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.

view of Milky Way from ULAS J0744+25
This illustration demonstrates how small the Milky Way would look from the location of ULAS J0744+25, a red giant nearly 775,000 light-years away. This star, along with ULAS J0015+01, are the most distant stars ever observed that are associated with our galaxy.
Visualization Software: Uniview by SCISS; Data: SOHO (ESA & NASA) / John Bochanski (Haverford College) / Jackie Faherty (AMNH and Carnegie Institute's Department of Terrestrial Magnetism)

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.

distant red giant stars
The red stars in the center of these Sloan Digital Sky Survey images don't look like much, but they're two distant red giants, ULAS J0744+25 (top) and ULAS J0015+01. These two stars are the most distant Milky Way stars ever observed.
Sloan Digital Sky Survey

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.

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Image of Robert-Casey


July 10, 2014 at 2:55 pm

Are these stars gravitationally bound to the Milky Way galaxy, or are they escaping? If escaping, they'd just be stray intergalactic stars. If bound, they must have really long orbital periods.

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July 11, 2014 at 9:45 am

According to the abstract "ULAS J001535.72+015549.6 and ULAS J074417.48+253233.0 are both moving away from the Galactic center at 52 ± 10 km s–1 and 24 ± 10 km s–1, respectively."

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John Bochanski

July 11, 2014 at 9:52 am

Hi Robert,

They are likely bound to the Milky Way. They are moving away from the Galaxy at speeds much lower than the escape velocity of the Galaxy. And you are correct, their orbital periods are very, very long. It took them at least a 1 billion years to get where they are now (if they formed in the disk).


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July 12, 2014 at 11:42 am

Are these stars in some sort of line or cone indicating a previous encounter between the Milky Was & something else, or are they randomly about? I would assume that you are looking in just one area so that answer might not be possible to answer until others are looked for and either found or not found elsewhere.

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Thomas Haas

July 13, 2014 at 6:04 am

I don't think our ancestors made fire 900 000 years ago - the earliest may have been about 400 000 years ...

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July 14, 2014 at 9:18 am

Unless I've been asleep for the last fifty years, the rough diameter of the Milky Way has always bee stated as 100,000 LY's. When numbers are thrown out as stars being roughly 500,000 light years from the galactic center, then we're talking a span of ovewr 1,000,000 light years as a diameter. What gives?

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