T Coronae Borealis

T Coronae Borealis (T CrB), nicknamed the Blaze Star, is a recurrent nova in the constellation Corona Borealis. It was first discovered in outburst in 1866 by John Birmingham, though it had been observed earlier as a 10th magnitude star. It may have been observed in 1217 and in 1787 as well. It is expected to undergo an outburst again very soon; it could erupt again before August 2024.

Description
T CrB normally has a magnitude of about 10, which is near the limit of typical binoculars. Well documented outbursts have been seen twice, reaching magnitude 2.0 on May 12, 1866 and magnitude 3.0 on February 9, 1946, though a more recent paper shows the 1866 outburst with a possible peak range of magnitude 2.5 ± 0.5. Even when at peak magnitude of 2.5, this recurrent nova is dimmer than about 120 stars in the night sky. It is sometimes nicknamed the Blaze Star.

T CrB is a binary system containing a large cool component and a smaller hot component. The cool component is a red giant that transfers material to the hot component. The hot component is a white dwarf surrounded by an accretion disc, all hidden inside a dense cloud of material from the red giant. When the system is quiescent, the red giant dominates the visible light output and the system appears as an M3 giant. The hot component contributes some emission and dominates the ultraviolet output. During outbursts, the transfer of material to the hot component increases greatly, the hot component expands, and the luminosity of the system increases.

The two components of the system orbit each other about every 228 days. The orbit is almost circular and is inclined at an angle of 67°. The stars are separated by $-4.22$.

2016–present activity
On 20 April 2016, the Sky & Telescope website reported a sustained brightening since February 2015 from magnitude 10.5 to about 9.2. A similar event was reported in 1938, followed by another outburst in 1946. By June 2018, the star had dimmed slightly but still remained at an unusually high level of activity. By mid-2023, it faded by 0.35 magnitude; the lowest brightness seen since 2016. A similar dimming occurred in the year before the 1946 outburst, indicating that it could erupt between January 2024 and August 2024.