Star explosion will be visible in the sky

The nova happens because material from the neighbouring star falls onto the surface of a white dwarf (above) and a thermonuclear explosion occurs there (illustration).

Experts are eagerly awaiting a unique event: a "new" star appears in the sky - a nova. All signs indicate that a gigantic explosion is imminent, which will also be clearly visible on Earth. With an apparent brightness of around two magnitudes, the eruption will be about as bright as the North Star in the sky. This can be predicted because the expected outburst of brightness of the pair of stars known as T Coronae Borealis is repeated regularly at intervals of several decades. The last time this happened was in 1946, when gas gradually rained down onto the surface of a white dwarf. And at some point, this shell becomes so dense and heavy that nuclear fusion ignites in it and sends an enormous flash of light out into the universe.

The two stars in the constellation Northern Crown (Latin: Corona Borealis), which orbit each other at a distance of almost 3000 light years, form an unequal pair. One is a red giant star, a celestial body at the end of its existence, whose comparatively cool shell reaches far out into space. The other is a hot but very compact stellar remnant with about the mass of the sun but barely more than the diameter of the earth: a white dwarf. They orbit each other at about half the distance between the Earth and the Sun. This is close enough for hydrogen-rich gas to constantly flow from the shell of the giant star to its partner. The gas orbits the star in a flat disc and gradually falls onto the surface of the white dwarf. At some point, the mass of this envelope is so great that hydrogen can fuse into helium on its underside. The energy released in the process heats the shell to up to 300 million degrees, hurls the gas into space and produces a flash of light that can be seen on Earth with the naked eye.

Novae are by no means rare. Such explosive binary stars flash in the universe all the time. The remarkable thing about T Coronae Borealis is that this happens again and again at comparatively short intervals - in 1787, 1866 and 1946, the "new star" flashed in the night sky for a short time. Detailed observations exist of the last two eruptions, which not only provide information about the nature of these explosions, but also enable a detailed prediction of the next explosion. The decisive warning sign: the star first becomes significantly brighter ten years before the eruption, and then immediately before that it becomes fainter again over a period of one year - until the explosion.

In 2015, experts observed that T Coronae Borealis actually became brighter - just in time to keep to the approximate 80-year rhythm of the last three outbursts. As expected, the star's light curve then began to drop significantly in the first half of 2023. From this, the American Association of Variable Star Observers (AAVSO) calculated the period from February to August 2024 as the most likely period for the outburst. This period has already elapsed, so no one can now estimate when the time will really come. One thing is certain, however: a star will come.

Spectrum of Science

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