We define stars in a few different ways that basically come down to, observationally, we look at how luminous a star is and what temperature it is shining, while mathematically, we ask what is it burning in its core and what mass is required to make that happen. If you have a solar mass of material giving off super hot ultraviolet light while shining faintly, that’s going to be a white dwarf, and it is going to have a core of hot gas, just hanging out. Take that same mass, expand it out to the radius of something like Mars and start shells of material glowing around its core, and you know you are going to see something glowing bright and cool as a red giant.

Throughout its life, a star will change both in what it is using as fuel, and in how it looks, and these two things are directly related to a point. It turns out, alterations are possible but only if a star gets a little unasked for help from its neighbors.

Many stars are formed in multi-star systems, and when two stars are sufficiently close, weird things can happen. Our own Sun, luckily, is a singleton, so when it one day expands out to become a larger, cooler, red star, it may eat some planets, but it doesn’t have to worry about anything eating it. For stars with near neighbors, this expansion can, at a certain point, find that the material is gravitationally more attracted to the neighbor than to the star in which it started.

We can see lots of examples of stars stealing material off of one another as we look around the sky. What we hadn’t seen, until now, are the smaller stars left behind after their outermost layers have been stripped off.

Now, in a new paper in Nature Astronomy with first author Yaguang Li, researchers describe the discovery of these smaller stars. As Li puts it: It’s like finding Waldo. We were extremely lucky to find about 40 slimmer red giants, hidden in a sea of normal ones. The slimmer red giants are either smaller in size or less massive than normal red giants.

It’s always good when the universe provides us with examples of the kinds of stars we know we should see. The actual observations needed for this discovery, which used data from the Kepler telescope, are actually far more complicated than this simple story would imply. To get at the stars’ sizes, they used a technique called asteroseismology to see how waves propagate through the stars. This allows their physical properties to be precisely determined and opens the door for this remarkable discovery.

More Information

The University of Sydney press release

“Discovery of post-mass-transfer helium-burning red giants using asteroseismology,” Yaguang Li et al., 2022 April 14, Nature Astronomy