In this Letter , we report the discovery of a new dwarf satellite to the Milky Way , located at ( \alpha _ { 2000 } , \delta _ { 2000 } ) = ( 158.72,51.92 ) in the constellation of Ursa Major .
This object was detected as an overdensity of red , resolved stars in Sloan Digital Sky Survey data .
The color-magnitude diagram of the Ursa Major dwarf looks remarkably similar to that of Sextans , the lowest surface brightness Milky Way companion known , but with approximately an order of magnitude fewer stars .
Deeper follow-up imaging confirms this object has an old and metal-poor stellar population and is \sim 100 kpc away .
We roughly estimate M _ { V } = -6.75 and r _ { 1 / 2 } = 250 pc for this dwarf .
Its luminosity is several times fainter than the faintest known Milky Way dwarf .
However , its physical size is typical for dSphs .
Even though its absolute magnitude and size are presently quite uncertain , Ursa Major is likely the lowest luminosity and lowest surface brightness galaxy yet known .