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Óscar del Barco Novillo
University of Murcia
Friday, 14 June 2024, 12:30
There are spectacular astronomical events that we may be lucky enough to see more than once in a lifetime, such as a total solar eclipse or the luminous trail of a supernova (dying star) in the night sky, and even the trails blazed by majestic comets. But a much more exceptional event is about to happen in our skies, and it could even start tonight. It is a stellar explosion, a nova, a 'brightening' of a star that will be visible to the naked eye.
Nasa has announced that this astronomical phenomenon will soon be visible to the naked eye for several days.
This rare but spectacular nova event consists of a sudden increase in brightness in a certain part of space. From Earth it will look as if a star has suddenly appeared. This nova event will appear in the small constellation of Corona Borealis (the Northern Crown), about 3,000 light years from our planet.
Several of the brightest stars in this constellation harbour planetary systems, including Negolu, a Jupiter-like planet in whose atmosphere water vapour has been detected. It is in this patch of sky that a new ball of light will emerge, the result of a stellar explosion in a binary system consisting of a white dwarf star and a red giant.
How and when do these nova-like explosions originate? What will this new object look like in the sky?
A new star is not forming, nor is it a "stellar death" (as is the case in supernova events). The observed increase in brightness is the result of thermonuclear reactions in a binary system of two stars orbiting each other: the T Coronae Borealis (T Cr) system.
One of them is a white dwarf (called T CrB) with a mass similar to the Sun, but with a diameter about 100 times smaller, giving rise to an extremely intense gravitational field. Its companion star, a red giant T CrA, is losing matter, mainly hydrogen, due to the strong gravitational pull of T CrB. Therefore the hydrogen is gradually deposited onto the surface of the white dwarf.
As a consequence, the concentration of hydrogen on T CrB rises steadily and there is a build-up of pressure and heat until it reaches its limit. It then explodes in a colossal thermonuclear explosion, similar to the power of a hydrogen bomb exploding on Earth.
The dwarf star will return to its original state after this violent event, collecting its companion's hydrogen once more, and so on until the nova repeats itself in about 80 years' time. This is why it is so difficult to witness this event twice in a lifetime - the last time this nova occurred was in 1946.
We cannot usually predict with certainty when such a stellar explosion will occur. However, there is a small group of binary white dwarf systems that generate novas periodically, i.e. they are repeated in cycles of a few decades. These are known as recurrent or periodic novas and, fortunately, T CrB belongs to this select club.
In addition to knowing the timing as to when T CrB (dubbed the Blaze Star) will have reached its limit, researchers have other clues about the imminence of this nova event. Thus, according to Nasa scientist William J Cooke, the dimming of a white dwarf for the preceding year (known as a pre-eruption dip) is a clear sign of an approaching stellar explosion and the Blaze Star began to dim in March 2023.
So what exactly will we see in the night sky this summer?
When cataloguing bright objects in the sky, such as planets or stars, astronomers use a well-known parameter called apparent magnitude.
Basically, it is related to the brightness of a star as it appears in the night sky and depends on the luminosity of the star and its distance from our planet. The lower the apparent magnitude value, the brighter the object in the sky.
Therefore, for example, the apparent magnitude of the full Moon is -12.6, that of the star Sirius (the brightest star in the night sky) -1.46, while the pole or polar star (also known as North Star) gives a value of +2.
The T CrB star system has an apparent magnitude of +10 but, when the next nova explosion occurs, its visibility will increase significantly to a value of +2, comparable to the brightness of the North Star.
It will be at that precise moment that we will be able to see this 'new star' with the naked eye for several days, no need for any optical device, before it dims again and disappears, possibly for another 80 years..
For anyone interested in looking at the stars, we will be witnessing a truly unique astronomical event: the thermonuclear outburst of a white dwarf star.
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