These Cannibalized Stellar Cores Could Explain Big Cosmic Mysteries

Some stars wear their hearts on their sleeve — not that they really have a choice. That’s because these unfortunate stellar victims have had their outer layers stripped away by the immense gravity of their cannibalistic partners.

Outer space can be a scary place.

Although theories have predicted that these stripped stellar cores should be fairly common in the universe, astronomers have found only a couple of possible examples. But in a new study, published on Thursday in the journal Science, astrophysicist Maria Drout of the University of Toronto and her colleagues pinpointed several of them in the Large and Small Magellanic Clouds, a pair of tiny dwarf galaxies that orbit the Milky Way.

The way these stars behave and evolve "will affect many things, from predictions for how many neutron star mergers the LIGO [Laser Interferometer Gravitational-wave Observatory] detectors should find in gravitational waves to the amount of ultraviolet light emitted from distant galaxies," Drout tells Inverse.

These exposed stellar cores, with their dysfunctional backstories, may be the key to understanding a variety of cosmological mysteries, including gravitational waves and certain types of supernovae.

The Voracious Appetites of Stellar Cannibals

In a survey of millions of stars in the Magellanic Clouds, Drout and her colleagues found 16 stars with exposed “entrails.” These innards are composed of elements forged by nuclear fusion deep in the star’s core but which are normally shrouded beneath deep, roiling layers of hydrogen. In this study, scientists noticed that all 16 stars showed slight wobbles as the stars moved toward us and away from us just slightly. Because of the Doppler Effect, this light shifts slightly bluer and slightly redder, which is how scientists can tell they're moving.

This suggests they are dancing around a shared center of gravity with a partner, likely a massive star, neutron star, or black hole. But this isn’t an uncommon phenomenon as about 70 percent of massive stars, ones that are at least eight times the mass of our Sun, are coupled up this way.

“This pairing, however, comes neither without risk nor without consequence,” write astrophysicists Jon Sundqvist and Hugues Sana of KU Leuven in a paper commenting on the new study.

The “consequence” is that sometimes one stellar partner takes relentlessly from the other, stripping away more than half its mass and leaving its core exposed. Such toxic stellar relationships can take tens or hundreds of thousands of years to reveal the burning core of the victim star, which blazes with brilliant ultraviolet light. This stage of a star’s life is fleeting and extremely weird, but models suggest that about a third of all massive stars experience it.

For something so ubiquitous throughout the universe, astronomers have had almost no luck in finding these stripped stars. Determined to seek out these stellar victims, Drout and her colleagues surveyed millions of stars in our neighboring galaxies using NASA’s Swift Observatory, a space telescope initially launched in 2004.

“We found these objects by looking for stellar systems that seemed to have ‘extra’ ultraviolet light,” Drout says. “This works because the stripped stars are predicted to be very hot.”

Once the team narrowed the search to 25 such stars, they took a closer look with the help of the Giant Magellan Telescope, which can view stars in wavelengths our eyes can’t see. As suspected, all 25 stellar candidates and been stripped of their hydrogen layers (leaving behind a much smaller stellar core of heavier elements), and 16 of them wobbled ever so slightly, revealing the slight tug of a binary companion.

Answers to Some Big Cosmic Questions

A few of these 16 stripped-down stars seem to be waltzing with the corpses. That’s because these stars have already burned up all their fuel and subsequently collapsed, forming either neutron stars or black holes. But like any zombie flick, the dead can feast on the living, and those ultra-dense celestial objects pull more mass away from their stripped but still-burning partners.

“These could be the evolutionary stage immediately prior to forming a neutron star and neutron star, or neutron star and black hole, system that will eventually merge,” says Drout.

Such mergers are some of the most dramatic partnerships in the universe and can churn up the kind of gravitational waves detected by LIGO. Learning more about this stellar cannibalism that creates these cosmic collisions could help reveal the origin story of gravitational waves.

Sadly, things don’t end well for these bizarre stars, which will eventually explode in a specific type of supernova that doesn’t contain as much hydrogen as normal.

"About 30 percent of core-collapse supernovas that we see in distant galaxies have no hydrogen; this is far more than we would expect if they just come from single stars, so we were pretty sure that they had to be coming mostly from stars that were stripped by binary companions,” says Drout, whose research has also focused closely on supernovae. “But we didn't actually know of any stars like that. So either we were just looking in the wrong way, or our understanding of what these supernovae were was completely off.”

The answer turns out to have been the former, and now that they’ve found them, Drout calls these 16 stripped stellar cores “the tip of the iceberg.”

“We found several hundred candidate systems that we are still in the process of confirming,” says Drout, “This is also part of what is next.”

Meanwhile, Drout and her team expect there to be many more stripped, cannibalized stars out there that astronomers just can’t see yet, concluding that two out of every three of them may be smaller than their companion stars and are effectively hidden by their brighter light. But with new confirmation of their ubiquitous existence, the help of ever-more advanced telescopes, and a little bit of luck, they won’t stay hidden for long.