A team of astronomers led by John Bochanski has found the farthest stars in our galaxy in the mysterious Milky Way halo, a rare discovery that may change our understanding of the formation of our galactic home. The two objects — known as ULAS J0744+25 and ULAS J0015+01 — are about 775,000 and 900,000 light-years from Earth, respectively, making them both about five times more distant than a satellite galaxy known as the Large Magellanic Cloud.
On July 3, Bochanski and his team, which includes Haverford College Associate Professor of Astronomy Beth Willman, published a letter in Astrophysical Journal Letters detailing the discovery of two cool red giants. These stars are extremely far away, at distances of 775,000 and 900,000 light years, respectively.
The giant stars were selected from observations contained in the UKIRT Infrared Deep Sky Survey and Sloan Digital Sky Survey.
“The distances to these two stars are almost too large to comprehend,” Bochanski said.
“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.”
The Milky Way extends far beyond its familiar disk, which is just 100,000 light-years or so wide. The galaxy is surrounded by a sparse “halo” of stars — perhaps stragglers left out there after the Milky Way’s many mergers with dwarf galaxies over the eons, researchers say.
Scientists know this halo extends to at least 500,000 light-years away, but its exact dimensions are unknown.
Red giant stars are relatively rare when compared to nearby cool red dwarf stars, which vastly outnumber giants. Yet, giants are nearly 10,000 times brighter than dwarfs, making them visible even at very large distances.
Using a combination of filters highlighting different parts of the optical and near-infrared light from these giants, the team was able to identify cool red giant candidates. The scientists then obtained spectroscopic confirmation of the identity of these stars using the 6.5m telescope at the MMT Observatory on Mt. Hopkins in Arizona.
“It really is like looking for a needle in a haystack,” Bochanski says. “Except our haystack is made up of millions of red dwarf stars.”
“Most models don’t predict many stars at these distances,”
“If more distant red giants are discovered, the models may need to be revised.”
The search in the outer reaches of our Milky Way goes on, using the brightest stars to guide the way.