The universe is a massive place, filled with crazy things like black holes and dark matter and theoretical wormholes and stars that shoot lasers at you. So, as Discover magazine put it back in 1994, “astronomers aren’t easily surprised by things astronomical.”
In that context, the idea of a star-within-a-star is weird. But it's not so weird as to be unbelievable.
Known as a Thorne-Żytkow Object, the idea of a nesting-doll star was first proposed by two physicists, Kip Thorne and Anna Żytkow, back in the '70s. But no one has ever seen one. Now, though, Alexandra Witze reports at Nature, astronomers have found the best candidate yet:
Theorists have proposed several ways in which a Thorne-Zytkow object could form, but the most likely scenario involves a red giant swallowing an orbiting neutron star.
Although a neutron star is only ten miles or so across, its gravitational pull is so strong that it could draw matter away from its huge but more diffuse companion. In the process, it would slow down, like a ship dragging an anchor. Eventually the neutron star, in an ever-shrinking orbit, would plow into the outer layers of its neighbor. After a few thousand years it would spiral down into the star’s center, demolishing the existing stellar core but leaving the rest of the star essentially intact. A red giant that has been violated in this way has come to be called a Thorne-Zytkow object--even though no one has ever seen one.
The scientist are keeping the name of their specific star(s) quiet until their research can be vetted by their peers, since there's always a chance there could be some other explanation for what they're seeing.