Every night a “new star” or nova will appear in the night sky. While it won’t set the sky ablaze, it is a special chance to see a rare event that is normally difficult to predict in advance.
The star in question is T Coronae Borealis (T CrB, pronounced “T Cor Bor”). It’s located 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.
Usually T CrB, which is 3,000 light-years away, is far too faint to see. But once every 80 years it bursts brightly.
A brand new star will suddenly appear, but not for long. Just a few nights later it will quickly disappear and disappear into the darkness again.
A burst of life
During the prime of their lives, stars are powered by nuclear fusion reactions deep in their cores. Most of the time, hydrogen is converted into helium, creating enough energy to keep the star stable and radiant for billions of years.
But T CrB is long past its prime and is now a stellar remnant known as a white dwarf. Its internal nuclear fire has burned out, allowing gravity to dramatically compress the dead star.
T CrB also has a stellar companion – a red giant that has bloated as it grew older. The white dwarf is clearing away gas from the bloated red giant, and this forms what is known as an accretion disk around the dead star.
Matter continues to pile up on a star that has already been squeezed to its limits, causing a steady rise in pressure and temperature. Conditions become so extreme that they mimic what would once have been found in the star’s core. The surface ignites in a runaway thermonuclear reaction.
When this happens, the released energy causes T CrB to shine 1500 times brighter than normal. Here on Earth, it appears briefly in the night sky. With this dramatic reset, the star has ejected the gas and the cycle can begin again.
How do we know when it’s time?
T CrB is the brightest of a rare class of recurring novae that repeat within a hundred years – a timescale that allows astronomers to determine their recurring nature.
There are currently only 10 known recurring novae, although there may be more recurring novae, but on much larger timescales that are less easy to track.
The earliest known date for an outburst of T CrB is in 1217, based on observations recorded in a medieval monastic chronicle. It is remarkable that astronomers can now predict outbursts so accurately, as long as the nova follows its usual pattern.
The star’s two most recent outbursts – in 1866 and 1946 – showed exactly the same features. About 10 years before the outburst, T CrB’s brightness increased slightly (known as a high state), followed by a brief decrease or dip about a year after the explosion.
T CrB peaked in 2015 and the pre-eruption dip was noted in March 2023, putting astronomers on high alert. What causes these phenomena is just one part 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 Corona Borealis as it is now, so you get the full impact of the “new” star.
Corona Borealis is currently reaching its best viewing position (known as a meridian transit) around 20:30 to 21:00 local time in Australia and Aotearoa. The further north you are, the higher the constellation will be in the sky.
The nova is expected to be fairly bright (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 location if you know where to look.
We don’t have much time
We don’t have long once it goes off. Its peak brightness lasts only a few hours; within a week, T CrB will have faded and you’ll need binoculars to see it.
It will almost certainly be an amateur astronomer who alerts the professional community to the moment when T CrB bursts.
These dedicated and knowledgeable people routinely monitor stars from their backyards on a “what if” basis, filling an important gap in night sky observations.
The American Association of Variable Star Observing (AAVSO) has a log of over 270,000 submitted observations on T CrB alone. Amateur astronomers here and around the world are working together to continuously monitor T CrB for early signs of an outburst.
Hopefully the nova will burst as expected sometime before October, because after that Corona Borealis will disappear from our evening sky in the Southern Hemisphere.
– By Tanya Hill and Amanda Karakas.
Tanya Hill is an extragalactic astronomer and Honorary Fellow of the University of Melbourne. Amanda Karakas is an Associate Professor in the School of Physics and Astronomy, Monash University.
This article has been republished from The conversation under a Creative Commons license.
