T Coronae Borealis Isn't the Only Star Ready to Blow — Meet U Gem

Every clear night before going to bed, I look out my front room window and check on T Cor Bor. T Corona Borealis is a recurrent nova that blows its top about every 80 years. The last eruption occurred in February 1946, when it rose to magnitude 2.0, briefly outshining the constellation's brightest star, Alphecca (mag. 2.2). So far, T CrB has refused to perform, even though a dip in its light in 2023 — a near-match to one that preceded the 1946 eruption — seemed to portend an imminent outburst last year.

We continue to wait, watch and hope. At times, it can feel like the star is yanking our chain, daring astronomers to make one more prediction to prove it has the upper hand. But as Yoda from Star Wars might say, "Predict astronomers will." Jean Schneider of the Paris Observatory picked up the gauntlet in his open access paper When Will the Next T CrB Eruption Occur? published in October 2024. Based on previous eruption dates and the binary star's orbital ephemeris (all novae and recurrent novae are binary systems), Schneider devised a method to tentatively anticipate "with a precision of a week or two" when T CrB will pull the trigger.

He expects the next opportunity to occur on March 27th. Should that fail, the next possibility would be one 228-day orbital period later on Nov. 10th. And if those don't pan out, T CrB completes additional orbits on June 25, 2026 and Feb. 8, 2027.

have a blast anyway

Variable star observers are accustomed to waiting. That's why we keep a list of targets to routinely check in hopes of catching at least one in outburst. Recurrent novae like T CrB are rare — there are only 10 known in our galaxy. But there are lots of other close binary stars, dubbed dwarf novae, that produce outbursts under similar circumstances. Their magnitude jumps may not be as extreme, but they're still dramatic, visible in modest instruments, and frequent enough to keep you on your toes.

U Geminorum, located near the Gemini-Cancer border, is the archetype of the dwarf nova class. English astronomer John Hind initially thought it was a nova when he discovered the star at 9th magnitude in 1855 while searching for asteroids. But just three months later, it was reobserved in outburst again and has been monitored by amateurs and professionals ever since.

About every 105 days, U Gem brightens a hundredfold from magnitude 14.0 to around 9.0 over the course of a couple nights, followed by a return to its dim state a week or two later. You can follow its entire cycle with an 8-inch telescope from a dark, moonless sky or catch the dwarf nova during its rise and fall in a 3-inch. The star links into an easily recognizable harp-shaped asterism of field stars, which I've outlined on the accompanying AAVSO finder chart.

T CrB and U Gem have much in common. Both are binary stars, with a hot, dense white dwarf and cooler companion in orbit about the pair's center of gravity. Also like T CrB, the white dwarf siphons hydrogen gas from its companion into a spinning accretion disk. In the former, the disk material get funneled down to the dwarf's surface, where it gradually accumulates until the heat and pressure become so extreme that the gas ignites in a runaway thermonuclear explosion. During a nova outburst, the white dwarf can brighten up to 100,000 times as the expanding debris cloud balloons into space at over 1,000 kilometers a second.

Shoveling coal in the furnace

U Gem's flare-ups originate from both the disk and transfer of material to the white dwarf. Mass transfer from the red dwarf pads the disk until it reaches a critical density, becomes ionized, and radiates light. The increase in density also causes the material to move through the disk more efficiently and spiral down to the dwarf's surface, where it's heated and consumed in a thermonuclear explosion — the light from which can last for days or even weeks. With the excess material transferred and ingested, the companion star proceeds to "restock" the accretion disk, setting the stage for the next outburst.

I've had the pleasure of watching U Gem vault to brilliance a couple times a year for decades. I especially enjoy catching the star rising from its slumbers to maximum. If you become a regular watcher, the rapid and dramatic change in brightness from minimum to maximum will never fail to amaze.

When might we expect U Gem's next flare? The more recent outburst occurred ~115 days ago around Dec. 1, 2024 — some 10 days beyond the usual waiting period. While its period can vary from as little as 62 days to 257 days, the time is definitely getting ripe for another flare-up. Hopefully, we won't have to wait long.