In the gravitational tug of war in a binary system, the hydrogen-rich outer layer of one star can sometimes be pulled away by its companion, leaving behind the compact, hot, and helium-rich core. Stripped helium stars should be abundant in the universe: Theory predicts that about a third of all massive stars should lose their hydrogen-rich outer layer and reach a range of 2–8 solar masses.

Once stripped stars collapse under their own gravitational forces, they’ll generate hydrogen-poor supernovae because of their lack of an outer layer. But despite observing the supernovae and despite years of searching for midsize stripped helium stars, scientists had found only one. (In 2008, astronomers identified it as a 4-solar-mass candidate,1 and its mass estimate was recently revised to the low end of the range, at 2 solar masses.2)

Although stripped stars emit visible light, it’s relatively faint and thus hard to...

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