Look Up! Once-in-a-Lifetime Explosion Set to Create ‘New’ Star in Sky
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Look Up! Once-in-a-Lifetime Explosion Set to Create ‘New’ Star in Sky

Now, every night, a “new star” or nova will appear in the night sky. While it won’t light up the sky, it is a unique opportunity to see a rare phenomenon that is usually hard to predict in advance.

The star in question is T Coronae Borealis (T CrB, pronounced “T Cor Bor”). Located in the constellation of the Northern Corona, it is visible in the Northern Hemisphere, but will also be visible in the northern sky from Australia and Aotearoa New Zealand for the next few months.

Most of the time, T CrB, which is 3,000 light-years away, is too faint to see. But once every 80 years or so, it erupts brightly.

A brand new star suddenly appears, although not for long. Just a few nights later, it quickly disappears, disappearing back into the darkness.

Explosion of life

At the peak of their lives, stars are powered by nuclear fusion reactions deep in their cores. Most often, hydrogen is fused into helium, producing enough energy to keep the star stable and shining for billions of years.

But T CrB has long since passed its prime and is now a stellar remnant known as a white dwarf. Its internal nuclear fire has been extinguished, allowing gravity to drastically compress the dead star.

Look Up! Once-in-a-Lifetime Explosion Set to Create ‘New’ Star in Sky
A white dwarf is about the size of Earth but about 300,000 times more massive, creating a powerful gravitational field.
ESA/NASA

T CrB also has a companion star—a red giant that has bloated into old age. The white dwarf wipes out the gas from the bloated red giant, and this creates what is known as an accretion disk around the dead star.

Material accumulates on a star that is already squeezed to its limits, forcing pressure and temperature to rise steadily. Conditions become so extreme that they resemble what once was in the star’s core. Its surface ignites in a runaway thermonuclear reaction.

When this happens, the released energy causes T CrB to shine 1,500 times brighter than usual. Here on Earth, it appears briefly in the night sky. With this dramatic reset, the star has thrown off its gas and the cycle can start over.

Animation of a nova exploding as thermonuclear reactions ignite on a smaller white dwarf star. Credit: NASA/Conceptual Image Lab/Goddard Space Flight Center.

How do we know it’s already due?

T CrB is the brightest star in a rare class of recursive novae that repeat themselves at intervals of a hundred years – a timescale that allows astronomers to detect their recursive nature.

Currently, only ten recursive novae are known, although there may be more – but on much longer time scales that cannot be tracked so easily.

The earliest known date for an outburst of T CrB is 1217, based on observations recorded in a medieval monastery chronicle. The amazing thing is that astronomers can now predict its outbursts so accurately, as long as the nova follows its usual pattern.

The star’s last two eruptions – in 1866 and 1946 – showed exactly the same characteristics. About a decade before the eruption, T CrB’s brightness increased slightly (known as a high state), followed by a brief dip in brightness about a year after the explosion.

Light curve of T CrB during the 1946 nova event, based on 6,597 observations recorded by the American Association of Variable Star Observers (AAVSO).
Museums Victoria/AAVSO

T CrB peaked in 2015, and a pre-eruption dip was seen in March 2023, putting astronomers on alert. The causes of these phenomena are just some of the current mysteries surrounding T CrB.

The latest light curve of T CrB shown in two filters or bands – V (green) and B (blue) – and compiled using 95,901 observations from the AAVSO. It is possible, especially in the B band, to see T CrB enter a high state in 2015 and now experience a decline before eruption.
Museums Victoria/AAVSO

How can I see this?

Start stargazing now! It is a good idea to get used to seeing the Corona Borealis as it is now to fully experience the impact of the “new” star.

Corona Borealis currently reaches its best viewing position (known as the meridian transit) around 8:30 to 9:00 a.m. local time in Australia and Aotearoa. The further north you go, the higher up in the sky the constellation will appear.

The further north you go, the higher Corona Borealis will appear in the northern sky. The new star will be about as bright as Alphecca in Corona Borealis or nearby Rasalhague in Ophiuchus.
Victoria/Stellarium Museums
While in the south in Hobart, the Corona Borealis remains low in the north. The bright star Arcturus acts as a good guide.
Victoria/Stellarium Museums
Throughout Aotearoa T CrB is best seen around 9pm in July. Additional constellations are shown for reference purposes.
Victoria/Stellarium Museums

The nova is expected to be of reasonable brightness (magnitude 2.5): about as bright as Imai (Delta Crucis), the fourth brightest star in the Southern Cross. So it will be easy to see even from a city, if you know where to look.

On July evenings the Southern Crux can be seen lying on its side, high in the southwest of Australia and Aotearoa.
Victoria/Stellarium Museums

We won’t have much time

We won’t have much time once it fades. Maximum brightness will last only a few hours; within a week T CrB will be gone and you’ll need binoculars to see it.

Almost certainly some amateur astronomer will alert the professional community to the moment the T CrB explodes.

These dedicated and knowledgeable people regularly observe the stars in their backyards, hoping for something to happen, and in so doing, fill a significant gap in night sky observations.

The American Association of Variable Star Observing (AAVSO) has a record of more than 270,000 submitted observations of T CrB alone. Amateur astronomers work together here and around the world to continually monitor T CrB for the first signs of an eruption.

In September, the Borealis Corona will move lower in the northwestern sky and will be best visible between 7:30 and 8:00 p.m. local time.
Victoria/Stellarium Museums

Let’s hope the nova erupts as predicted before October, because that’s when Corona Borealis will leave our evening sky in the Southern Hemisphere.