Unearthing Ancient Stars: The Hidden History of the Boötes I Dwarf Galaxy

Boötes I is one of the faintest and oldest known galaxies, classified as an ultra-faint dwarf galaxy (UFD). In their 2025 study, Muratore and collaborators used both the Hubble Space Telescope (HST) and the James Webb Space Telescope (JWST) to uncover new details about the metallicity (chemical composition) and binary star population of this tiny galaxy. Because UFDs are relics from the early Universe, understanding them offers clues about how the first stars and galaxies formed.

Ancient Galaxies and the Mystery of Dark Matter

Ultra-faint dwarf galaxies like Boötes I are small, dim systems whose total brightness is less than one hundred-thousandth that of the Sun. Despite their size, they are dominated by dark matter, invisible material that makes up most of their total mass. Previous studies found that their stars move faster than expected, suggesting large amounts of dark matter. However, this measurement can be affected by binary stars, pairs of stars orbiting each other, since their motions can mimic the effects of dark matter. Determining how many stars in Boötes I are binaries is therefore essential to properly measuring the galaxy’s true mass.

Observing Boötes I with Hubble and JWST

The authors combined high-precision images from HST and JWST, using several filters that capture both visible and infrared light. By measuring how bright stars are in different wavelengths, they built a color–magnitude diagram (CMD), a plot showing how stars’ brightness relates to their color. This diagram revealed a clear main sequence of stars and even a distinct secondary track, produced by binary systems. To ensure accurate results, Muratore’s team used software that carefully analyzed each star’s brightness, motion, and position to remove background stars not belonging to Boötes I.

Tracing Metallicity Through Starlight

“Metallicity” measures the abundance of elements heavier than hydrogen and helium in a star, often expressed as [Fe/H]. Because the first stars formed from pure hydrogen and helium, lower metallicities indicate older stellar populations. The faint, low-mass stars in Boötes I are especially sensitive to metallicity, allowing the authors to estimate the galaxy’s chemical makeup without relying solely on spectroscopic data, which is difficult for such dim objects. By comparing the observed CMD to theoretical isochrones (model tracks for stars of different ages and compositions), they found that most stars in Boötes I have [Fe/H] below −2, and about 17% have values below −3, among the lowest in the Universe. This means Boötes I preserved stars that formed soon after the Big Bang.

Mapping Binary Stars

To study binaries, the researchers created what they call a Binary Map, an adaptation of a technique used for globular clusters. This map identifies where stars fall on the CMD depending on how their combined light shifts in color and brightness. From this, Muratore’s team determined that roughly 20% of Boötes I’s stars are binaries with mass ratios above 0.4, leading to a total binary fraction of about 30%. This result is similar to binary rates in star clusters of comparable stellar mass, implying that the presence of dark matter does not influence how binaries form or survive.

What the Findings Mean

Boötes I’s metallicity distribution and binary fraction reveal that it is a truly ancient system, with a stellar population that formed between 11 and 14 billion years ago. The consistency of its binary rate with that of normal star clusters suggests that dark matter, though dominating the galaxy’s total mass, does not disrupt local star formation processes. These findings confirm Boötes I as a “fossil galaxy”, a preserved remnant of the early Universe, offering astronomers a direct glimpse into cosmic history.

In Summary

Through deep imaging from HST and JWST, Muratore et al. achieved one of the most detailed looks yet at an ultra-faint dwarf galaxy. They demonstrated how precise photometry can reveal the metallicity and binary properties of ancient stars, even when spectroscopy is not possible. Their results strengthen the view of Boötes I as an untouched relic of the first galaxies, quietly orbiting our Milky Way and holding clues to the dawn of star formation.

Source: Muratore