Massive Binary Stars Huddle Up Surprisingly Quickly

Dancing is a favorite pastime of many couples.  Swinging around a dance floor, using the laws of physics to twirl at just the right moment, and hopefully not step on any toes, is an art unto itself.  The same laws of physics that govern couples on a dance floor also govern (to some extent) the much larger dance of stellar objects.  And recently astronomers have started to understand the intricacies of how binary stars dance with each other – turns out it’s not quite as simple as doing the tango.

As with many dance styles, two stars are required to begin.  Binary star systems usually form in clouds of dust and gas known as stellar nurseries.  Sometimes those nurseries birth massive stars approximately eight times the size of the sun.  Usually these massive new stars form a smaller star in their stellar neighborhood, for reasons that are as yet unknown, and the two pair up to become a binary star system.

Young binary stars. Image credit: NASA

Scattered throughout the universe, these stellar nurseries can vary widely in age.  The team, led by Dr. Maria Claudia Ramirez-Tannus from the Max Planck Institute for Astronomy, found that the orbital speeds of these binary star systems seem to vary significantly based on the age of the nursery they are birthed from.

Data supporting this idea was gathered by the spectrographs at the Very Large Telescope, where the researchers used shifts in the spectral lines of stars to calculate their radial velocity.  What that data showed is that massive binary stars in younger nurseries are more likely to have slower radial velocity between the paired stars than those born from older nurseries.

The researchers point out that the most likely explanation for the differences in radial velocity comes from a commonly understood physical phenomena – the conversation of angular momentum.   This simply states that when two bodies more closely orbit each other, their radial velocity increases.  The paper also notes that, while the results they have found are not due solely to this physical law, it certainly does play some part.

There are two possible explanations that might cause the two stars to rapidly decrease their orbital size around another, thereby increasing their radial velocity.   One is due simply to friction, while the other might result from the interference of another dance partner.

MAXI J1820+070 is a binary pair that has one black hole and one star. The black hole is emitting relativistic jets, and Chandra made a movie of it. Image Credit: Chandra X-Ray Observatory

A third star might disrupt the more mundane motion of the two stars in a binary system, in an elaborate example of the three-body problem.  The gravity well of this additional dance partner could throw the two stars of the binary system into a more elliptical orbit that eventually degenerates into a much smaller orbital radius.  In some cases, that interference throws the third partner completely off the dance floor and out of the gravitational fields of the two remaining stars, making it difficult for astronomers to tell if it was ever even there.

The other potential explanatory scenario is slightly more mundane, and can be thought of as the dance floor not being waxed well enough.  Friction with the gas and dust that remains in the stellar nursery can cause the orbital distance of the two stars in a system to decrease dramatically, thereby increasing their orbital velocities.

The results from this study points to the complexities in trying to understand not only how binary star systems move in relation to one another, but even how they started doing so in the first place.  No matter how hard it might seem to not step on your partner’s foot during the samba, the intricacies of interstellar physics are much more complicated.

Learn More:
MPIA: The dance of massive stellar couples
UT: This is a Binary Star in the Process of Formation
UT: Binary Stars Orbiting Each Other INSIDE a Planetary Nebula

Lead Image: Illustrations of the two scenarios explained. Credit: MPIA Graphics Team