T CrB map
Will Corona Borealis the Northern Crown add a new jewel this year? Some astronomers predict that the famous recurrent nova T CrB will erupt soon.
Bob King

On every clear night, I face Arcturus, then slide north and east to Corona Borealis hoping to see another jewel in the Crown. That gem would be the Blaze Star, which also goes by its variable star moniker T Coronae Borealis, or T CrB. It's one of just 10 recurrent novae known in the Milky Way; these are novae that erupt on time scales of less than a century. T CrB is the brightest of the bunch with a peak magnitude of 2.0, on par with its neighbor Alphecca.

The star erupted in 1866 and did so again 80 years later in 1946. While it's only been 78 years since the last blast, T CrB's behavior over the past year has mimicked what was observed just prior to its February 9, 1946, outburst. Should it continue along that path, astronomers think another eruption could be imminent sometime between now and September.

Find details of the star's past behavior and predictions for the future nova in the March 2024 issue of Sky & Telescope.

Nova anatomy
Novae occur in close binary systems where one star is a white dwarf and the other a Sun-like star or a red giant. The dwarf pulls material into a swirling disk, some of which spirals to the surface and ultimately ignites in a nova explosion. Most nova binaries complete their orbits in just a few hours; T CrB is unusual in having a much longer orbital period of 227 days.

Novae involve interacting, close binary systems. T CrB consists of a red giant in orbit about a white dwarf 1.35 times the mass of the Sun. The compact dwarf siphons gas from the giant into an accretion disk. Instabilities within the disk cause material to spiral down to the surface, where it's heated and compressed until it ignites in a runaway thermonuclear explosion. The blast creates a brilliant flash that quickly fades as the material expands and cools.

T CrB wide map
The recurrent nova T CrB nestles just below 4th-magnitude Epsilon CrB, 5.5° degrees east of Alpha (α) CrB, also called Alphecca. Star magnitudes are shown with decimals omitted.
Sky & Telescope

All of this happens at a rapid pace, so T CrB is expected to rise from around 10th magnitude to 2nd magnitude in just a few hours. Peak brightness lasts only about half a day. Within a week, the system will fade below the naked-eye limit. So when the moment comes, hope for good weather! If necessary I'll hop in the car and drive to clear skies to witness this once-in-a-lifetime event. I've kept my eye on the star for years and don't want to get skunked, like the famous variable-star observer Leslie Peltier did back in 1946. After diligently observing T CrB for more than 25 years, hoping to catch it during outburst, he chose to go to bed and sleep (for his health's sake) on the very night the star flared.

T CrB 1944-46 light curve
About a year before the 1946 eruption, T CrB entered a "high state," in which it was slightly brighter than normal, before fading again in a pre-eruption dip.
Courtesy of the AAVSO

Brad Schaefer (Louisiana State University) has been watching T CrB and estimating its brightness since high school. Examining historical records, he identified a likely eyewitness report of an even earlier outburst in 1787. Adding this observation to the others yields an average interval of 79 years between flares. That would imply we'd see the next blast in 2025. However, a careful comparison of the system's 1945–46 pre-outburst light curve with its recent behavior suggests the blast will come earlier, in 2024.

T CrB pre-eruption dip
This light curve of T CrB plots the system's changing brightness in visual (V) and blue (B) magnitudes (green and blue circles, respectively). Dust in the system may have blocked the red giant star's light, causing the large dip in V magnitudes seen in early 1945. Dimming began around 1945.0 (with an uncertainty of 0.3 years), about 1.1 years before the eruption.
Courtesy of the AAVSO

Just before the 1946 outburst, T CrB underwent a mysterious dimming from its usual magnitude of ~10.1 to around 12.3 — a plunge of more than 2 magnitudes! Soon after this pre-eruption dip, it blew its top. The time from the initial dip to the nova blast was 1.1 years, give or take a third of a year. Prior to the dip, the star was in its "high state" — a few tenths of a magnitude brighter than normal.

T CrB current pre-eruption dip
This plot of B- and V-band magnitudes shows T CrB's latest pre-eruption dip, which began in March 2023. The falloff is particularly obvious in the B-band. If the system follows the same pattern as it did in 1945 and 1946, we would expect the next blast 1.1±0.3 years after the start of the 2023 pre-eruption dip, or sometime between now and September.
Courtesy of the AAVSO

Amateur astronomers around the world routinely monitor T CrB and submit their magnitude estimates to sites such as American Association of Variable Star Observers (AAVSO). Starting in 2015, the star climbed to its high state. Then, in late March, it began to fade. If this is the pre-eruption dip, though, it's not as deep as the one in 1945, especially in the V-magnitude range.

Assuming the nova follows the same pattern as before, we should see it flare in 1.1 ± 0.3 years, or in 2024.4 ± 0.3. After the 1946 outburst, the star quickly plummeted in brightness, then underwent a second, smaller eruption to 8th magnitude, before returning to its high state and ultimately to quiescence. Will it follow the same drum beat in 2024–25?

T CrB detail chart
This map will help pinpoint T CrB as it fades after outburst. As with the map above, use the labeled comparison stars to estimate its changing brightness.
Sky & Telescope

How to report

It's always possible T CrB could break with tradition and do something else entirely. I encourage you to check on the star every night with binoculars. A pair of 10×50s can reach down to 9th magnitude and enable you to track any changes in brightness. If you see T CrB at 9.5 magnitude or brighter, immediately report your observation to the AAVSO site. You can register for a free account by clicking the Sign-in button at the top of the page. This allows you to access the T CrB Time Sensitive Alerts Forum thread. Post your observation there with the date, time and star's estimated brightness and ask for confirmation. If confirmed, that news will be relayed to The Astronomer's Telegram (ATEL) and distributed around the planet. You can subscribe to the ATELs for free at the link and stay current on T CrB's behavior.

If you don't plan on submitting observations and just want to know when it blows, go to My Account, Profile, Email Settings and check the box for Alert Notices. You'll now receive an email when novae are discovered and rare stars like T CrB show unusual activity.

Good luck! I hope you're the first to spot the nova.


Image of Genac


July 14, 2024 at 6:51 am

There is no passive alerting system for celestial events? Somebody has to come up with an app for that. Actively monitoring a forum is a weak substitute.

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Bob King

July 14, 2024 at 1:21 pm

Hi Genac,
As long as you subscribe to the Astronomer's Telegram that I described at the end of the article you'll be alerted as soon as anyone reports the star in outburst. The observation might show up on the Forum first but I'm sure it would be immediately communicated to ATEL, then you'd see a message pop up in your email.

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July 16, 2024 at 12:58 pm

I created a free account at the American Association of Variable Star Observers (AAVSO) website. From My Account, Profile, Email Settings, check the box for Alert Notices. I get an email for discovery of novae, unusual activity of variable stars, and requests from astronomers for simultaneous AAVSO observations.

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Bob King

July 16, 2024 at 1:30 pm

Dear phenk,

I was unaware that the free registration also included these benefits. This is great to know. I will add this information to the story. Much appreciated!

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