Waiting for the Blaze Star

When will T Corona Borealis go nova?

The Blaze Star, corrected for the southern hemisphere! (Credit: Adapted from The Farmers’ Almanac).

By Dirk Goës

The Blaze Star, whose scientific name is T Corona Borealis (T CrB), is a star which experiences a thermonuclear explosion on its surface about every 80 years.  The last explosion occurred in 1946 and according to observations and measurements made by astronomers in the last few years it is set to explode any day now.  This kind of event is called a Recurrent Nova.  T CrB is the brightest known recurrent nova in the night sky and therefore is a once in a lifetime event. 

At about 3000 light years distant from Earth the Blaze Star is normally too faint to see with the naked eye.  However, when the explosion occurs it will be visible as a bright star in the northern constellation of Corona Borealis.  Afterwards it will fade away, probably over a period of a few days. 

The Blaze Star has a companion star, and the two stars orbit each other in a binary configuration.  The Blaze Star is a white dwarf star whose strong gravity is pulling hydrogen gas off its red giant companion.  This gas builds up on the surface of the white dwarf and gets compressed.  As more gas builds up and gets compressed the temperature increases.  Once the gas reaches the critical temperature for nuclear fusion to commence a runaway thermonuclear explosion occurs causing the star to dramatically brighten for a short time. 

The explosion only occurs on the surface of the white dwarf star, and the star remains intact.  Once the accumulated hydrogen has burned away the process will start again with the next explosion occurring in another 80 years.  Hence it is called a recurrent nova.  The word nova or novae (plural) originates from the Latin nova stella for new star. 

Figure 1: Two stills from an animation simulating the recurrent nova T Coronae Borealis.  On the left (A) the white dwarf star is pulling material, mainly hydrogen, from its red giant star companion.  Once a critical threshold is reached the accumulated material on the white dwarf explodes as shown on the right (B) (Credit: NASA's Goddard Space Flight Center Conceptual Image Lab).

When will it explode?

The eruption of T CrB (The Blaze Star) has been observed twice in modern times.  Once in 1866 and once in 1946, making an interval of 80 years between outbursts.  Therefore, it is expected to occur again in 2026.  In addition, a recent study of historical records identified two additional observations of T CrB outbursts, one in 1787 and one way back in 1217 by German Monks.  These years also coincide with the explosion occurring approximately every 80 years. 

Historical observations of T Corona Borealis

• 1217 - By German Monks in Southern Germany: Reported in the Ursperger Chronicle, written in 1225 by the Abbott Burchard.

• 1787 - By English Astronomer Francis Wollaston: Francis Wollaston specialised in astrometry and measured a bright star at the exact location of T CrB.

• 1866 - By Irish Astronomer John Birmingham: The first eruption for which a light curve was recorded.

• 1946 - By 15-year-old schoolboy Michael Woodman: A letter from The Astronomer Royal to Michael Woodman confirming his discovery can be seen in this article from the BBC.

Professor Emeritus Bradley Schaefer from Louisiana State University has studied the light curve of T CrB in detail.  A light curve shows the measured brightness of an astronomical object over time.  Professor Schaefer has determined that the light curves for the 1866 and 1946 explosions are identical as are the timing of the different observed phases including the pre-eruption high state, the pre-eruption dip, the eruption itself, the secondary eruption and the post eruption high state.  In addition, at the peak of the eruption T CrB had a maximum brightness of 2.0 on the visual magnitude scale for both 1866 and 1946.  

In the last decade two events have occurred that caused Professor Schaefer to bring forward his estimates for the nova occurring.  In 2015 T CrB entered the pre-eruption high state causing the estimate to be revised to around May 2025.  In March 2023 T CrB commenced the pre-eruption dip causing the estimate to be revised to April 2024.  The revised estimates are based on matching the light curve from the 1866 and 1946 eruptions to the observed light curve since 1946 to the present date, as shown in figure 2.  These estimates have not come to pass but Professor Schaefer points out that the nova may not behave exactly as before and that astronomers do not currently have a good understanding of all the physical mechanisms behind the different observed phases. 

Figure 2: The light curve of the 1946 eruption of T CrB plotted from AAVSO data showing the main phases.  The apparent magnitude is the brightness as seen from Earth (Credit: Adapted from PopePompus - Own work, CC BY-SA 4.0). 

Nova or Supernova?

A nova, like the Blaze Star, is a thermonuclear explosion that occurs on the surface of a star, but the star remains intact.  On the other hand, a supernova is a thermonuclear explosion that completely destroys a star or leaves behind a remnant in the form of a neutron star or a black hole.  A thermonuclear explosion is a fusion reaction which converts existing elements into heavier elements.  For example, hydrogen being converted into helium.    

Both a nova and a supernova can eventuate from a binary star system where one star is a white dwarf, and the other star is a red giant or a main sequence star (like the Sun).  If a supernova occurs from this type of binary star system, it is specifically called a Type-1a supernova. 

What then determines if a nova or Type-1a supernova occurs?  If the mass of a white dwarf exceeds 1.4 times the mass of the Sun, it will explode as a Type-1a supernova.  This is called the Chandrasekhar Limit.  If the mass of a white dwarf is near the Chandrasekhar Limit, then the accumulation of hydrogen drawn from a binary companion may push the mass of the white dwarf over the Chandrasekhar Limit causing a supernovae Type-1a explosion.  If, however, the mass of the white dwarf is somewhat lower than the Chandrasekhar Limit then the build-up of hydrogen may first reach the critical temperature for nuclear fusion to commence causing a nova explosion. 

It is also possible that neither a nova nor Type-1a supernova explosion will occur in this type of binary system.  This can occur if the gas being pulled in by the white dwarf is arriving at just the right steady rate and it is burning off in a fusion reaction as it arrives. 

A Type-1a supernova can also occur when two white dwarfs in a binary system undergo a merger.  When two white dwarfs are closely orbiting each other, their strong gravity can cause them to spiral into each other.  When they collide and start to merge their combined mass will exceed the Chandrasekhar Limit causing a Type-1a supernova. 

Figure 3: Four different scenarios where a binary star system containing one or two white dwarfs can result in a nova, a supernova Type-1a or a steady burn (Diagram: Dirk Goës). 

It is important to note that the scenarios presented above are the subject of intense research and not yet fully understood. 

Brightest of the nova

The Blaze Star (T CrB) is the brightest of the ten known recurrent novae in our home galaxy, the Milky Way.  At a peak brightness of magnitude 2.0 it is visible even under light polluted skies.  The next brightest is RS Ophiuchi with a peak brightness of magnitude 4.8, which is difficult to see under light polluted skies without binoculars or a telescope.  However, RS Ophiuchi experiences an outburst more often with recent eruptions in 2021, 2006 and 1985. 

A unique feature of T CrB over the other recurrent novae is the secondary eruption that occurs after the main one as shown in figure 2. 

As well as recurrent novae there are also classical novae.  A classical nova is one where only one eruption has been observed.  If a second eruption occurs it would be reclassified as a recurrent nova.  The brightest classical nova ever observed was V603 Aquilae in 1918 with a peak magnitude of -0.5.  This made it brighter than every star except for Sirius and Canopus.  The most abundant type of novae are dwarf novae which have more frequent outbursts but are not as bright. 

All these different types of novae come under the larger category of cataclysmic variables.  A cataclysmic variable is a close binary system consisting of a white dwarf accreting material from a main sequence or red giant companion star. 

Finding the Blaze Star

Figure 4: The constellation of Corona Borealis as viewed from the southern hemisphere with the location of T Coronae Borealis marked with a dashed circle. (Credit: Stellarium).

From the southern hemisphere the constellation Corona Borealis is visible in the evening sky from June to September and in the early morning sky from mid-January to May.  For example, from Sydney Australia it will be located due north about 30 degrees above the horizon in mid-July around 8pm.  It will be in a similar position around 4am in mid-March. 

Use the star chart in figure 5 to find Corona Borealis.  First look for the brilliant and fourth brightest star in the sky Arcturus in the constellation of Boötes.  Look below Arcturus and a little east to find the second brightest star in Boötes called Izar.  Now move your eyesight directly east until you encounter the brightest star in Corona Borealis called Alphecca.  Let your eyes adjust to the night sky and you should be able to make out the seven main stars of Corona Borealis (“Northern Crown”).  When the nova occurs, you will see the Blaze Star to the east of Corona Borealis as shown in figure 5.  It is expected to be about as bright as Alphecca. 

Figure 5: Star chart showing how the Blaze Star (T CrB) may appear in the northern sky in the southern hemisphere in relation to the constellations Corona Borealis and Boötes. (Credit: Stellarium). 

If you are viewing under light polluted skies then you should still be able to make out the bright stars Arcturus, Izar and Alphecca as shown in figure 6.  Using binoculars, you should be able to make out the other six main stars of Corona Borealis and then estimate where the Blaze Star will appear. 

Figure 6: Star chart showing how the Blaze Star (T CrB) may appear under light polluted skies.  This view is of the northern sky in the southern hemisphere in relation to the constellations Corona Borealis and Boötes. (Credit: Adapted from Stellarium).

The beauty is in seeing the night sky change

The Blaze Star, T Cornona Borealis or T CrB is also known colloquially as “T Cor Bor” amongst professional and amateur astronomers.   It is highly likely that an amateur astronomer will be the first to spot it.  This is because amateur astronomers are in a better position to undertake long term monitoring of a single star system as opposed to professional research telescopes which are constantly being switched around to strict schedules to observe different objects for different research projects. 

There is a lot of misinformation on the internet which implies that the eruption of T CrB will light up the sky like a firework display.  The real beauty in appreciating this event is in learning where in the sky T CrB is located and then observing the amazing change when what looks like a new bright star appears where previously there was nothing visible to the naked eye. 

Further information:

The Science Show on the ABC Australia Listen App: Ready for the big flash by Dr Laura Driessen (University of Sydney) together with Dr Bradley E. Schaefer (Louisiana State University)  and amateur astronomer Peter Williams (Sutherland Astronomical Society).