Willman 1 is an ultra low-luminosity dwarf galaxy or a

star cluster Star clusters are large groups of stars. Two main types of star clusters can be distinguished: globular clusters are tight groups of ten thousand to millions of old stars which are gravitationally bound, while open clusters are more loosely clust ...

. Willman 1 was discovered in 2004. It is named after

Beth Willman Beth Willman is an American astronomer who is the Executive Officer of the LSST (Large Synoptic Survey Telescope) Corporation, an astronomical organization notable for its support of the Vera C. Rubin Observatory. She was previously the deputy dir ...

Haverford College Haverford College ( ) is a private liberal arts college in Haverford, Pennsylvania. It was founded as a men's college in 1833 by members of the Religious Society of Friends (Quakers), began accepting non-Quakers in 1849, and became coeducational ...

, the lead author of a study based on the

Sloan Digital Sky Survey The Sloan Digital Sky Survey or SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project began in 2000 a ...

data. The object is a satellite of the Milky Way, at ~120,000

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s away. Willman 1 has an elliptical shape with the half-light radius of about 25 pc. Its heliocentric velocity is approximately −13 km/s. As of 2007, it was declared the least massive galaxy known, opening up a new category of ultra-low-mass galaxies, lower than the then-theoretical minimum of 10 million solar masses thought to be needed to form a

galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

.New Scientist

David Shiga, 4 June 2007 (Retrieved 12 June 2013) As of 2016, it is the third dimmest likely galaxy known, after

Segue 1 Segue 1 is a dwarf spheroidal galaxy or globular cluster situated in the Leo constellation and discovered in 2006 by Sloan Digital Sky Survey. It is located at a distance of about 23 kpc (about 75,000 light years) from the Sun and moves awa ...

and Virgo I, and is over ten million times less luminous than the Milky Way. It has an absolute magnitude of . Observations indicate its mass is about 0.4 million

solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

es, which means that Willman's 1

mass to light ratio In astrophysics and physical cosmology the mass-to-light ratio, normally designated with the Greek letter upsilon, , is the quotient between the total mass of a spatial volume (typically on the scales of a galaxy or a cluster) and its luminosity ...

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 In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...

of these stars is also very low at , which means that they contain 110 times less heavy elements than the Sun.

Notes

: a. 15.2 ± 0.4 apparent magnitude − 5 * (log₁₀(38 ± 7 kpc distance) − 1) = −2.7 absolute magnitude

References

Inline General * * * * {{Sky, 10, 49, 22.3, +, 51, 03, 03.6, 120000 Dwarf galaxies Dwarf spheroidal galaxies Globular clusters Local Group Milky Way Subgroup ? Ursa Major