Willman 1
Encyclopedia
Willman 1 is an ultra low-luminosity dwarf galaxy
or a star cluster
. It is named
after Beth Willman
of Haverford College
, the lead author of a study based on the Sloan Digital Sky Survey
data. The object is a satellite of the Milky Way
; ~120,000 light-year
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 2011, it is 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 . Observations indicate its mass is about 0.4 million solar mass
es, 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 , which means that they contain 110 times less heavy elements than the Sun.
Dwarf galaxy
A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Way's 200-400 billion stars...
or a star cluster
Star cluster
Star clusters or star clouds are groups of stars. Two types of star clusters can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars which are gravitationally bound, while open clusters, more loosely clustered groups of stars, generally contain less than...
. It is named
Galaxies named after people
A small number of galaxies or galaxy groups have been named after individual people. In most cases, the named individual was the person who discovered the object, who first brought attention to it, or who first studied it scientifically....
after Beth Willman
Beth Willman
Beth Willman is an American astronomer and is an assistant professor at Haverford College. Working with Sloan Digital Sky Survey data, she and her team have discovered two Milky Way satellites so far, SDSSJ1049+5103 and UMa dSph....
of Haverford College
Haverford College
Haverford College is a private, coeducational liberal arts college located in Haverford, Pennsylvania, United States, a suburb of Philadelphia...
, the lead author of a study based on the Sloan Digital Sky Survey
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter 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 was named after the Alfred P...
data. The object is a satellite of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
; ~120,000 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
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 2011, it is the second dimmest likely galaxy known, after Segue 1
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 the distance of about 23 kpc from the Sun and moves away from the Sun with the velocity of about 206 km/s. Segue 1 has a...
, and is less than one ten millionth the Milky Way's luminosity. It has an absolute magnitude
Absolute magnitude
Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...
of . Observations indicate its mass is about 0.4 million solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es, which means that Willman's 1 mass to light ratio
Mass to light ratio
In astrophysics and physical cosmology the mass to light ratio, normally designated with the symbol \Upsilon is the quotient between the total mass of a spatial volume and its luminosity...
is around 800. A high mass to light ratio implies that Willman 1 is dominated by dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
. 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 , which means that they contain 110 times less heavy elements than the Sun.