Willman 1 is an ultra-faint dwarf galaxy discovered in 2005. It is named after Beth Willman of Haverford College who first identified it as an over-density of stars in the Sloan Digital Sky Survey (SDSS). At the time, it was unclear whether Willman 1 was a globular cluster or a very faint dwarf spheroid galaxy.
Combining spectroscopy from the W. M. Keck Observatory with imaging from the Kitt Peak National Observatory, they reported the presence of stellar multi-directional tails, suggesting possible tidal interactions with the Milky Way. Despite tentative evidence for mass segregation, a feature only observed in globular clusters, they leaned towards a dwarf galaxy classification based on a metallicity spread from three red giant branch stars. This metallicity spread indicates multiple epochs of star formation because stars with different metal abundances, or metallicities, must have formed at different times. These successive generations of stars enrich the surrounding gas with heavier elements leading to an observed spread in metallicity for stars in the object. This extended chemical evolution is a property of galaxies but not globular clusters because the dark matter in galaxies allows them to retain gas as a long-term reservoir for continued star formation.
Further spectroscopy in 2007 on the Keck Telescope identified seven red giant branch members, also with a significant metallicity spread. However, follow-up spectroscopy from the Hobby-Eberly Telescope in 2008 determined that up to five of these members were likely Milky Way foreground dwarf stars whose presence was inflating the metallicity spread.
In 2011, Beth Willman and collaborators re-analyzed spectroscopy from the Keck Telescope and carefully characterized Milky Way contaminates, identifying 45 member stars (although a later study showed that this includes four binaries and seven Milky Way stars This sample contains an additional red giant branch member and two horizontal branch members whose metallicity spread was once again indicative of a dwarf galaxy. Photometry from the Hubble Space Telescope confirmed this spectroscopic metallicity spread and measured an updated distance to the object. These data also show evidence for an irregular kinematic distribution and a tentative excess of stars at large half-light radii that could point to a disturbed morphology.
In 2026, Marla Geha homogeneously re-reduced over 20 years of archival spectroscopy from the Keck Telescope for 78 Milky Way satellites, including Willman 1. From these new data, 56 member stars were identified and used to measure the kinematic and chemical properties of the object. It is over ten million times less luminous than the Milky Way. As seen from Earth, Willman 1's location is several arcseconds west of galaxies NGC 3406, NGC 3406, and NGC 3410.