The Mysterious Case of the Resurrected Star

The star HD 65907 is not what it appears to be. It’s a star that looks young, but on closer inspection is actually much, much older. What’s going on? Research suggests that it is a resurrected star.

Astronomers employ different methods to measure a star’s age. One is based on its brightness and temperature. All stars follow a particular path in life, known as the main sequence. The moment they begin fusing hydrogen in their cores, they maintain a strict relationship between their brightness and temperature. By measuring these two properties, astronomers can roughly pin down the age of a star. But there are other techniques, like measuring the amount of heavy elements in a stellar atmosphere. Older stars tend to have fewer of these elements, because they were born at a time before the galaxy had become enriched with them.

Going by its temperature and brightness, HD 65907 is relatively young, with an age right around 5 billion years old. And yet it contains very little heavy elements. Plus, its path in the galaxy isn’t in line with other young stars, which tend to serenely orbit around the center. HD 65907 is much more erratic, suggesting that it only recently moved here from somewhere else.

In a recent paper, an international team of astronomers dug into archival data to see if they could resolve the mystery, and they believe that HD 65907 is a kind of star known as a blue straggler, and that it has its strange combination of properties because of a violent event in its past, causing it to be resurrected.

If two low-mass stars collide, the remnants can sometimes survive as a star on its own. At first that newly merged star will be both massive and large, with its outer surface flung far away from the core due to the enormous rotation after the collision. But eventually some astrophysical process (perhaps strong magnetic fields might be to blame) drag down the rotation rate of the star, causing it to slow down and settle into equilibrium. In this new state the star will appear massive and incredibly hot: a blue straggler.

No matter what, blue straggler stars get a second chance on life. Those mergers transform small stars into big stars, and they’re just now enjoying their hydrogen-burning main sequence lives.

The astronomers believe this is the case for HD 65907. What makes this star especially unique is that it’s not a member of a cluster, where frequent mergers can easily lead to blue stragglers. Instead, it’s a field star, wandering the galaxy on its own. It must have cannibalized a companion five billion years ago, leading to its apparent youthful age.

Work like this is essential to untangling the complicated lives of stars in the Milky Way, and it shows how the strangest stars hold the keys to unlocking the evolution of elements that lead to systems like our own.