Is T CrB About to Blow its Top?

The recurrent nova T Coronae Borealis last made a splash just after World War II. Does its current restive state hint at an imminent outburst?

Making Time for T

This finder chart covers about as much sky as the field of view in a typical pair of 7-power binoculars. It includes both R CrB (currently at ~14 magnitude) and T CrB. The italic numbers next to stars are their visual magnitudes to the nearest tenth (with the decimal point omitted), for comparison purposes. North is up and east is left. 

We've been struggling lately in northern Minnesota to get past winter and get on track with spring. That's why I was so surprised to step out my door the other night and hear the frogs in full, throaty chorus.

Variable stars can be like that, too. You can watch a particular variable for months, even years, and its brightness might fluctuate by a few tenths of a magnitude. Then all of a sudden, it blows up like a firecracker when you least expect.

Take T Coronae Borealis (T CrB). It's one of only about 10 stars in the entire sky classified as a recurrent nova, with two recorded outbursts to its name. Normally, the star slumbers at 10th magnitude, but on May 12, 1866, it hit the roof, reaching magnitude +2.0 and outshining every star in Corona Borealis before quickly fading back to obscurity. Eighty years later, on February 9, 1946, it sprang back to life, topping out at magnitude +3.0.

Many variable star observers include it in their nightly runs because it's easy to find 1° south-southeast of Epsilon (ε) in Corona Borealis and only requires a 3-inch telescope. Not to mention the huge payoff should you happen catch the star during one of its rare explosions. Famed comet hunter and variable lover Leslie Peltier faithfully kept an eye on T CrB for over 25 years, hoping to catch it in outburst. On that fateful February morning in 1946 he'd set his alarm clock for 2:30 a.m., planning to check in on several favorite stars before dawn. But when he awoke and looked out the window, he felt a cold coming on and allowed himself instead to go back to bed. Big mistake. That very morning, T CrB came back to life.

In his book Starlight Nights, Peltier writes:

"I alone am to blame for being remiss in my duties, nevertheless, I still have the feeling that T could have shown me more considerationWe 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 a wary eye. There is no warmth between us any more."

T CrB Sudden Shift

Light curve depicting T CrB's behavior between April 2011 and April 2016. Until February 2015, T CrB's brightness was almost constant. Notice the slight increase in brightness in February 2015 and the much more dramatic rise this winter and spring. The system's now a magnitude brighter than normal. Is a nova-like outburst in the offing?

T stayed under the radar for the next 69 years, holding steady around magnitude +10.2–10.3. That began to change in February 2015, when it inched up to +10.0 and remained there until early February this year. That's when things kicked into high gear with the star steadily growing brighter from late winter through early spring to reach its current magnitude of ~9.2.

Alongside the brightening trend, T's become bluer as well. Astronomers describe its recent unprecedented activity as a star entering a "super active" state. This last happened in 1938, eight years before its last great outburst.

T CrB followers can't help but wonder if the next night we look up, Corona Borealis will twinkle with "new" second-magnitude star.

Two For T

Stars like T CrB involve a red giant closely paired with a white dwarf. The giant feeds hydrogen gas into a swirling accretion disk around a massive, compact white dwarf at a rate a million times greater than the solar wind. Material funnels from the disk onto the dwarf's surface until it ignites in a thermonuclear explosion similar to a nova.

Recurrent novae are similar to nova and dwarf nova types but with unique characteristics that set them apart. All three types occur in close binary stars and involve mass transfer from a normal star to a small but gravitationally powerful white dwarf. Classical novae have only been seen in outburst once and typically brighten by 8-15 magnitudes before slowly fading back to their pre-outburst brightness. Dwarf novae outburst frequently — every 10-1,000 days — with moderate increases of 2-6 magnitudes. Recurrent novae fall in between and typically vary by 4-9 magnitudes over a 10-100 year period.

Use this finder chart to close in on T CrB. The "42 star" is Epsilon CrB. South is up. Click for a large version. AAVSO

Use this detailed finder chart to close in on T CrB. Numbers are star magnitudes with the decimals omitted. The star marked "42 star" is Epsilon CrB. South is up. Click for a large version.

T CrB has two components: a red giant star in a close, 227-day orbit with a planet-sized white dwarf. Material spills from the giant and accumulates in an accretion disk around the dwarf. Some of that gas gets funneled down to the dwarf’s surface, becomes compacted and heated, and eventually ignites in a spectacular thermonuclear explosion. We see the results as a sudden brightening of the star.

It's even theoretically possible for enough matter to accumulate on the dwarf to push it past the 1.4 solar mass Chandrasekhar Limit, forcing the entire star to burn explosively as a Type Ia supernova. At T CrB's 2,500 light-year distance, it would easily cast shadows!

Maybe we'll have to wait until 2026 (80 years after the 1946 eruption) for T's next upheaval. Or maybe not. Either way, let Leslie Peltier's story serve as a cautionary tale. Keep a close eye on this star every clear night, and expect surprises.

9 thoughts on “Is T CrB About to Blow its Top?

    1. Bob KingBob King Post author

      You’re welcome, Bob-Patrick. Anytime I can quote Leslie Peltier it brings back good memories of reading and so thoroughly enjoying his book “Starlight Nights”. It inspired me to start observing variable stars.

  1. William-Moser

    Great article, Bob. I was wondering as I read it if you would mention Leslie Peltier, and you didn’t let me down. I’ve read Starlight Nights many times, and I think the only part I liked better than his T Coronae Borealis story was the one about how he reported his first comet discovery. Clear Skies.
    Bill Moser

  2. OwlEyeOwlEye

    I had a look at T this morning at about 10 UT, and it appeared brighter than the 9.9 mag. star, and and nearly equal to or just barely fainter than the 9.4.

    Will be keeping an eye on this one – for another decade, if necessary! Thanks for the post.

    Doug Z

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