Free Astronomy Magazine January-February 2016

SPACE CHRONICLES red giant is left with only its core, making it into a white dwarf. The partner — ini- tially the less massive of the pair, but now the heavier one — becomes a blue strag- gler. Gosnell’s method is lim- ited by the fact that it will not detect white dwarfs that have cooled down enough so that they don’t glow in UV light detectable by Hub- ble. That means that only those white dwarfs formed in the last 250 million years (youngsters, astronomically speaking) are detectable. Knowing more about how these stars form is important because astronomers use their assumptions to model the stellar populations of distant galaxies, where the light from all the stars blends together. “You don’t want to be ignoring 25 per- cent of the evolved stars in those galaxies,” Gosnell said. Such models are important because distant galaxies figure into many different types of cosmological stud- ies. “Right now — Gosnell said — the models have a lot of room for improvement.” “If we tweak the way models treat mass transfer, that would bring the observations and theory together,” Gosnell said. “They would agree. And we can use this to inform our understanding of unresolved stellar populations” — that is, those stars in galaxies so far away that all their light is blend- ed together. Gosnell plans to con- tinue studying these stars using the 2.7-meter Harlan J. Smith Telescope at McDonald Observatory and its IGRINS spectrograph to constrain the number of blue stragglers that could form through mergers in tri- ple systems. P revious page, a normal star in a binary system gravitationally pulls in matter from an aging companion star that has swelled to a bloated red giant that has expanded to a few hundred times of its original size. Below, after a couple hundred million years the red giant star has burned out and collapsed to the white dwarf that shines intensely in ultraviolet wavelengths. The compan- ion star has bulked up on the hydrogen siphoned off of the red giant star to become much hotter, brighter and bluer than it was previously. [NASA/ESA, A. Feild (STScI)] n to a quarter of the oldest stars “are not evolving like we think they’re supposed to,” Gosnell said. Stars that astronomers expected to be- come red giants (like Aldebaran, the eye of Taurus, the bull) instead became “blue stragglers,” unexpect- edly bright, blue stars with a host of strange characteristics. Gosnell wanted to find out what happened to them. So she, along with Bob Mathieu at the University of Wisconsin-Madison and their col- laborators, designed a study using Hubble Space Telescope’s Advanced Camera for Surveys to try to diffe- rentiate between three theories of how these stars became blue strag- glers. The theories included: colli- sions between stars in the cluster (with debris coalescing to form a blue straggler), the merger of two of the stars in a triple star system, or mass transfer between two stars in a binary pair. In a binary pair of stars, the larger star will evolve faster, Gosnell said. That star becomes a red giant. A red giant is so bloated that the outermost layers of gas on its surface are only tenuously held by the star’s gravity. They can be pulled off by the gravity of the companion star. This is mass transfer. As the gas is siphoned off by the partner, the