Beta Andromedae, a Red Giant

Mirach is Beta Andromedae, apparent magnitude about 2.05, at roughly 197 light years. It is a red giant of spectral type M0, cooler and vastly larger than the Sun, and it varies slightly and irregularly as red giants do. It sits at the hip of Andromeda and serves observers as a signpost: a line through Mirach points toward both the Andromeda Galaxy and the smaller Triangulum Galaxy. A faint lenticular galaxy, NGC 404, lies so close to the star in the sky that it is nicknamed Mirach's Ghost, an unrelated background object glimpsed past the foreground glare.

At magnitude 2.05 it sits almost exactly on the traditional boundary between Ptolemy's first and second magnitude classes, and indeed ancient sources are not perfectly consistent about which class they place it in, a reminder that the six-step Hellenistic magnitude scale was always a judgment by eye rather than a measured quantity, and stars near a class boundary could be sorted differently by different observers centuries apart. Modern photometry places it steady at 2.05 without the variability sometimes claimed for it in older amateur reports, which most likely arose from comparison-star error rather than any real change in the star.