The rare star explosion predicted for the coming months – 04/09/2024 – Science

As the world’s attention turns to Monday’s total solar eclipse, the distant Corona Borealis binary system is busy preparing for its own moment of glory: one of its stars will generate a spectacular nova explosion.

Located 3,000 light years from Earth, the system is in the sky of the northern hemisphere of our planet. It is made up of a dead white dwarf star and an aging red giant.

And the white dwarf — called T Coronae Borealis, or T CrB — is about to undergo a nova explosion that, according to NASA, occurs only once in a human lifetime.

The rare cosmic event is expected to happen sometime before September 2024. It will likely be visible to the naked eye in the Northern Hemisphere. NASA claims that expensive telescopes will not be needed to witness the cosmic event.

T CrB explosions occur only once every about 80 years. The last one was in 1946.

“I’m very excited. This is a bit like Halley’s Comet — it happens once every 75 to 80 years — but the new ones don’t get the same attention from the press that Halley did,” says the manager of the meteoroid environmental program at Nasa, William J. Cooke. “Comets always get the most headlines.”

How do scientists know when nova explosions will occur?

In most cases, according to Cooke, NASA experts have no idea when nova explosions will occur. But there are about ten known as “nova recurrents”.

“A recurring nova is a nova that explodes periodically,” explains Cooke. “And T Coronae Borealis is one of its main examples.”

But how does NASA know with such certainty that T CrB will specifically explode in the coming months? Well, it’s a matter of mathematical calculations and visible evidence.

The last time the T CrB suffered a new one, for example, was in 1946 — 78 years ago. And time doesn’t stop.

There is also another sign that T CrB is about to explode, according to Cooke.

“We know that before the nova explodes, it dims for about a year. And T Coronae Borealis started to dim in March 2023,” Cooke continues. “So we think it will explode by the end of September.”

The reliable frequency of nova outburst recurrence makes T CrB unique among the many novae identified over the years. And this is one of the reasons why the star explosion is so special.

“There are many novae that have been discovered, but most of them have no known recurrence — or they go very long periods of time without recurrence, so that we don’t know when they will explode again,” explains Professor Meredith MacGregor, an expert in stellar activity. from the William H. Miller III Department of Physics and Astronomy at Johns Hopkins University, in the United States.

The period of time until the nova spectacle repeats can vary from one to millions of years, adds astronomy professor Richard Townsend, from the University of Wisconsin-Madison, also in the United States.

What causes the event?

In addition to predicting when some of the most predictable events will occur, such as the T CrB explosions, NASA experts also know why these explosions happen.

The white dwarf T CrB, for example, is part of a binary system. This means it is 1 of 2 stars in orbit around each other. The other star in the system is a red giant.

White dwarfs have a mass similar to that of the Sun, but their diameter is about a hundred times smaller. Therefore, its size is comparable to that of Earth, according to Townsend.

The white dwarf’s large mass and relatively small size make its gravity very strong.

As the red giant of the Corona Borealis system ejects matter, the gravity of T CrB attracts or collects this material and retains it on the white dwarf’s own surface. This happens year after year, until a limit is reached.

“What happens in the system is that the red giant star dumps all this material onto the surface of the white dwarf,” explains Cooke. “And when the surface of the white dwarf (T CrB) receives excess material, you literally have a thermonuclear reaction, like a bomb, and the white dwarf expels that material.”

Townsend provides a similar description. He explains that when T CrB accumulates enough material and its temperature reaches a few million degrees Celsius, a nuclear fusion reaction begins to occur, creating the clearly visible nova event that many are eagerly awaiting in the coming months.

“These are the same reactions that occur in the core of the Sun and release tremendous amounts of energy in the surface layers of the white dwarf,” according to Townsend.

“The release of energy causes the white dwarf to temporarily shine brighter than its red giant companion and the total light emission from the two stars, when observed here on Earth, increases by 1,000 to 100,000 times.”

This type of explosion helps NASA experts understand the mass transfer that occurs between stars in binary systems and the thermonuclear explosions that result when the white dwarf explodes. It is a process that, in the case of T CrB, happens repeatedly.

“It continues to go through this continuous cycle of accreting material from the larger star,” explains MacGregor. “Normally, it takes thousands of years for this accumulation to reach the point of generating a nova. But T Coronae Borealis appears to do this much faster, making it a rare star.”

What can be seen in the explosion

Normally, the Corona Borealis star system has visibility magnitude +10 in terms of brightness, according to NASA.

But when the T CrB nova explosion occurs, visibility will increase significantly, reaching what is known as magnitude +2, which is much brighter than +10.

In comparative terms, +2 is a degree of brightness similar to the North Star. Therefore, when the explosion happens, T CrB will be visible to the naked eye.

People in the Northern Hemisphere who want to try to see the nova explosion should look to the sky for the constellation Corona Borealis, or Northern Crown — a small semicircle arc near the constellations Boieiro and Hercules, according to NASA. “This is where the explosion will emerge as a ‘new’ bright star,” explains the space agency.

But make no mistake: what is happening is not the formation of a new star. In fact, T CrB is simply becoming visible to us thanks to these distant nuclear reactions.

“It’s a star that already exists,” explains MacGregor. “The star has always been there, but to us it seems like a new star has suddenly appeared because we can’t always observe it.”

“White dwarfs are so small that we can’t see them with the naked eye. But because of the fusion reaction that’s going on, we can see it temporarily. You can go out at night and observe the event.”

When T CrB’s brightness reaches its peak, it could be as bright as the planet Mars, according to Cooke. It is expected to remain flaming and visible to the naked eye for at least a few days, but its explosive event could very well last more than a week.

And, when the white dwarf gets rid of all the accumulated material from its larger neighbor (the red giant), T CrB will again fall into obscurity, unseen for decades.

_{Read the original version of this report (in English) on the BBC Innovation website.}