Waiting for T Coronae Borealis (T CrB) to erupt: what could we see?

The astronomical community is waiting for the predicted outburst of T Coronae Borealis (T CrB, dubbed the Blaze Star), the brightest recurrent nova out there. Should the prediction turn out to be correct, what we will see?

Corona Borealis and the T CrB recurrent nova. 12 June 2024.

Corona Borealis and the T CrB recurrent nova. 12 June 2024.

* See T CrB with asteroid Pallas LIVE here on 24 June! *

The image above comes from the combination of 5, 120-second exposure, remotely taken with the Samyang 135+Paramount ME+ZWO ASI 6200MC Pro robotic unit available as part of the Virtual Telescope Project facility in Manciano, Italy.

The Corona Borealis (Northern Crown) constellation is gently marked, with T CrB indicated by an arrow on the bottom left of the picture (click on the image to see it at full resolution). Asteroid (2) Pallas is also visible and marked on the left side of the picture. To better see the recurrent nova, check the close-up below.

T CrB: close-up.

T CrB: close-up.

T Coronae Borealis, aka T CrB, is a truly important astrophysical object, a so-called recurrent nova, a system showing huge increases in brightness from time to time, called outbursts. The last eruption occurred in 1946, after the first one discovered in 1866; there were also possible outbursts in the XIII and XVIII Centuries. The next eruption is expected between June  and Sept. 2024, after the object behaved like just before the 1946 outburst. Normally around magnitude 10 and visible at least through binoculars, T CrB can become as bright as mag. 2, like Polaris, easily visible to the naked eye: among the 10 similar objects, T CrB is the one able to reach the brightest magnitude in our sky. It is the queen of its class ob objects.

Looking a bit more in detail, T CrB is a binary system, including a red giant and a smaller white dwarf. Material is transferred from the colder, red component to the hot white dwarf, via an accretion disk. In quiescence, the luminosity of the system is dominated by the red giant, as it is visible in the close-up above. The increasing amount of transferred material triggers from time to time the mentioned outbursts, because of a thermonuclear runaway, which at this point dominates the overall luminosity of the system.

To make it clear, the star involved will not be destroyed, as the recurrent nature of the event clearly indicates. The phenomenon in action here is very different for the explosion of a supernova.

Now, we would like to address the big question, perhaps the one of real interest to most sky enthusiasts: what will we see when the star is finally “in action”?

Based on what we know from previous outbursts, the expected maximum apparent brightness is around magnitude 2.0-2.5, not very different from that of the North Star (magnitude 2.0). Clearly, it could be even dimmer, like magnitude 3. Therefore, it will not be a spectacular star. Of course, it won’t be negligible either (the North Star is among the 50 brightest stars in the sky), but if you expect to see a dazzling point of light, you will be disappointed. In short, it won’t be a star that we notice by chance due to its brightness.

A critical issue is light pollution. To prepare for the appearance of T CrB, it is advisable to check how well the North Star can be seen in our sky. If it is difficult to discern, so will be the expected nova.

Furthermore, it will be useful to become familiar with the region of the sky in question, the one where the elegant and ancient constellation Corona Borealis is located. Greek mythology refers to it as the wedding gift of Dionysus to Ariadne of Knossos, cast into the firmament by Hephaestus. The celestial diadem is located in an easily identifiable portion of the starry vault, between the bright stars Arcturus (Alpha Boötis) and Vega (Alpha Lyrae).

The brightest star in the Corona Borealis is its alpha, Gemma, known in Arabic as Alphecca. It is worth locating this star because when T CrB is at its peak brightness, its apparent luminosity and color will be quite comparable to it.

Note that the recurrent nova will not remain at maximum brightness for long, fading in an handful of days, so be ready and let’s also hope for favorable weather!

A final note: on 24 June, asteroid (2) Pallas will be less than 1/4 of a degree from T CrB. A wonderful opportunity to have a look at and discover the Blaze Star before it erupts, so we will show the appulse live here!

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