Two stars have been discovered, lying at the very edge of the Milky Way galaxy. These are the two most-distant stellar bodies ever recorded within our family of stars.
Once of the distant stars, ULAS J0744+25, is located almost 775,000 light years away from Earth. The objects are located around five times further away from us than is the Large Magellanic Cloud that orbits the Milky Way. Along with ULAS J0015+01, set 900,000 light years away from Earth, the pair is the most-distant stars ever associated with our home galaxy.
John Bochanski, teaching at Haverford College, began a study of the outer halo of the Milky Way. This feature is a thin collection of stars surrounding our native galaxy. This system extends more than 500,000 light years from the center of the galaxy.
Until the discovery of these two stars, only seven such objects were known to be part of our galactic system that lay more than 400,000 light years from Earth.
The newly-discovered stars are each red giants. These cool stars are not as common as red dwarfs, but due to their size, they are up to 10,000 times brighter, and can be observed from much greater distances then their smaller cousins.
Targets were chosen from the Sloan Digital Sky Survey and the UKIRT Infrared Deep Sky Survey. The team used a series of filters on the data, in order to identify red giants. They then used the 21-foot telescope at MMT Observatory, located on Mount Hopkins in Arizona, to identify the stars using a spectroscope.
"The distances to these two stars are almost too large to comprehend. To put it in perspective, when the light from ULAS J0015+01 left the star, our early human ancestors were just starting to make fires here on Earth," Bochanski said.
In addition to discovering these record-breaking stars, the find could also help astronomers answer other questions. Our own Milky Way Galaxy is believed to have formed, in part, through mergers with other, smaller, families of stars. Many astronomers believe the halo in which these red giants live could hold remnants of these long-dead galaxies. Investigation of the stars and similar bodies in the halo could assist astrophysicists in understanding how our galaxy formed, and developed over time.
"Theory predicts the presence of such an extended stellar halo, formed by the destroyed remains of small dwarf galaxies that merged over the cosmic ages to form the Milky Way itself. The properties of cool red giants in the halo thus preserve the formation history of our Milky Way. These stars are truly ghosts of galaxies past," Beth Willman, associate professor of astronomy at Haverford College, explained in a press release.
Discovery of the distant stars is detailed in The Astrophysical Journal Letters.
