Despite earlier predictions that it would explode by September 2024, we’re still waiting. But it will happen — and when it does, it will brighten only briefly, so try not to miss it.
This closeup of Corona Borealis shows the location of T CrB with a green dot. In this view, North is up and East is left. Credit: Alison Klesman (via TheSkyX)
If I’m being honest, the title of this story should be “When is T Coronae Borealis going to explode again?”
T Coronae Borealis (T CrB) is classified as a recurrent nova — a star that blows its top over and over. Such objects are rare; fewer than a dozen have been identified in our galaxy. And despite predictions that it would explode last year, we’re still waiting.
For now.
Why T CrB explodes
T CrB is part of a binary system where a white dwarf and a red giant orbit one another. The red giant, near the end of its stellar life, is continually shedding hydrogen, some of which falls on the white dwarf. As the smaller star collects this gas, the pressure and temperature on its surface increase until the hydrogen ignites and, BOOM!
That boom happens roughly every 80 years. In fact, the last time an earthling saw T CrB with just their eyes was in 1946, the year of its most recent outburst. Normally, it hovers around 10th magnitude, some 40 times too faint for even seasoned sky observers to detect. (Also, in case you’re curious, the first time it was seen — or, at least, recorded — was way back in 1217.)
What we’ll see
When the star does have its outburst, it will rise in brightness to 2nd magnitude, equal to that of the North Star, also known as Polaris. How long will it stay that bright? Less than one day. Yep, that’s it. As soon as you read about it exploding, start making plans to head out that night. It might remain visible to naked eyes for a few days, but there are no guarantees about that.
This column by Stephen James O’Meara contains a side-by-side comparison of how Corona Borealis will look to the naked eye when T CrB is visible.
Why we still think it will explode soon
In April 2024, observers noted a dip in the brightness of T CrB. Such a dip is usually followed by the blast a few months later. Most scientists agreed that September was the most likely month. But September has come and gone and still nothing has happened, at least as of this writing.
Just this month, however, a team of scientists using the Alfred Jensch Telescope in Tautenburg, Germany, saw an increase in the strength of several lines in T CrB’s spectrum. This indicates the white dwarf is accreting matter at an increased rate, a state that should precede the explosion. They had previously noted this in November 2024, and that finding corresponded to a slight increase in the star’s brightness.
So far, however, there is no brightening of T CrB associated with the current spectral line strengthening. But the news has put teams of astronomers worldwide on the alert.
How to find T CrB
T CrB’s location is easy to find. It lies just south of a line between the northern sky’s two brightest stars, Arcturus and Vega. Starting at Arcturus, T CrB is about one-third of the way to Vega, although a bit below that line. As the name indicates, T Coronae Borealis is within the boundaries of the constellation Corona Borealis the Northern Crown. It lies roughly 5½° east of that star pattern’s brightest star, Alphecca.
So, keep your eyes on Astronomy.com for news that T CrB has briefly become bright enough to observe. Then run outside that night and gaze on a sight you’ll never see again.
