An incredibly bright star has lost its gas and is just a leftover core

One of the brightest stars we see in the night sky has lost its outer layers of gas and now it is just a rare, stripped-back core

By Alex Wilkins

Gamma Columbae looks like a normal star to the naked eye, but it has been stripped of its outer layers of gas

ESO/Digital Sky Survey

One of the Milky Way’s brightest stars visible with the naked eye, Gamma Columbae, is actually a stripped-back stellar core.

We don’t know how the cores of most typical stars function because they are obscured by outer layers of gas, which generate huge amounts of energy and light from merging hydrogen atoms to make helium, known as nuclear fusion. Clouds of gas normally continue to obscure cores even after they run out of fuel.

Now, Norbert Przybilla at the University of Innsbruck in Austria and his colleagues have used spectroscopy to analyse the elements in Gamma Columbae, which has a mass about five times that of our sun. They realised that the signature, which was lacking in carbon and rich in nitrogen, only made sense if what they were seeing was the surface of a recently extinguished core.

“It exposes, on the surface, the chemical signature of a stellar core,” says Przybilla. “Of course, fusion is not taking place on its surface, but what we see today must have been the layers where nuclear fusion was going on in the past.”

The star is churning products of past fusion to the surface, which is what has led to the carbon and nitrogen signature. The team says it is in a restructuring phase that might last for 10,000 years, a very short time for a star that might live for many millions of years.

Although there were high-quality measurements of Gamma Columbae, the abundances of different chemicals there had never been properly analysed. Przybilla and his team were looking through a database of stars and their chemical signatures when they spotted Gamma Columbae’s unusual elements. “The spectrum was lying on the hard disc for some time until we realised what we had,” says Przybilla.

Further observations of Gamma Columbae should help us better understand how stars evolve once they have exhausted their supply of hydrogen for fusion, as well as how Gamma Columbae might have been stripped back. The team thinks it could be from a companion star removing its outer layers, but it isn’t certain.

The star is just 870 light years from the sun, and this proximity should allow astronomers to gather more high-quality information about the star in the coming weeks and months, says Derek Ward-Thompson at the University of Central Lancashire. “[Gamma Columbae] is relatively nearby and has gone relatively unnoticed – that’s actually the surprise about it, that it’s taken till now for anyone to spot it.”

Journal reference: Nature Astronomy, DOI: 10.1038/s41550-022-01809-6

Sign up to our free Launchpad newsletter for a voyage across the galaxy and beyond, every Friday