T Coronae Borealis is one of the few known recurring novae…

Observation History

The star was first noticed in an outburst for certain in 1866 during an outburst by John Birmingham: earlier historical records may have also recorded it in 1787, and as far back as 1217.
The outburst in 1946 (seen by a Welsh teenager) led to the hypothesis that it was a recurrent nova, with the expectation that another outburst might happen before 2027.

The System

T CrB is a binary system with a red giant star and a white dwarf, in a nearly-circular orbit. The separation is about the same as the Earth and the Sun (0.96 AU), and they’re locked in a 227.58-day orbit.

System Parameters: T Coronae Borealis

The red giant is about 69% of the Sun’s mass, but with 63x the size - it reaches out to ~0.29 AU, almost one-third the separation, and the pull of the white dwarf stretches it out like tides, so that’s more egg-shaped than spherical. This causes light variations of half the orbital period (since you’re seeing different projections of the “egg” shape at different phases of the orbit), with an amplitude of ~0.5 magnitudes.

The white dwarf is the heavyweight of the system: 1.37 M☉, and in the distant past was a red giant star itself.

In its “quiescent phase” - most of the time - the light we see comes primary from the red giant (though in ultraviolet light, the white dwarf contributes more flux). During an outburst the transfer of material to the white dwarf increases, causing it to expand, and the overall brightness increases substantially.

What causes the eruptions?

Mass loss from the red giant coalesces around the white dwarf as an accretion disk, slowly dumping matter onto the white dwarf.

Why hasn’t it erupted yet?

Since the mass loss is almost certainly variable, the mass transfer to the white dwarf isn’t on a clock: therefore, “it happens when it happens”.

So - when will it happen?

An interesting study of the timings of recorded outbursts done by Jean Schneider shows that they appear to happen at the same orbital phase (which is odd, given that the binary’s orbit is nearly circular), give or take a month. The recorded eruptions seem to happen every 125-130 orbits.

Orbit 129 happens on June 27, 2026.

Or not?

Remember that no eruptions are recorded between 1787 back to 1217. Telescopic observations wouldn’t be possible before the early-to-mid 17th century, and while it defintely is visible to the naked eye during an eruption, they only last for a day or so. Thus, it’s possible that the (maybe) six eruptions between those two years just weren’t observed, or the records of anyone seeing “a new star” in Corona Borealis were lost.

Or - perhaps - the 80-year-ish “spacing” that we’ve seen since 1787 is a coincidental “fluke”. (There has been some thinking that it might be a triple system, and the third object has an 80-year orbit, so when it’s at its closest approach, that might contribute to a flare up - but no third star has been observed.

Stellar Activity?

Another possibly has to do with stellar activity. Cool stars, like the red giant have surface activity: starspots, which have cycles like the 11-year sunspot cycle. The measured rotational velocty of the red giant suggests that the rotational period is under 275 days, and if the star’s equator is aligned to its orbit, then < 240 days.
Stars also have differential rotation: on the Sun the equator spins faster than the poles. So it’s possible there’s a latitude where the surface rotation is close to the orbital period - that would mean that the star is rotating at the same rate it’s orbiting the white dwarf. If the activity cycle is ~80 years (it would vary a little), then at maximum activity the star would also experience other effects associated with higher activity: larger starspots and an increased frequency of flares and coronal mass ejections. If everything “lines up” it could be the case that an energetic flare up on the star drives an episode of mass loss, hitting the accretion disk, and subsequently dumping more mass onto the white dwarf, “igniting” it.

Finding T CrB

With the naked eye - if you’re hoping to catch the outburst (they only last a couple of days close to peak brightness), it’s just a little SE of the central star of the Northern Crown.

With a telescope, here’s a map showing faint nearby stars. The numbers are the magnitudes with the decimal point removed (so “108” would be mag 10.8).

So, Corona Borealis being a late-Spring constellations just East of Boötes rises just after sunset, making continued monitoring of the star convenient. Keep you eyes peeled! The “fuzzy” window of opportunity is probably anywhere from mid-May to mid-July (after which we’ll have to wait until 9 Feb 2027). Maybe you’ll get lucky!