Segue 1

Segue 1 is a dwarf spheroidal galaxy or globular cluster located at a distance of about 23 kpc. Segue 1 has a noticeably elongated (ratio of axes ~ 2:1) shape with the half-light radius 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.

Segue 1 is one of the smallest and faintest satellites of the Milky Way — its absolute visible magnitude of about −1.5 is much smaller than the luminosity of a typical globular cluster.

Observations indicate its mass is about 0.6 million stellar masses, which means that Segue's 1 mass to light ratio is around 3400. Segue 1 has one of the highest known mass-to-light ratios of any observed galaxy, implying that Segue 1 may be dominated by dark matter.

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 [Fe/H] ≈ −2.5 ± 0.8. Currently there is no star formation in Segue 1. Measurements have so far failed to detect neutral hydrogen in it.

If Segue 1 is a galaxy it may have been a satellite of Sagittarius Dwarf Elliptical Galaxy in the past.