Hercules Dwarf Galaxy

Hercules, or Her, is a dwarf spheroidal galaxy situated at a distance of about 140 kpc from the Milky Way. The Hercules Galaxy has a noticeably elongated (ratio of axes ~ 3:1) shape with a half-light radius of about 350 pc. This elongation may be caused by tidal forces acting from the Milky Way galaxy, meaning that Her is being tidally disrupted now. Her also shows some gradient of velocities across the galaxy's body and is embedded into a faint stellar stream, which also points towards its ongoing tidal disruption.

Her is one of the smallest and faintest satellites of the Milky Way — its absolute visible magnitude of about −6.6 is comparable to the luminosity of a typical globular cluster. However, its total mass is about 7 million stellar masses, which means the galaxy's mass to light ratio is around 330. A high mass to light ratio implies that Her is dominated by dark matter.

The stellar population of Her 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] ≈ −2.58 ± 0.51. The stars of Her were probably among the first stars to form in the Universe. Currently there is no star formation in Her. Measurements have so far failed to detect neutral hydrogen in it.