Analysis: Once every 88 years, a cataclysmic eruption makes the T Coronae Borealis appear so bright you can see it with the naked eye
An old friend of Irish astronomy will soon be appearing in the constellation of Coronae Borealis. After 88 years of dormancy, T Coronae Borealis (aka T Cor Bor, the Blaze Star) is expected to reappear, and will be visible to the naked eye as a "new" 2nd magnitude star. It's impossible to know when exactly it will become visible, but it's expected to be any day now. So what is T Cor Bor, why is it becoming visible, and why do we care about studying such systems?
A "cannibal" star
In 1866 the Irish astronomer John Birmingham, living in Tuam, Co. Galway noticed a "new" star in the constellation of Coronae Borealis. But what he was witnessing wasn't a new star, it was the cataclysmic eruption of an otherwise very faint star. You see, T Cor Bor is a cataclysmic, variable binary star system. Translated, that means it’s a system of two stars orbiting each other, and every now and again, one of the stars undergoes a "cataclysmic" event where it suddenly gets a lot brighter.
The stars themselves are a white dwarf star and a red giant star. The term "red giant" refers to a star which has exhausted all of the hydrogen fuel in its core, which powers nuclear fusion in stars such as our Sun, and has instead moved on to burning helium. The increased energy coming from helium burning has thus inflated the star compared to something like the Sun, making it a "giant" star.
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The white dwarf star is a more exotic type of star. It is the fate of all stars within the Universe, that form with less than eight times the mass of our Sun, to end their lives as a white dwarf. You can think of these as the exhausted cores that stars leave behind when they've spent all available fuel. They’re "white" because they tend to be incredibly hot, and a "dwarf" because they are very, very dense and very small. Typically, a white dwarf has the same mass as the Sun (roughly 300,000 times the mass of the Earth), but is the same size as the Earth.
So, in T Cor Bor, we have these two stars orbiting each other. In addition they are so close together that the white dwarf is ripping material off of the red giant, which spirals in towards the white dwarf, eventually settling down onto its surface - this is a process which is called accretion. This type of system (a white dwarf slowly consuming a nearby companion star) is what we call a cataclysmic variable.
The cataclysmic event itself
When the white dwarf in T Cor Bor is accreting matter from the nearby red giant, the entire system isn’t particularly bright. In astronomer parlance, its visual magnitude is normally about +10 (roughly 100 times fainter than the faintest star visible with the naked eye). Unfortunately, we’ve inherited a system for brightness from the ancient Greeks in which fainter objects have a higher magnitude.
So why does the system suddenly brighten to have a magnitude of +2? And why does it do this every 88 years or so? Remember when I said that white dwarfs are the exhausted, burnt out cores of dead stars? This means there isn’t any available fuel to power nuclear fusion in the star. But, remember that I said it's also ripping material away from the nearby star? The material that the white dwarf is pulling from the nearby star is nearly pure hydrogen - the perfect fuel to power nuclear burning. However, as this hydrogen settles onto the white dwarf's surface, the conditions aren’t quite right for fusion to occur. So it just sits there.
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As more hydrogen accumulates on the surface, the pressure and temperature of this thin layer of fuel slowly increases, until the conditions for nuclear fusion to occur are met, and then, suddenly, a cataclysmic event occurs: the entire surface of the white dwarf ignites, increasing the temperature and brightness of the white dwarf, as that thin layer of fuel is quickly consumed and processed into other elements. This whole process happens very quickly - the system will brighten from its normal magnitude of +10 to +2 overnight, and then will gradually fade as the hot layer of fused material slowly cools back down. This event is called a nova, which is short for "nova stella", the latin for "new star".
T Cor Bor is an example of a recurrent nova - that is, once it has settled back down, the white dwarf will resume the process of accumulating hydrogen from the companion star until the conditions to undergo a nova happen again. The timescale for this varies from system to system, and in T Cor Bor's case, it takes 88 years. We know this because the first time T Cor Bor was observed as a nova was in 1866, by John Birmingham, the Irish astronomer, and another nova outburst from T Cor Bor was observed in 1946. This is an 88 year gap, and its recent eruption now confirms this as the recurrence period for these systems.
So what do cataclysmic variables let us study?
There are many, many different types of cataclysmic variables. Some have red giant companions, such as T Cor Bor, but others have companion stars that are more similar to our Sun, or stars that have been stripped of their outer layers. The most extreme examples of these systems are that last one, as in this case, the binary systems are incredibly small - the stars complete a single orbit around each other in minutes (rather than the 1 year it takes for us to orbit the Sun).
It’s thought that these systems should produce very strong gravitational wave radiation, and they’ll become very important in verifying the performance of the European Space Agency’s Laser Interferometer Space Antenna (LISA), which is due to launch in the 2030s. These systems are also thought to produce type Ia supernovae, observations of which lead to the discovery of the acceleration of the expansion of the Universe in 1998 (and which was awarded the Nobel prize in 2011).
So understanding these systems and how they behave has important consequences, as they are natural probes for understanding the very nature of our Universe. Regardless, when you go outside to look at this "new star," remember - you are witnessing a white dwarf erupting with one last blast of energy, as if it is screaming out into the dark of the Universe, defying its very nature as a burnt-out, dead core.
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The views expressed here are those of the author and do not represent or reflect the views of RTÉ
