Stars are Messy Cannibals

by | May 8, 2024, 6:00 AM | Stars & Nebulae

This image, taken with the VLT Survey Telescope hosted at ESO’s Paranal Observatory, shows the beautiful nebula NGC 6164/6165, also known as the Dragon’s Egg. The nebula is a cloud of gas and dust surrounding a pair of stars called HD 148937. Credit: ESO/VPHAS+ team. Acknowledgement: CASU 

In the southern sky, about 3,800 light-years away toward the constellation Norma, the well-named Dragon’s Egg nebula has been hiding a secret. Surrounded by a cloud of carbon, nitrogen, and oxygen are a pair of mismatched stars. Despite forming from the same original material, these stars have very different chemistry, and that just shouldn’t be possible. It’s like getting one double chocolate cookie in a pack of regular cookies. Something had to happen. 

The clue to what happened is the stunning nebula, which is made of materials not generally shed by stars — that carbon, nitrogen, and oxygen are generally locked away in stellar cores.

According to a new paper published in the journal Science with lead author Abigail Frost, this system formed with three sibling stars, but at some point in their past, one of the siblings, much like a shark, woke up and chose violence and ate its sibling. It wasn’t neat either; that surrounding material was the leftover guts of the consumed star. 

As described by researcher Hugues Sana, “The two inner stars merged in a violent manner, creating a magnetic star and throwing out some material, which created the nebula. The more distant star formed a new orbit with the newly merged, now-magnetic star, creating the binary we see today at the center of the nebula.”

There are lots of reasons for a star to eat another star. In terms of science, the reason is pretty simple – gravity can and will draw two stars together if the stars are aligned just right. But, to continue our anthropomorphizing of these great balls of plasma, the consumption of companions can make a star look significantly younger. In fact, the cannibal star appears 1.5 million years younger than its remaining sibling. This kind of merger also grants the remaining star a powerful magnetic field: a feature not otherwise found in large stars like this one.

Remarkably, researchers were able to determine when this siblicide took place. The mix of elements in the nebula indicates it is only about 7,500 years old. 

If you’re in the Southern Hemisphere, this object can be readily observed with a 16” telescope. To replicate this research, however, you’re going to need the amazing spectrometers on the Very Large Telescope and nine years’ worth of data.