The star system T Corona Borealis (T CrB) belongs to a category of stars called recurrent novae. These objects periodically suffer large explosions that cause their luminosity to increase sharply and, for a few days, they can shine with unusual strength in the sky.
Previous T CrB bursts were documented in 1866 and 1946, although there is a suspicion that the phenomenon had already been observed much earlier. During these episodes, T Corona Borealis increases its light more than three thousand times, going from being a star barely visible with binoculars and only on dark nights to shining powerfully in the sky, perfectly observable with the naked eye.
T CrB is a binary system, composed of two stars with masses similar to those of the Sun and that are at different times in their evolution. One of the components of the pair is a white dwarf, a very hot and compact object that forms in the last stage of life of a solar-mass star. Its companion is a red giant, a star that has not yet reached the white dwarf phase and is greatly expanded. The two objects are very close to each other, which makes it easier for the white dwarf to attract material from the outermost layers of the red giant. And the accumulation of this material on its surface ends up causing cyclic thermonuclear explosions.
The periods of recurrent novae are difficult to predict with complete accuracy. Despite this, constant observations of T CrB seem to indicate that it is close to a new seizure, an event that could take place any time between now and September.
When the explosion occurs, T CrB will appear as a “new” star visible in the sky (hence the term nova with which this phenomenon is called), right in the constellation of the Corona Borealis to which it belongs. This constellation, which can already be observed on March nights above the eastern horizon, will gain height as the months go by and in summer it will be practically vertical at midnight. The figure of the Corona Borealis is easily recognizable in the sky as a semicircle formed by five stars.
The increase in brightness of T CrB is expected to turn it, for a few days, into a star similar in brightness to Polaris, our North Star. However, and although it should be possible to contemplate the phenomenon from almost anywhere, it is best to move away from urban centers and nearby light sources.
This is a unique opportunity to be able to observe, without the help of instruments, a phenomenon of this type (only a dozen recurring novae are known in our galaxy), so it is advisable to keep an eye on the news as it emerges.
A star lives in a delicate balance between the force exerted by its gravity, which pulls it inward, and the energy it generates in the core, through nuclear fusion, which opposes the former. This situation can continue for billions of years. The most common nuclear fusion reaction generates helium atomic nuclei from hydrogen nuclei, and operates in the heart of the star at a temperature of about 15 million degrees.
When the center of the star becomes depleted of hydrogen, the rate of nuclear fusion declines and the pull of gravity causes an increase in temperature that is sufficient to ignite hydrogen fusion again, but this time in the layer that surrounds the core of the star. The energy released by this process causes the outermost layers of the star to expand and cool, and the star becomes what we call a red giant.
This stage is characterized by frequent seizures in which large amounts of material are released into space. The evolution of the star continues with the fusion of the helium inside to form carbon, this being the last resource that the star will have to support its own weight (the most massive stars, although they live much less time, can sustain other fusion reactions complex nuclear).
Finally, with the core stripped, the star becomes a white dwarf, a very hot (about 100,000 degrees) and dense object, made up mainly of carbon (the result of the last nuclear fusion that took place in the center of the planet). dying star). Unable to generate more energy, the fate of a white dwarf is to slowly cool over billions of years.
However, some stars live in binary systems, in which two stars orbit around their common center of gravity. In this case, it may happen that one of the components of the pair evolves more quickly and becomes a white dwarf, while its partner is still in the expanded phase. So, if the two objects are close enough, the white dwarf can attract the gas that forms the outermost layers of the red giant.
Progressively, the white dwarf gains mass as more material falls, in a spiral path, towards its surface. But this situation has a limit, marked by the resistance capacity of the white dwarf to support its own weight.
When the mass of the white dwarf exceeds 1.44 times the solar mass (a limit called Chandrasekhar in honor of its discoverer), an uncontrolled fusion chain reaction suddenly begins that completely destroys the star in the middle of a spectacular explosion. thermonuclear. This phenomenon is known as a type Ia supernova.
But recurring novae are able to avoid this end. Before the Chandrasekhar limit is reached, the star can suffer violent but limited bursts on the surface that expel a large amount of matter and lighten the mass of the star. In this way, the object will survive to continue attracting material from the companion star and thus begin a new cycle that will lead to the next explosion.
This binary system is located about 3,000 light years away. Its components, the white dwarf and the red giant, revolve around their common center in 228 days, and are separated by a distance that is barely half that between the Earth and the Sun.
Previous outbursts were recorded in May 1866 and February 1946, and astronomers have estimated that these events repeat approximately every 80 years. In fact, some researchers have suggested that there are historical descriptions, for the years 1787 and 1217, that could fit perfectly with the observation of this phenomenon in the sky.
The forecast that various organizations, including NASA, have made for the next explosion places it at any time between now and the month of September. This estimate has been possible thanks to the detailed study of the variation in the light intensity shown by T CrB.
As the two stars of the binary system rotate around each other, they periodically obscure each other from our perspective. This causes the light we receive from the object to vary cyclically in its intensity. But analysis of data from the most recent explosion, that of 1946, shows that, a few months before the event, a marked decrease in T CrB light could be measured, clearly below that expected in the normal cycle. variability of the object.
This behavior is precisely what is observed now: a drop in luminosity that began approximately in the first quarter of last year and that continues to develop currently. For this reason, the explosion is considered imminent.
