Distant Red Giants Discovered in the Milky Way’s Outer Halo

Artist’s conception of Mira, a luminous M giant star.

Red giant stars are the most luminous ones found in a population of old stars, and so are particularly useful to study the early history of the Milky Way. “We use these stars like fossils, because in many cases their chemistry and motions have been unchanged since they were formed more than 10 Gyr ago”, says CWRU astronomer Heather Morrison. She and her collaborators have spent more than a decade identifying these rare red giants as part of the Sloan Digital Sky Survey’s SEGUE project, and have found over 5,000 giant stars, some of them as far away as 100 kiloparsecs (kpc; for comparison, the Milky Way’s brightest satellite companion galaxies, the Magellanic Clouds, are only 50 kpc away.) In 2016, former CWRU Astronomy undergraduate Bill Janesh, now in the graduate program at Indiana University, first-authored a paper using this sample of stars to find traces of the formation of the halo in streams of stars shed by satellites as they fall into the Milky Way.

The coolest and largest of these red giant stars are known as M giants.  Morrison and her collaborators have stayed away from studying these stars, preferring to target the slightly warmer K giants whose spectra are easier to analyze. “An M giant spectrum looks like the top of a picket fence,” says Morrison. “There are so many molecular lines that it is hard to get a handle on the basic properties of the star.” However, recently Morrison thought that it might be worthwhile searching for warmer K giants in the M giant sample, just in case some of them had slipped through due to uncertainties in their measured colors. And in doing so, she found an unexpected bonus: 8 more stars from the outer halo, with distances of more than 50 kpc. These outer halo stars are quite rare, so the serendipitous extras are very welcome; Morrison and her collaborators are now using these stars to study the assembly history of the Milky Way in ever more detail.

Spectra of K and M stars. Because of their cooler temperatures, M stars have many more spectral lines due to molecules such as titanium oxide (TiO), which make their spectra much harder to interpret.