) relative to the horizon changes. Sometimes it's upright; other times it's tilted to the right or left. Sometimes it's below the ground or too low toward the horizon. If you stick to the Dipper method you'll succeed in finding CrB, provided you can find the Big Dipper.
You can also use a phone app like SkyView®, which allows you to "see through" the screen and orient against foreground objects in the field of view, like the trees below. The app shows the general direction to look and to photograph, even if you can't see all the stars shown.
In early spring, Corona Borealis is rising before midnight (below). Toward the eastern horizon, the Crown is oriented as a backward C. As the morning hours pass the constellation rises and moves into the southern quadrant of the sky, appearing as a tilted U (above, in SkyView).
In the actual night sky, Izar and Gemma stand out as two similarly-bright stars, more obvious than the photograph suggests.
In early spring after midnight, star hop from the upside-down Big Dipper to find T CrB rising in the east, looking like a backwards C;
By summertime, if the star hasn't exploded, the sport takes on more civil hours. Rather than being a wee hours pursuit, you can simply wait until it's dark after sunset and CrB is well positioned. Come November, after sunset it's low above the western horizon.
Again, stick to the Big Dipper star hop and you'll find it readily. Here it is told better: https://www.mos.org/article/t-corona-borealis-recurrent-nova.
In early spring before morning twilight, star hop from the Big Dipper to find T CrB, looking like a C; March 10 at 6:00 AM.
Site of the Nova
Coordinates for T Coronae Borealis:
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R.A.=15 59 30.16
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Declination= +25 55 12.6 (2000.0)
T CrB is a faint 10th magnitude star (circled in red) that brightens to 2nd magnitude as a recurring nova every 80 years. Image: Marc Listello Astrophotography.
When reading star charts, remember the recurring nova is the star T in Corona Borealis, not to be confused with designated stars Tau or Theta.
In the real sky, the stars Izar and Gemma (a.k.a. Alphecca) will readily appear to be a similar brightness, whereas the cell phone photos make the stars look slightly more homogenous.
Observation reports by members of the American Association of Variable Star Observers (AAVSO) reveal a running graph of the star's magnitude.
AAVSO finder charts are at https://www.aavso.org/tcrb-charts-photometry-tables. The numbers show the magnitudes of the comparison stars, though the decimal points are removed for clarity. For example, a star labeled 84 denotes its magnitude equals 8.4.
T Coronae Borealis
Jim Kaler describes (2011) the dynamics of T Coronae Borealis (T CrB), and hints at an extreme case of nearby nova candidate Epsilon Coronae Borealis accreting mass that exceeds a tipping point:
...The "recurrent nova" T Coronae Borealis...is one of only eight known. Normally of tenth magnitude, T CrB consists of a class M3 giant in a 227-day orbit with a massive white dwarf. Tidally distorted by its companion (and thus variable as it presents different-sized cross-sections with different temperatures to us), the giant feeds matter onto the white dwarf (at a rate close to a million times that of the solar wind). When the layer of fresh hydrogen is sufficiently great, the heat of compression causes the layer to blow up like a hydrogen bomb. Normal novae are made of lower mass white dwarfs that feed off low mass ordinary dwarfs, and after the new layer is thick enough, should pop off every few tens or hundreds of thousands of years. But T's white dwarf is so massive that the intervals between explosions are short. In 1866 the star hit magnitude 2, and in 1946 mag 3, taking but a few days to drop back to invisibility. Are we due for another blowup around 2026?
Nobody really knows, but keep your eye on Epsilon [note: Epsilon Coronae Borealis is a nearby nova candidate]. The event could be far grander. The accreted matter could push the white dwarf over the fabled 1.4 solar mass limit, beyond which the WHOLE STAR would collapse and blow up in a grand (Type Ia) supernova that -- even at a distance of 2500 light years -- could become as bright as a crescent Moon!