A Star is About to Explode: Bright Recurrent Nova T Coronae Borealis Makes its Return This Year

 Every 80 years, faint T Coronae Borealis erupts to shine in bright magnitude 2, temporarily altering the night sky — and we have all signs that the system is about to burst again. 

The constellation Corona Borealis where T CrB will erupt this year appears as in a crescent moon shape; Source: BBC Sky at Night

In 2016, astronomers noticed something odd about the binary star system T Coronae Borealis (T CrB). The star is so faint it usually is only visible through a telescope; but that year, T CrB brightened by more than an entire magnitude from 10.5 to 9.2. 

It was kind of a déjà-vu. In 1938, astronomers observed exactly the same brightening. And in 1946, the star erupted, glowing as bright as magnitude 2, equal in brightness to Polaris. 

T CrB is the prime example of a recurrent nova — nova eruptions that seem to follow a strict cycle. Novae are different from supernovae in that they aren’t as energetic, and therefore don’t destroy the progenitor star. 

In the T CrB sytem, chaos reigns. A red giant and a white dwarf orbit each other fairly close, closer than Earth is to the Sun. The red giant, having reached the end of its lifetime, slowly sheds its outer layers, which the white dwarf greedily accretes. 

During these eruptions, the accretion of material onto the white dwarf increases, subsequently brightening the system in the nightsky. 

With the brightening event observed in 2016, it quickly became clear to astronomers: T CrB would make a drama again. And, as predictable as the system is, the star temporarily dimmed to 12.3 last year, just like it did a year before the outburst of 1946. Based on our knowledge, the star should erupt again any moment — it could be right now, tomorrow, in a week, or in a month. 

The “deadline” astronomers have for the eruption is September — the timeframe is ideal for observation. 

T CrB is 3000 light-years from Earth in the constellation Corona Borealis, which can be easily found as it lies between the bright stars Arcturus and Vega. At the moment, Corona Borealis can be found in the Northern Hemisphere’s east in the late evening hours. 

When T CrB erupts, it will shine as a relatively bright star just below Epsilon Coronae Borealis, a magnitude 4 star. 

As a recurrent nova, T CrB is a unique system. We only know of a few recurrent novae in our galaxy, and most of them have been too faint to be seen with the unaided eye. The next-brightest recurrent nova was seen in 2021, when RS Ophiuchi erupted. However, it was still a magnitude 5 eruption, making it difficult to see it in light-polluted skies without any binoculars. 

Being relatively bright, T CrB is one of the rare examples of a celestial event observed prior to the age of telescopes. There’s evidence that an outburst from the system was observed in 1217 from Bavaria in the constellation then known as Ariadne’s Crown, the modern-day Corona Borealis. 

In 1787, the astronomer Reverend Francis Wollaston observed the constellation while working on a stellar catalogue, and one star in the catalogue has an equal position as T CrB in eruption. 

So, astronomers already knew centuries before T CrB was officially discovered that the system is a recurrent nova! The significance of this makes this year’s nova even more special. 

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