Stellar Halo around Mirfak

On mid-December evenings the constellation of Perseus is climbing the northeastern sky. Just for 2024, bright Jupiter will also be gleaming to the lower right of Perseus. The outer rim of our Milky Way galaxy runs through Perseus’ stars, filling its territory with rich star clusters. The largest of those surrounds his brightest star, Mirfak, or Alpha Persei. That elderly yellow supergiant star has evolved out of its blue phase and is now fusing helium into carbon and oxygen in its core. Melotte 20, also known as the Alpha Persei Moving Group and the Perseus OB3 Association, is a collection of 100 or so young, massive, hot B- and A-class stars sprinkled over several finger widths (or 3 degrees) of the sky around Mirfak. The cluster can be seen with unaided eyes, but it’s especially dazzling in binoculars (orange circle). Its stars are approximately 600 light years from the sun and are moving as a group – Mirfak along with them.