Australians may catch a glimpse of a stellar explosion in the next few months — if predictions are correct.

T Coronae Borealis, aka “the Blaze Star”, last lit up the sky 80 years ago, and astronomers believe it’s ready to go off again. 

While scientists are divided on exactly when it will explode, they agree it will be an exciting event, and have their telescopes trained on it.

“Pretty much a hydrogen bomb explodes on the surface [of a star],” Tanya Hill, an astronomer and curator at the Melbourne Planetarium says. 

“I think that’s pretty neat.”

Whether or not we’ll be able to see it will come down to timing.

Here’s what you need to know about this astronomical event, and how to maximise your chances of seeing it in Australia.

How bright will it be? 

T Coronae Borealis is a pair of stars in the constellation Coronae Borealis, which, in Australia, can be seen in the evening sky from May to the end of September.

The stars, which are about 3,000 light-years away, are normally too faint to see with the naked eye. At about +10 in magnitude — a logarithmic measurement used by astronomers — you usually need a hobby telescope to glimpse the pair.

But about every 80 years or so, one of the stars lights up in a stellar explosion known as a nova (more on this later) before returning to its normal brightness in a few weeks.

When the star explodes, it shoots up in brightness, ending up at magnitude +3 or +2.

According to Dr Hill, magnitude +3 will be about as bright as Delta Crucis, the dimmest of the four major stars in the Southern Cross. That’s easily visible to the naked eye, even in light-polluted areas. 

Once the blaze star explodes, it may be as bright as Delta Crucis, the blue star on the right side of this image of the Southern Cross. (Getty Images: Haitong Yu)

“It’s pretty exciting that there will be a star where there wasn’t one before,” Dr Hill says.

“But it won’t set the night sky ablaze.

“You probably want to go out and do some homework first, so that you can see what the [Corona Borealis] constellation looks like without the star, and then you’ll easily be able to see it.”

Once the explosion occurs, T Coronae Borealis might look like a normal star to the naked eye, but astronomers are excited to use high-powered telescopes for the first time on the stellar explosion.

“The whole community, space telescopes, everything, will be pointed at this star.”

When will it happen?

Astronomers are split on whether we will see it in the next couple of months, or if the explosion will happen over the next few years. 

“While it could happen as early as this year, it is more likely to happen in the range of 2025 to 2027,” astronomer Alex Kemp predicts. 

The explosion was last seen in the first few days of 1946, when it became visible without a telescope, before slowly fading back into the cosmos over the following month. 

It was the second time in 80 years that star was visible with the naked eye from Earth. 

According to Dr Kemp, who is based at KU Leuven in Belgium, it stands to reason that we will see the next explosion in 2026 — 80 years after the last one. 

But there’s another group of astronomers who believe that the nova will occur significantly earlier than that, potentially between now and September this year.

This is because of a telltale dip that occurs before the star shoots up in brightness.

Eighty years is a long time between explosions, so although the nova was spotted in 1866, we only have detailed information from the 1946 nova. In May 1945, the Blaze Star dimmed slightly before skyrocketing up to full brightness in February 1946. 

 In 1946 T Coronae Borealis dipped in brightness a few months before exploding.    (Wikimedia: PopePompus/CC BY 4.0 SA)

This time around, astronomers saw that dip in brightness in March 2023, suggesting a nova this year, Dr Hill says.

“Of course, the star might decide to do something completely different. That always has to be a caveat.”

While this lack of certainty may seem a little frustrating for anyone who just wants to see an exploding star, even rough predictions are exciting for astronomers like her.

“The fact that we can narrow it down to six months when the stars are 3,000 light-years away, I think that’s a really pretty remarkable achievement.” 

Is T Coronae Borealis visible from Australia?

The timing of the nova will be important in determining whether or not Australians will be able to see it.

Luckily, the constellation is visible from Australia in the evening for a few hours over the next couple of months.

In more northern latitudes around the country, like Brisbane and Darwin, the constellation will be higher in the sky, but any explosion should still be visible right across Australia around 8:30pm to 9pm local time.

T Coronae Borealis will be visible in the Australian night sky for the next few months. (Stellarium/ABC)

According to Dr Hill, the star will be close to the horizon facing north between bright stars Vega and Arcturus until the end of September.

But it will duck below the horizon in October, and won’t be visible to Australian viewers again until next year.

“By late October and through November, no-one [worldwide] will be able to see it because the Sun will be in that part of the sky,” Dr Hill says.

“That would really be the worst luck.”

What is a nova?

Unlike our Sun, sometimes stars, such as those that make up T Coronae Borealis, come in pairs.

An artist’s concept of a nova shows a red giant star and white dwarf orbiting each other.(NASA: Goddard Space Flight Center)

Novas are made up of a white dwarf — an old Sun-like star that has exhausted its fuel leaving only a core behind — and a companion star, Dr Kemp says.

“Despite being star-like in mass, white dwarfs are approximately Earth-like in size, meaning they are extremely dense with brutally strong gravity on the surface.”

In the case of T Coronae Borealis, the second star is a red giant, which is a younger version of a white dwarf, but is in a later phase of life than our Sun.

Normally, the stars spin around each other, bound by gravity.

Occasionally, though, they get too close. The white dwarf pulls material, mostly hydrogen, off its companion, which accumulates on its surface.

As the hydrogen piles up on the white dwarf, temperature and pressure rises too.

With enough hydrogen on its surface, the white dwarf explodes, producing a runaway Sun-like fusion reaction.

It glows incredibly brightly over a short period until the hydrogen is all burned away.

“The nova eruption lifts off almost all of the accumulated material, but the underlying white dwarf is relatively unharmed,” Dr Kemp says.

“After the nova, it will begin to [accumulate] material from the companion star again until the next event.”

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