What a celestial explosion will reveal about the universe

Abbott Burchard of Upsberg in Germany was a good chronicler of events that took place in the world in the early 13th century. One of his recordings was of a celestial event in 1217 when a star suddenly seemed to have appeared in the sky. Such events were later noticed by astronomers, and most certainly by people from other cultures in the world. Tycho Brahe, the 16th-century Danish astronomer who made painstaking observations of stars and planetary positions, saw one such event on November 11, 1572. He called it Nova or a new star. It later transpired that the novelty was not in its existence but visibility from the Earth. However, Brahe's name stuck. Since then, all such events have been called novae.

A nova is now understood to be a sudden brightening of a star that may or may not have been visible from the Earth till then. The energy for the brightening comes from an explosion. Astronomers later discovered other explosions that were far more drastic than those of novae and hence called them supernovae. Both novae and supernovae have been seen by many cultures for a long time. Although the names are similar, modern physics tells us that novae and supernovae are very different events with entirely different underlying mechanisms. Nova repeats itself at regular intervals; a supernova occurs only once in the lifetime of a star.

The nova Burchard noticed was in the constellation Corona Borealis in the northern hemisphere. The name translates in English to 'northern crown'. There are several stars in the constellation, at various distances from the Earth, and the one spotted by Burchard is called T Corona Borealis. This star, roughly 2,600 light years from the Earth, brightens through an explosion roughly once in 80 years. The next explosion is likely to happen between September and November 2024. It can be seen with the naked eye for a little less than a week.

T Corona Borealis is a system of two stars rotating around each other. Such binaries are very common in the universe. In fact, a single star like the Sun is the exception; most stars consist of binaries or multiple stars gravitationally bound to each other. One of the stars of T Corona Borealis is a white dwarf, and the other is a red giant. A white dwarf is a dead star that has spent all its fuel and then exploded, leaving a very small and dense core. The Sun will eventually become a white dwarf after more than 5 billion years, leaving a core the size of the Earth but a few hundred times as dense. A spoonful of white dwarf matter will weigh as much as an elephant.

A star will die as a white dwarf only if its mass is not more than 1.4 times the mass of the Sun. This limit was calculated by S. Chandrasekhar, theoretical physicist and nephew of Indian physicist C.V. Raman. If a white dwarf exceeds this mass limit, known as the Chandrasekhar Limit, it will explode as a supernova and result in a neutron star, where protons and electrons have been pushed against each other so hard by gravity that they have become neutrons. Supernovae normally occur when a massive star dies through an explosion. They are extremely common in the universe: there is one every second.

The second star in the T Corona Borealis system is a red giant, an intermediate stage when a star expands before it collapses into a white dwarf. The Sun is expected to become a red giant after about 5 billion years. A red giant is a hot and thin ball of expanding gas. When the Sun becomes a red giant, the Earth will be inside it. In a binary system, when a red giant and a white dwarf revolve around each other, the surface of the expanding red giant can get too close to the white dwarf. Then, the white dwarf will pull the gas out of the red giant towards itself. This gas will revolve around the white dwarf and keep accumulating till it reaches a threshold. At some point, the gravitational pressure from the accumulated gas ring becomes so high that it ignites through a nuclear reaction, just as it happens in the core of the Sun.

This ignition happens in an instant, close to the surface of the white dwarf. The ignition results in an explosion that expels the rest of the gas into space. This explosion is powerful and can instantaneously release as much energy as a million suns. After a while, gas starts accumulating again, and the process repeats itself. In the case of T Corona Borealis, the cycle happens once every 80 years. Astronomers have noticed the expanding gas rings from previous explosions. It will continue the cycle until the gas from the red giant is exhausted. A red giant usually lasts about a billion years before exploding into a white dwarf.

The T Corona Borealis explosion will be visible from the Earth at night. In fact, since it is roughly 2,600 light years from the Earth, the explosion would have happened about 2,600 years ago. Compared to other stars, it will shine about as bright as the Pole Star. Such stars are common in the night sky. So, if we didn't know the exact positions of stars at night, we would not notice anything unusual in the night sky. However, it is an invaluable event for the astronomy community. Telescopes on the ground and in space are being focused on the star. Celestial events happen unexpectedly, often when no one is looking in that direction, and sometimes valuable data can be lost before telescopes are oriented towards the object. For T Corona Borealis, the entire community with an interest in the phenomenon will be watching it go.

Studying explosions is useful to astronomers because they reveal a lot of physics that is not apparent at any other time. Gas accumulating around a white dwarf does not always result in a nova. If the white dwarf is very close to the Chandrasekhar Limit, and if more gas falls into it or another white dwarf merges with it, there is enough pressure to ignite a nuclear reaction in the core of the white dwarf instead of its surface. Then, the part of the star can collapse into a neutron star, expelling the rest in a giant supernova explosion. Such supernovae are Type Ia supernovae. This explosion is usually a million times more powerful than a nova explosion. Or more.

Many supernovae in the universe are not born this way. They result when the original star is already very big, well above the Chandrasekhar Limit. Such a star produces heavier and heavier elements in its core as it burns, each time also producing heat that pushes the star stuff outward and counteracts gravitational collapse. This process ends when the star produces iron, which results in no heat. So, the core of the star collapses instantly, expelling the outer material in an explosion whose power is hard to comprehend from the Earth. Such explosions are called Type II supernovae, and their precise mechanism is more complicated than described here. Type II supernovae as far as 9 billion years away have been seen from the Earth with the naked eye.

Supernova explosions are invaluable to astronomers as they reveal much about the universe. Scientists learn about the life cycle of stars by studying supernovae. They reveal information about the early history of the universe. By studying supernovae, scientists learnt that the expansion rate of the universe was increasing. For a long time in the history of human beings, celestial explosions were seen but not understood as such. Now, human beings know about them in advance, and will get to watch one of them as it happens.