A once-in-a-lifetime explosion is about to create a “new” star in the sky

Every night a “new star” or nova appears in the night sky. Even if it doesn’t set the sky on fire, it’s a special opportunity to see a rare event that’s usually difficult to predict in advance.

The star in question is T Coronae Borealis (T CrB, pronounced “T Cor Bor”). It lies in the constellation of the Northern Coronet, prominent in the Northern Hemisphere but also visible in the northern sky from Australia and Aotearoa New Zealand over the next few months.

Most of the time, T CrB, which is 3000 light-years away, is too faint to be seen. But once every 80 years or so it clearly explodes.

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

An explosion of life

During the prime years of their lives, stars are powered by nuclear fusion reactions deep within their cores. Most often, the hydrogen turns into helium, creating enough energy to keep the star stable and glowing for billions of years.

But T CrB is long past its prime and is now a stellar remnant known as a white dwarf. Its inner nuclear fire was extinguished, allowing gravity to dramatically compress the dead star.

T CrB also has a companion star – a red giant that has bloated as it enters old age. The white dwarf sweeps up the gas of the bloated red giant and it forms what is known as an accretion disk around the dead star.

Mass continues to accumulate on a star that is already compressed to its limit, forcing a constant increase in pressure and temperature. The conditions are so extreme that they mimic what would once be found in the core of a star. Its surface ignites in an uncontrolled thermonuclear reaction.

When this happens, the energy released causes the T CrB to shine 1,500 times brighter than usual. Here on Earth, they appear briefly in the night sky. With this dramatic reset, the star expels the gas and the cycle can begin again.

How do we know it’s due?

T CrB is the brightest of a rare class of recurrent novae that repeat themselves every hundred years—the time scale that allows astronomers to detect their recurring nature.

Only ten recurring novae are currently known, although more novae may be recurring—just on much longer timescales that aren’t as easily tracked.

The earliest known date for the eruption of T CrB is from 1217, based on observations recorded in a medieval monastic chronicle. Remarkably, astronomers can now predict its eruptions so accurately if the nova follows its usual pattern.

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

T CrB entered its high state in 2015, and a pre-eruption decline was noted in March 2023, putting astronomers on alert. What causes these phenomena are just some of the current mysteries surrounding T CrB.

How can I see it?

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

Corona Borealis currently reaches its best viewing position (known as the meridional transit) around 20:30 to 21:00 local time across Australia and Aotearoa. The further north you are, the higher the constellation will be in the sky.

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 well visible even from the city if you know where to look.

We won’t have much time

It won’t be long before it breaks out. Maximum brightness will only last a few hours; by week T CrB will fade and you’ll need binoculars to see it.

It will almost certainly be an amateur astronomer who alerts the professional public to the moment of the T CrB explosion.

These dedicated and knowledgeable people routinely stargaze from their backyards with the “what if” hope, filling an important gap in night sky observation.

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

Hopefully the nova will explode as expected sometime before October, as the Corona Borealis will then leave our Southern Hemisphere evening sky.

This article is republished from The Conversation under a Creative Commons license. Read the original article.