Andromeda I

Andromeda I is a dwarf spheroidal galaxy (dSph)^[5] about 2.40^[4] million light-years away in the constellation Andromeda. Andromeda I is part of the local group of galaxies and a satellite galaxy of the Andromeda Galaxy (M31). It is roughly 3.5 degrees south and slightly east of M31.^[6] As of 2005, it is the closest known dSph companion to M31^[7] at an estimated projected distance of ~40^[4] kpc or ~150,000^[6] light-years.

Andromeda I
Andromeda I by the HST
Observation data (J2000 epoch)
Constellation	Andromeda
Right ascension	00h 45m 39.8s[1]
Declination	+38° 02′ 28″[1]
Redshift	-368 ± 11 km/s[1]
Distance	2.40 ± 0.08 Mly (735 ± 23 kpc)[2][3][4]
Apparent magnitude (V)	13.6[1]
Characteristics
Type	dSph
Apparent size (V)	2.5′ × 2.5′[1]
Notable features	satellite galaxy of M31
Other designations
And I,[2] Anon 0043+37,[1] PGC 2666[1]

Andromeda I was discovered by Sidney van den Bergh^[8] in 1970 with the Mount Palomar Observatory 48-inch telescope.^[5] Further study of Andromeda I was done by the WFPC2 camera of the Hubble Space Telescope. This found that the horizontal branch stars, like other dwarf spheroidal galaxies were predominantly red.^[9] From this, and the abundance of blue horizontal branch stars, along with 99 RR Lyrae stars detected in 2005,^[7] lead to the conclusion there was an extended epoch of star formation. The estimated age is approximately 10 Gyr. The Hubble telescope also found a globular cluster in Andromeda I, being the least luminous galaxy where such a cluster was found.^[10]