This is a cowboy shoujo manga set in the Wild West. Miriam is an orphan. Her father was a drunken gambler; she only had one dress and they moved from place to place, until one day her father died and Grace took her in. The series follows Miriam's life, growing up, and falling in love with her life-long friend, the skillful sharpshooter Douglas.
First of the Miriam series (3 volumes), followed by sequels Jikan wo Tomete Matteite
(3 volumes) and Sore nari ni Romantic!
(1 volume). Seven volumes total.
My Milky Way Railroad is a oneshot sequel to Hoshi no Harmony