Astronomers have discovered two stars that lie more than 700,000 light-years from Earth, making them the most distant stellar members of our galaxy ever detected.
Well, I thought this might happen eventually. Today, instead of blogging about other researchers’ work, I get to tell you about my own astronomical discovery! During the last two years, I have led a team of astronomers searching for the most distant stars in the Milky Way Galaxy. We’re now publishing a study in the July 20th Astrophysical Journal Letters, detailing our discovery of two of the most distant stars in the Milky Way, ULAS J0744+25 and ULAS J0015+01.
We haven’t been hunting the farthest stars merely because they’re curiosities. The distant outskirts of the Milky Way harbor valuable clues for understanding the formation and evolution of our galaxy. Yet, due to overwhelming distances and an extremely sparse population of stars, many objects have not been identified beyond 400,000 light-years from Earth. Prior to our search, only seven stars were known beyond this limit.
We began by targeting stars in the Milky Way’s outer halo, which is a sparse shroud of stars that surrounds the disk of our galaxy and stretches at least 500,000 light-years out from the Milky Way’s center. We focused on cool red giants, selecting them from observations contained in the UKIRT Infrared Deep Sky Survey and Sloan Digital Sky Survey.
Red giant stars are relatively rare when compared with nearby cool red dwarf stars, which vastly outnumber giants. These two types of stars are at completely different stages in their development: dwarfs are plodding away at fusing hydrogen in their cores, whereas giants have used up their core hydrogen and puffed up as they move to burning the hydrogen shell around the core. Both stars are about the same temperature and color. Yet giants are nearly 10,000 times brighter than dwarfs, making them visible even at very large distances.
Even so, looking for giants in the halo is like looking for a needle in a haystack — except our haystack is made up of millions of red dwarf stars.
Using a combination of filters highlighting different parts of the optical and near-infrared light from these giants, our team was able to identify about 400 cool red giant candidates in the sky surveys. We then obtained spectroscopic confirmation of the identity of these stars using the 6.5-meter telescope at the MMT Observatory on Mt. Hopkins in Arizona.
During a visit last November to the MMT Observatory, my team and I, which includes astronomers from Boston University, Michigan State University, the Harvard-Smithsonian Center for Astrophysics, and the Kapteyn Astronomical Institute in The Netherlands, observed ULAS J0744+25 and ULAS J0015+01. We used a variety of methods to estimate the distances to these stars, but every method pointed to the same conclusion: these stars are extremely far away, at distances of 775,000 and 900,000 light-years, respectively. That’s more than 50% farther from the Sun than any other known star in the Milky Way, or about five times more distant than the Large Magellanic Cloud. In fact, they lie about one third of the distance to the Andromeda Galaxy, the Milky Way’s sister spiral in the Local Group.
To put these distances in perspective, think of them this way: when the light from ULAS J0015+01 left the star, our early ancestors were just starting to make fires here on Earth.
The significance of ULAS J0744+25 and ULAS J0015+01 goes beyond their record-holding distances, because they inhabit the Milky Way’s halo. Some astronomers think that the halo is like a cloud of galactic crumbs, the result of the Milky Way’s merger with many smaller galaxies over our galaxy’s lifetime. As my collaborator Beth Willman (Haverford College) explained in our team’s press release, “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.”
By assembling a larger sample of distant red giants, we hope to test predictions for the formation of the Milky Way. Our results may already be able to put some of these models to the test. Most models don’t predict many stars at these distances, and if more distant red giants are discovered, astronomers may need to start revising their ideas.
The search in the outer reaches of our Milky Way goes on, using the brightest stars to guide the way.
John J. Bochanski et al. "The Most Distant Stars in the Milky Way." Astrophysical Journal Letters. July 20, 2014.
Explore the stars of the Milky Way with our popular Pocket Sky Atlas. Great for skygazing enthusiasts and beginners alike!