Astronomers discover binary stars in 'impossibly' tight orbits

A team of scientists using the United Kingdom Infrared Telescope (UKIRT) in Hawaii have found four different pairs of binary systems whose stars are so close to each other that it takes them less than four hours to orbit one another. Previous estimates indicated that stars shouldn't be allowed to do this — they should consume each other, creating one giant mass.

This observation forces astronomers to throw away previously held notions of what binary star systems might look like.

These stars, which are red dwarfs, are up to ten times smaller and a thousand times dimmer than our Sun — though they are the most common type of star in the Galaxy. When you look up at the sky at night, you're only able to observe a very small fraction of what's actually up there.

That's why these astronomers used UKIRT, a state-of-the-art camera that can pick up their near-infrared light. And it was this device that was used to detect these four unprecedented binary star systems.

Prior to the discovery, astronomers hadn't seen anything shorter than a five hour orbit. Much to their surprise, one of the pairs featured an orbit of 2.5 hours; these stars are rapidly falling into each other, soon to coalesce into a single star.

Writing in Universe Now, Nancy Atkinson explains more:

Since stars shrink in size early in their lifetime, the fact that these very tight binaries exist means that their orbits must also have shrunk as well since their birth, otherwise the stars would have been in contact early on and have merged. However, it is not at all clear how these orbits could have shrunk by so much.

One possible scenario is that cool stars in binary systems are much more active and violent than previously thought.

The astronomers said it is possible that the magnetic field lines radiating out from the cool star companions get twisted and deformed as they spiral in towards each other, generating the extra activity through stellar wind, explosive flaring and star spots. Powerful magnetic activity could apply the brakes to these spinning stars, slowing them down so that they move closer together.

"The active nature of these stars and their apparently powerful magnetic fields has profound implications for the environments around red dwarfs throughout our Galaxy, " said team member said David Pinfield from the University of Hertfordshire.

The paper was published in the journal Monthly Notices of the Royal Astronomical Society.

Artistic impression of the binary star system via J. Pinfield, for the RoPACS network.