Every night now, a “new star” or nova will appear in the night sky. While it won’t set the sky on fire, it’s a unique opportunity to see a rare event that’s usually hard 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 Crown, prominent in the Northern Hemisphere, but also visible in the northern sky from Australia and Aotearoa New Zealand over the coming 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 explodes brightly.
A brand new star seems to emerge out of nowhere, though not for long. Only a few nights later it will have faded rapidly, disappearing back into the darkness.
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An explosion of life
During the prime of their lives, stars are powered by nuclear fusion reactions deep within their cores. Most often, hydrogen is converted into helium, creating enough energy to keep the star stable and shining for billions of years.
But T CrB has passed its prime and is now a stellar remnant known as a white dwarf. Its inner nuclear fire has died down, 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 this forms what is known as an accretion disk around the dead star.
Matter continues to accumulate in a star that is already compressed to its limit, forcing a steady increase in pressure and temperature. Conditions become so extreme that they mimic what would once have been found inside the star’s core. Its surface ignites in a runaway thermonuclear reaction.
When this happens, the energy released makes the T CrB glow 1500 times brighter than usual. Here on Earth, it appears briefly in the night sky. With this dramatic reset, the star has then expelled 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 within a hundred years—a time scale that allows astronomers to detect their recurring nature.
Only ten recurrent novae are currently known, although more novae may be recurrent—just over much larger timescales that are not as easily tracked.
The earliest known date of the T CrB eruption is from 1217, based on observations recorded in a medieval monastic chronicle. It is remarkable that astronomers can now predict its outbursts so accurately as long as the nova follows its usual pattern.
The star’s two most recent explosions – in 1866 and 1946 – showed exactly the same characteristics. About ten years before the explosion, the brightness of T CrB rose slightly (known as a high state) followed by a brief dimming or decline about a year after the explosion.
T CrB entered its peak state in 2015 and its pre-explosion decline was observed 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, so you can get the full impact of the “new” star.
Corona Borealis currently reaches its best viewing position (known as a meridian 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 a reasonable brightness (magnitude 2.5): almost 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 location if you know where to look.
We won’t have much time
We won’t have much time after it goes off. Maximum brightness will last only a few hours; within a week T CrB will have faded and you will need binoculars to see it.
It will almost certainly be an amateur astronomer alerting the professional community to the moment T CrB explodes.
These dedicated and knowledgeable people routinely monitor the stars from their backyards for the “what if” possibility and therefore fill an important gap in night sky observations.
The American Society for Variable Star Observation (AAVSO) has a registry of over 270,000 observations presented in T CrB alone. Amateur astronomers are collaborating here and around the world to continuously monitor T CrB for the first signs of an explosion.
Hopefully the nova will explode as expected sometime before October, because after that the Corona Borealis leaves our evening sky in the Southern Hemisphere.
Tanya Hill, Senior Curator (Astronomy), Museums Victoria and Honorary Fellow, University of Melbourne, Victoria Research Institute and Amanda Karakas, Associate Professor, School of Physics and Astronomy, Monash University
This article is republished from The Conversation under a Creative Commons license. Read the original article.