Nearly overhead in the after-dinner hours of a northern winter night, the rich constellation Perseus offers even a modestly-equipped amateur astronomer many hours of pleasant stargazing. Named after the great hero of Greek mythology, Perseus finds itself in the starry plane of the Milky Way Galaxy where thousands of brilliant blue-white stars have coalesced in the the last few tens of millions of years. Near the star Mirfak, or α (alpha) Persei, the brightest star in Perseus, lies a particularly dazzling collection of associated blue-white stars that make up a loose cluster often called the “Attendants of Mirfak”. This little group is a beautiful sight in binoculars.
The Attendants of Mirfak are what astronomers call a “moving group”, in this case the Alpha Persei Moving Group, which is a collection of stars that formed together and continue to move together through space as they slowly disperse into the galaxy. More formally cataloged as Melotte 20 or Collinder 39, this group of stars is spread across roughly 3° of sky. There are some 40 to 50 stars in the group and they lie at a distance of about 600 light years.
Most of the group’s brightest members lie between the stars α (alpha) Persei and δ (delta) Persei. At least a dozen shine brighter than 6th magnitude. In dark sky, look directly towards Mirfak without optics, and you’ll see a vaguely luminous region. Avert your vision slightly and individual stars will snap into view.
In binoculars or a wide-field telescope at low power, Melotte 20 is a beautiful sight. Look directly in and around the region of Mirfak to see a profusion of lovely stars cast about in all manners of shapes and patterns. Look also for a long, winding shape of the brighter stars which resembles a small sea monster — a mini-version of the constellation Cetus, perhaps. The monster’s bulbous head is formed by bright Mirfak and a ring of 6th magnitude stars; σ (sigma) Persei marks the hump of the beast, and δ (delta) marks the tip of the tail.
(Publisher’s Note: The Attendants of Mirfak is just one of hundreds of celestial objects covered in the detailed Cosmic Pursuits course entitled “Fundamentals of Stargazing” which will be offered in late February 2017).
While the position of this star favors northern observers, Mirfak is also visible about 10° to 15° over the northern horizon as darkness falls at the latitudes of South Africa and southern Australia in January and February. It’s west of the much brighter star Capella. Mirfak is closer to the horizon and harder to see from southern New Zealand.
The star’s name comes from the Arabic for “elbow”. It was also an important star in Hawaiian celestial folklore. Called Hinali’i, the star was beheld as the point of separation between Earth and Sky as the Milky Way was formed.
Most visible members of the Alpha Persei Moving Group are massive blue-white stars that make up a much larger conglomeration of stars called the Alpha Persei OB3 Association. There are many X-ray sources here, too, which hints at neutron stars and black holes created by massive stars that have already detonated as supernovae.