Nice but Dim: A bevy of stars found beyond our Milky Way

The Muñoz 1 globular cluster is seen to the right of the Ursa Minor dwarf galaxy in this image from the Canada-France-Hawaii Telescope MegaCam imager. Credit: Geha & Muñoz

A team of American, Canadian and Chilean astronomers have stumbled onto a remarkably faint cluster of stars orbiting the Milky Way that puts out as much light as only 120 modest Sun-like stars.

The tiny cluster, called Muñoz 1, was discovered near a dwarf galaxy in a survey of satellites around the Milky Way using the Canada-France-Hawaii Telescope (CFHT) and confirmed using the Keck II telescope, both of which are on Mauna Kea, Hawaii.

“What’s neat about this is it’s the dimmest globular cluster ever found,” said Ricardo Muñoz, an astronomer at the University of Chile and the discoverer of the cluster. A globular cluster is a spherical group of stars bound to each other by gravity so that they orbit around a galaxy as a unit.

“While I was working on the Ursa Minor dwarf galaxy I noticed there was this tiny little object close by,” Muñoz recalled. He made the discovery while he was a postdoctoral associate at Yale University.

Most globular clusters have in the range of 100,000 stars. Muñoz 1 has something like 500 stars. “This is very surprising,” he said.

“It’s ridiculously dim,” agreed Yale astronomer Marla Geha. “There are individual stars that would far outshine this entire globular cluster.” That puts Muñoz 1 head-to-head with the Segue 3 globular cluster (also orbiting the Milky Way) as the dimmest troupe of old stars ever found.

Muñoz 1’s discovery was the result of a survey done with the CFHT MegaCam imager in 2009 and 2010. It was then confirmed by spectroscopic study using the Deep Extragalactic Imaging Multi-Object Spectrograph (DEIMOS) on the Keck II telescope. The researchers will be publishing their results soon in The Astrophysical Journal Letters.