Open clusters are groups of stars that have formed within a small region of space within a relatively narrow time window. As a result, their resident stars are all about the same age and have reached similar points on the Main Sequence of stellar evolution. Most of them, at least. Many open clusters also play host to what have been called "blue stragglers." These are stars that shouldn't be there—bright and hot, they look like they've formed much more recently than the rest of the cluster (hence the straggler term).

Researchers have come up with a number of ways that the blue stragglers can form, all of which involve an old star that gets a new infusion of gas, making it look young again. These range from a merger of two middle-aged stars to a dense star stripping gas off a more diffuse companion. But there hasn't been observational evidence of any of these events in progress. Now, new observations have detected otherwise-invisible companions of the blue stragglers, and the size of the companions indicate that mass has in fact been transferred to the blue stragglers.

The observations focused on the cluster NGC 188, which contains over 3,000 stars, including over 20 blue stragglers. The authors took detailed measurements of the blue stragglers' spectroscopy, and performed an analysis identical to the one we use to identify planets: look for subtle shifts in the wavelengths of light emitted by the blue stragglers. These shifts can be caused by an invisible orbiting companion, which will pull the blue straggler in different directions as it orbits. This motion creates a Doppler shift in the light that reaches us, providing an indication that an otherwise invisible body is present.

Their observations showed that most of the blue stragglers (16 of them) had indications of a companion star orbiting nearby, with most of them having an orbital period of less than 1,000 days. Two of them had companions so close that their orbits took less than 100 days. The authors didn't analyze those two further, since the companion star is considered so close that it would necessarily have participated in the formation of a blue straggler.

Based on the orbital behavior, it's possible to produce the likely mass distribution of the star's companion. These calculations show that the companions all fall within a narrow mass range, centered at about half the mass of our Sun.

That's rather convenient, since theoretical calculations had already been done to model the mass transfer needed for a giant star on the main sequence to feed its outer gas to a blue straggler. Those calculations predict that, after the hydrogen and helium from the giant star is fed to the straggler, the process would leave behind a carbon-oxygen rich white dwarf with a mass about half that of the Sun—precisely what is seen here. Models of collision events, in contrast, indicate that any companion stars should probably be about solar mass, or twice as large as what's seen here.

The process of mass transfer should probably even out any orbital eccentricities in the binary system, leaving the orbit roughly circular. And, in fact, three of the blue straggler binaries have circular orbits. In contrast, none of the binaries involving regular stars in the same cluster are circular, since the dense packing of stars in the cluster makes sure that orbital interactions with neighbors keeps things from staying circular long. This probably explains why the remaining blue straggler systems no longer have circular orbits.

Although a binary merger seems to be ruled out by the authors' data, there's another option that will leave behind a white dwarf companion about 15 percent of the time: a merger within a system of three orbiting stars. Although that's a less likely scenario than mass transfer, the statistics don't rule it out based on this cluster.

If the authors are right, they should still be able to detect energetic gas being transferred between the dwarf remnant and the blue straggler. They've lined up time with the Hubble to look in the UV area of the spectrum to see if they can confirm that the mass transfer is still taking place.

Nature, 2011. DOI: 10.1038/nature10512 (About DOIs).