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T Coronae Borealis

April 1, 2024

There are about a quarter of a million stars within 250 light years of Earth, most visible to space observatories such as the European Space Agency's Gaia. The space observatory, Gaia, which I wrote about in a previous article (Gaia Asteroid Census, August 8, 2022), can detect stars brighter than apparent magnitude 20 in a wavelength range of 320-1000 nanometers; that is, from near ultraviolet light, through the visible spectrum, and including short wavelength infrared light.

Gaia sky coverage visualization

This is a visualization of Gaia's sky coverage. It would make a pleasant wallpaper image for a cellphone or tablet computer. (A still frame from a Wikimedia Commons animated image by B. Holl of the University of Geneva, Switzerland, and A. Moitinho and M. Barros of CENTRA, University of Lisbon.)

Before telescopes, ancient astronomers had only their unassisted eyes available for observation, and this placed a limit on the number of visible stars. The best human eyes can discern stars down to magnitude 6.5, and there are 9,110 such stars, as tabulated in the Yale Bright Star Catalog.[1] There's one star, T Coronae Borealis, that's briefly visible to the unassisted eye. This star is a recurrent nova in the constellation, Corona Borealis, and it exhibits sudden and extreme outbursts in luminous intensity. For T Coronae Borealis, the interval between outbursts is about 80 years, and an outburst increases its apparent magnitude from 10 to 2.5 ± 0.5.

The location of T Coronae Borealis (circled in red)

The location of T Coronae Borealis, as circled in red.

Variable star such as this are not labeled with a Greek letter; rather, they are labeled with Latin letters R through Z. This means that T Coronae Borealis was the third discovered variable star in Coronae Borealis.

T Coronae Borealis is located at right ascension, 15h 59m 30.1622, and declination, 25° 55' 12.613", and it's 2629 light years (806 parsecs) from Earth.

(Wikimedia Commons image by the International Astronomical Union and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg). Click for larger image.)

An outburst of T Coronae Borealis was first discovered by Irish astronomer and polymath, John Birmingham (1816–1884), in 1866, reaching peak magnitude of 2.0 on May 12, 1866. It had been a known star long before that time, and outbursts may have been observed in 1217 and in 1787. A subsequent outburst to magnitude 2.5 ± 0.5 was detected on February 9, 1946. The star is the white dwarf companion of a red giant in a nearly circular orbit of period 228 days at 0.54 astronomical units (AU) spacing, somewhat larger than the orbit of Mercury around the Sun. The red giant transfers material to T Coronae Borealis.

T Coronae Borealis is one of 10 known recurrent novae in the Milky Way Galaxy, and the mechanism for its outburst is known.[4] As the white dwarf companion of a red giant, it collects matter ejected from its unstable red giant companion.[2] This is facilitated by the white dwarf's high mass and relatively small size that makes it gravitationally strong.[3] Eventually, this matter infall causes a runaway thermonuclear reaction when its temperature reaches a few million degrees Celsius.[2-3] Since the nova eruption involves just a small fraction of the mass of the binary star system, this process can repeat.[4]

An artist's conception of a recurrent nova

An artist's conception of a recurrent nova in a binary star system composed of a white dwarf and a red giant. The white dwarf is hidden in an accretion disk around the red giant, while a stream of material flows from the red giant to the white dwarf. Eventually, enough matter is captured by the white dwarf to ignite the nova. (NASA Goddard Space Flight Center animation.)

According to NASA, the next eruption of T Coronae Borealis is expected this year. This prediction comes not just from the historical 80 year interval between eruptions, but by the observation that the star dims about a year before eruption.[3,5] Also detected was a flicker of up to 0.2 magnitude on November 14, 2022, and on April 23, 2023.[4] A recent paper in the Monthly Notices of the Royal Astronomical Society examined the light curve of T Coronae Borealis from 1842 to 2022 to predict an eruption at 2024.4 ± 0.3; that is, any time between now and September.[2-3,5]

At eruption, the anticipated magnitude of 2.0 will give the nova a similar brightness to Polaris, the North Star, making it visible to the unaided eye.[2-3] The peak brightness could even be as much as that of the planet Mars, and it is expected to remain visible to the unaided eye for a few days, and visible with binoculars for about a week.[2-3]


  1. Yale Bright Star Catalog, via Harvard University.
  2. Lauren Perkins, "View Nova Explosion, 'New' Star in Northern Crown," NASA Marshall Space Flight Center, February 27, 2024.
  3. Mia Taylor, "Move over, solar eclipse: Scientists predict a once-in-a-lifetime nova explosion in the coming months," BBC, March 22, 2024.
  4. Jeremy Shears, "Get set for the next eruption of the recurrent nova T Coronae Borealis!" British Astronomical Association, August 22, 2023.
  5. Bradley E. Schaefer, "The B & V light curves for recurrent nova T CrB from 1842-2022, the unique pre- and post-eruption high-states, the complex period changes, and the upcoming eruption in 2025.5 ± 1.3," Monthly Notices of the Royal Astronomical Society, Vol. 524, No. 2 (March 14,2023), pp. 3146-3165, https://doi.org/10.1093/mnras/stad735.

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