Leo I

Leo Triplet

The Leo Triplet. Image courtesy Richard Crisp.

Leo I is a large, prominent group of predominantly spiral galaxies in Leo. It consists of two main subgroups: the M66 Group and the M96 Group. The former lies about 35 million light-years away and is centered on the interacting spirals M65 (NGC 3623), M66 (NGC 3627), and the edge-on NGC 3628, also known as the Leo Triplet (see accompanying photo). Not far from the M66 Group, and almost certainly physically related to it, is the much larger M96 group dominated by M96 itself, about 41 million light-years away, M95, M105, and NGC 3384. Fainter members of the M96 Group include NGCs 3299, 3377, 3377A, 3384, 3412, and 3489. The slightly more distant S0 or early Sa galaxy NGC 3593 is probably also a member.


Leo I is also the name of a dwarf spheroidal (dE3) galaxy that, at a distance of about 880,000 light-years, is the most remote satellite galaxy of the Milky Way and hence also a member of the Local Group. Discovered in 1950, it has a diameter of about 6,000 light-years. Because it lies close to Regulus in the sky (making study of it difficult in the bright star's light) it is sometimes known as the Regulus Dwarf, and also catalogued as DDO 74, UGC 5470, and Harrington-Wilson 1.