Segue 1 is a

dwarf spheroidal galaxy A dwarf spheroidal galaxy (dSph) is a term in astronomy applied to small, low-luminosity galaxies with very little dust and an older stellar population. They are found in the Local Group as companions to the Milky Way and to systems that are comp ...

globular cluster A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of membe ...

situated in the Leo constellation and discovered in 2006 by

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 ...

. It is located at a distance of about 23 kpc (about 75,000 light years) from the Sun and moves away from the Sun with the velocity of about 206 km/s. Segue 1 has a noticeably elongated (ratio of axes ~ 2:1) shape with the

half-light radius Galaxy effective radius or half-light radius (R_e) is the radius at which half of the total light of a galaxy is emitted. This assumes the galaxy has either intrinsic circular symmetry, spherical symmetry or is at least circularly symmetric as vie ...

of about 30 pc. This elongation may be caused by the tidal forces acting from the Milky Way galaxy if Segue 1 is being tidally disrupted now. The name is due to the fact that it was found by the SEGUE program, the Sloan Extension for Galactic Understanding and Exploration.

Properties

Segue 1 is one of the smallest and faintest

satellites A satellite or artificial satellite is an object intentionally placed into orbit in outer space. Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotop ...

of the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

—its integrated luminosity is about 300 times that of the Sun (absolute

visible magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's lig ...

of about −1.5), which is much smaller than the luminosity of a typical

. Observations indicate its mass is about 600,000

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 Segue'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 3400. Segue 1 had the highest known mass-to-light ratio of any observed galaxy as of 2011. A high mass to light ratio implies that Segue 1 may be dominated by

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...

. It is difficult, however, to estimate the mass of such faint objects due to significant foreground contamination, which inflates the velocity dispersion. In addition, 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 Segue 1 consists mainly of old stars formed more than 12 billion years ago. The metallicity of these old stars is also very low at , which means that they contain 300 times less heavy elements than the Sun. Currently there is no star formation in Segue 1. Measurements have so far failed to detect

neutral hydrogen The hydrogen line, 21 centimeter line, or H I line is the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation has a precise frequency of , w ...

in it—the upper limit is 13 solar masses. There is an estimate of roughly 1000 stars within the object. Of these, 7 have been found to be in the

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...

stage of their life. The chemical composition of Segue 1 indicates no substantial chemical evolution has occurred since the galaxy formed, supporting the idea that it may be a surviving first galaxy that experienced only one burst of star formation, a fossil galaxy from the early universe. Segue 1 is located in the middle of the

Sagittarius Stream In astronomy, the Sagittarius Stream is a long, complex structure made of stars that wrap around the Milky Way galaxy in an orbit that nearly crosses the galactic poles. It consists of Tidal stripping, tidally stripped stars from the Sagittarius Dw ...

and at approximately the same distance from the Sun. It may once have been a globular cluster of the

Sagittarius Dwarf Elliptical Galaxy The Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph), also known as the Sagittarius Dwarf Elliptical Galaxy (Sgr dE or Sag DEG), is an elliptical loop-shaped satellite galaxy of the Milky Way. It contains four globular clusters in ...

, which was later stripped from it by the

tidal force The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenomen ...

s acting from the Milky Way galaxy. However, more recent studies concluded that Segue 1 is not actually associated with the Sagittarius stream and that it is not being tidally disrupted. If Segue 1 is a galaxy it may have been a satellite of the Sagittarius Dwarf Elliptical Galaxy in the past.

Notes

References

External links

* {{Milky Way Local Group Milky Way Subgroup Dwarf spheroidal galaxies Leo (constellation) Globular clusters ? 4713559