T Coronae Borealis

Not to be confused with Tau Coronae Borealis.

T Coronae Borealis (T CrB), nicknamed the Blaze Star, is a binary star and a recurrent nova about 3,000 light-years (920 pc) away in the constellation Corona Borealis.^[11] It was first discovered in outburst in 1866 by John Birmingham,^[12] though it had been observed earlier in quiescence as a 10th magnitude star.^[13] It may have been observed in 1217 and in 1787 as well.^[14][15] In February 1946, Michael Woodman, a 15-year-old schoolboy from Wales, observed a flare up, subsequently writing to the Astronomer Royal and leading to the theory that the star flares every 80 years.^[16]

T Coronae Borealis

Location of T Coronae Borealis (circled in red)
Observation data Epoch J2000      Equinox J2000
Constellation	Corona Borealis
Right ascension	15h 59m 30.1622s[1]
Declination	+25° 55′ 12.613″[1]
Apparent magnitude (V)	2.0–10.8[2]
Characteristics
Evolutionary stage	Red giant + white dwarf[3]
Spectral type	M3III+p[4]
Variable type	recurrent nova[2]
Astrometry
Radial velocity (Rv)	−27.96±0.07[5] km/s
Proper motion (μ)	RA: −4.461 mas/yr[1] Dec.: 12.016 mas/yr[1]
Parallax (π)	1.0920±0.0275 mas[1]
Distance	2,990 ± 80 ly (920 ± 20 pc)
Absolute magnitude (MV)	+0.16 (min.)[6]
Orbit
Period (P)	227.55 d[7]
Semi-major axis (a)	0.54 AU[8]
Eccentricity (e)	0.0[7]
Inclination (i)	67[9]°
Details[5]
Red giant
Mass	0.69+0.02 −0.01 M☉
Radius	63.5±0.3 R☉
Luminosity	583±4 L☉
Surface gravity (log g)	0.672+0.009 −0.007 cgs
Temperature	3,561±3 K
Metallicity [Fe/H]	+0.20+0.05 −0.03 dex
Rotational velocity (v sin i)	11.6±0.1 km/s
White dwarf
Mass	1.37±0.01 M☉
Luminosity	~100[9] L☉
Other designations
T CrB, AAVSO 1555+26, BD+26°2765, HD 143454, HIP 78322, HR 5958, SAO 84129, 2MASS J15593015+2555126, T Cor Bor[10]
Database references
SIMBAD	data

Description

The light curve of T Coronae Borealis during the time surrounding its 1946 eruption, plotted from AAVSO data

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,^[17] though a more recent paper shows the 1866 outburst with a possible peak range of magnitude 2.5 ± 0.5.^[18] Even when at peak magnitude of 2.5, this recurrent nova is dimmer than about 120 brightest stars in the night sky.^[19] It is sometimes nicknamed the Blaze Star.^[20]

Diagram of T Coronae Borealis based on a description given by Kraft^[3] using the updated mass ratio given by Stanishev^[9]

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.^[3] 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.^{[8][9][21][22]}

AAVSO light curve of recurrent nova T CrB from 1 Jan 2008 to 17 Nov 2010, showing rotating ellipsoidal variability. Up is brighter and down is fainter. Day numbers are Julian day.

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 radius of primary component orbit around the center of mass is 0.54 AU.^[23]

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.^[24] 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 or about 28 percent^[a]; the lowest brightness seen since 2016.^[25] A similar dimming occurred in the year before the 1946 outburst,^[26] suggesting an eruption before September 2024.^[27] As of April 2025, such a nova has not yet been observed, although some have predicted it is imminent.^[28]

Predictions of the next nova:

• 2026-2027 (made in 1946 either by N. F. H. Knight or W. M. Lindley)^[29]
• Mid February 2024 to end September 2026 (made in March 2023)^[30]
• Beginning January 2024 to mid August 2024 (made in June 2023)^[31] (lapsed)
• January 2024 (made in August 2023)^[32] (lapsed)
• End of October 2024 (made in June 2024)^[33] (lapsed)
• Around 27 March 2025 (lapsed), 10 November 2025, 25 June 2026 or 8 February 2027 (made in October 2024)^[34]

Notes

1. 100^-0.35/5 = 0.7244