Heavyweight Stars Change Composition in Pairs express
In stellar nurseries, massive stars are not born alone but in pairs. Two stars orbit a common center, and the more massive one expands first and begins to shed its outer layers. Some of this material doesn't escape into space but flows onto the companion, like water from an overflowing cup into a cup held beneath. As a result, the receiving star becomes enriched with helium that accumulated in the donor's core.
Astronomers from the IACOB project tested this picture on a large sample: 318 massive O-type stars in our Galaxy. In 22% of them, the helium in the atmosphere was significantly higher than the cosmic standard (about 0.098 relative to hydrogen). Moreover, most of these stars show signs of companionship: either a companion is visible nearby, or the star is moving through the Galaxy at an unusually high speed — a trace of past interaction.
This is important for understanding the fate of the hottest and shortest-lived stars. Massive stars explode as supernovae, leaving behind neutron stars or black holes. If a significant fraction of them went through a mass exchange stage, then both the explosions and their remnants will be different. A seemingly single star may be the product of a violent binary history.
In 22% of massive stars, helium makes up more than 13% of atoms in the atmosphere, like having one and a half spoons of sugar in a glass of water instead of one.