Willman 1

Willman 1 is an ultra low-luminosity dwarf galaxy or a star cluster. The object is a satellite of the Milky Way, at ~120,000 light-years away. Willman 1 has an elliptical shape with the half-light radius of about 25 pc.

It is one of the least massive galaxies known, opening up a new category of ultra-low-mass galaxies, lower than the then-theoretical minimum of 10 million stellar masses thought to be needed to form a galaxy. It is also the second dimmest likely galaxy known, after Segue 1, and is less than one ten millionth the Milky Way's luminosity. It has an absolute magnitude of −2.7 ± 0.7.

Observations indicate its mass is about 0.4 million stellar masses, which means that Willman's 1 mass to light ratio is around 800. A high mass to light ratio implies that Willman 1 is dominated by dark matter. It is difficult, however, to estimate the mass of such faint objects because any mass estimate is based on an implicit assumption that an object is gravitationally bound, which may not be true if the object is in a process of disruption.

The stellar population of Willman 1 consists mainly of old stars formed more than 10 billion years ago. The metallicity of these stars is also very low at [Fe/H] ≈ −2.1.