Astronomers discover a star in Andromeda that's exploded every year for millions of years

A white dwarf in Andromeda has erupted so often that it created a 400-light-year-wide shell.

Key Takeaways:

Astronomers found a star in Andromeda that has erupted every year for over a million years.
This white dwarf, M31N 2008-12a, is the most frequently recurring nova ever discovered.
Its explosions have formed one of the largest known remnants, stretching 400 light-years across.
The star may eventually explode as a type Ia supernova or collapse into a neutron star.
Finding similar nova remnants could help scientists better understand cosmic distances and the universe’s evolution.

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Astronomers have discovered an extraordinary white dwarf in the Andromeda galaxy that has been erupting annually for millions of years. This discovery, published in Nature, is the first of its kind and could help scientists detect more recurrent novae, offering new insights into how the universe evolves.

The Most Frequent Nova Ever Found

The star responsible for this phenomenon, known as M31N 2008-12a, is a white dwarf—a stellar remnant that has lost its outer layers. However, this particular white dwarf is part of a binary system, where it pulls hydrogen from its companion star. As the hydrogen accumulates on the white dwarf’s surface, it triggers nuclear fusion, causing a nova explosion that brightens the star by a million times and ejects material outward at 3% the speed of light.

GK Persei, seen above, is a prime example of a nova remnant. Unlike supernovae, which blow stars apart from within, nova explosions occur on the surfaces of accreting white dwarfs. Though the previous record for largest nova remnant is about 3.5 light-years wide, researchers recently discovered M31N 2008-12a, a nova remnant that spans over 400 light-years. X-ray: NASA/CXC/RIKEN/D.Takei et al; Optical: NASA/STScI; Radio: NRAO/VLA

Unlike typical recurrent novae, which erupt about once per decade, M31N 2008-12a explodes at least once a year. Over millions of years, these repeated eruptions have created a massive shell of gas and dust that spans an astonishing 400 light-years—larger than most supernova remnants. The discovery of this enormous structure provides strong evidence that the star has been undergoing high-frequency explosions for an incredibly long time.

A Future Supernova in the Making?

Although M31N 2008-12a has survived countless eruptions, it won’t last forever. Once it accumulates enough mass to exceed the Chandrasekhar limit—1.4 times the mass of the Sun—it will undergo a catastrophic transformation. Astronomers predict it will either explode as a type Ia supernova or collapse into a neutron star within the next 40,000 years.

Nova remnant M31N 2008-12a, the largest nova remnant ever discovered, is about 450 light-years long by 300 light-years wide. M. J. Darnley et al. (Nature volume 565, pages 460–463 (2019))

Studying remnants like the one around M31N 2008-12a could help researchers understand the origins of type Ia supernovae, which serve as crucial “standard candles” for measuring cosmic distances. If similar super-remnants exist around other novae, they could play a vital role in refining our understanding of the universe’s expansion.

For now, astronomers are working to determine whether M31N 2008-12a is an exception or if more stars like it are waiting to be discovered. If these recurrent novae are common, they may be key to unlocking new insights about stellar evolution and the history of the cosmos.