Astronomers Spot Mismatched Twin Stars Being Born

Astronomers have caught a pair of fraternal, twin stars in the midst of being born. The researchers have spotted a new pair of stars that's actually a new class of binary stars; one is fully formed while the other is still in its infancy.

"We caught them at just the right time," said Maxwell Moe, the lead author of the new study, in a news release. "In effect, we're seeing these stars in the delivery room."

In this case, the researchers were searching for extreme mass-ratio binary star systems to learn a bit more about them. The larger a star is, the bright it shines. This makes it difficult to identify extreme mass-ratio binaries because the heavier star outshines and hides the lighter star. In order to combat this particular effect, the researchers looked for eclipsing systems in which the two stars lined up in such a way that they periodically passed in front of each other as seen from Earth.

After sifting through thousands of eclipsing systems, the scientists found 18 extreme mass-ratio binaries in a neighboring galaxy, called the Large Magellanic Cloud. The stars circle each other tightly in orbits that range from three to nine days.

These systems, though, are young. The fainter star shows illumination phases, just like phases of the moon, as the two stars orbit each other. This indicates that the companion is reflecting the light of the brighter, more massive star. However, these phases are only seen because the smaller companion is not yet a full-fledged star.

When stars are born, they contract. In the young systems, the more massive star is already on the main sequence, while the less massive companion is not. As a result, the companion is puffier than it would be, since it's still contracting. This effectively lets the pre-main sequence star act as a giant mirror, reflecting the brilliance of its partner.

The findings reveal a bit more about these stellar twins. This, in turn, may tell researchers a bit more about the formation and evolution of massive stars, close binaries and star nurseries.

The findings are published in The Astrophysical Journal.

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