Willman 1 is an ultra low-luminosity

dwarf galaxy A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is so ...

or a

star cluster A star cluster is a group of stars held together by self-gravitation. Two main types of star clusters can be distinguished: globular clusters, tight groups of ten thousand to millions of old stars which are gravitationally bound; and open cluster ...

. Willman 1 was discovered in 2004 and is located near Ursa Major in the night sky. It is named after

Beth Willman Beth Willman is an American astronomer who is the Chief Executive Officer of the LSST Discovery Alliance, an astronomical organization notable for its support of the Vera C. Rubin Observatory. She was previously the deputy director of the Nation ...

Haverford College Haverford College ( ) is a private university, private Liberal arts colleges in the United States, liberal arts college in Haverford, Pennsylvania, United States. It was founded as a men's college in 1833 by members of the Religious Society of Fr ...

, 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 The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

, at ~120,000

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

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 Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...

. 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 away ...

and

Virgo I Virgo I is an extremely faint satellite galaxy of the Milky Way. It was discovered in the Subaru Strategic Survey. Virgo I has an absolute visual magnitude of −0.8 making it one of the least luminous galaxies confirmed thus far. The galaxy has a ...

, 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 frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...

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 galaxy cluster, cluster) and i ...

is around 800. A high mass to light ratio implies that Willman 1 is dominated by

dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...

. 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 the chemical elements, abundance of Chemical element, elements present in an object that are heavier than hydrogen and helium. Most of the normal currently detectable (i.e. non-Dark matter, dark) matt ...

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

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

Notes

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

References

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