Across the Middle of Perseus

The central portion of Perseus is loaded with celestial treasure.

Perseus char

Visible to the unaided eye, the large star-rich region that defines the middle of the constellation Perseus is mostly a star cluster designated Melotte 20, about 600 light-years distant. Mel 20 includes the constellation’s brightest star, Mirphak. The region is a fine sight in any size binocular. (Click on the chart for a larger version.)

The lead star in Perseus, Mirphak (sometimes spelled Mirfak) anchors the midsection of the stick-figure star pattern representing the mythic hero. As the New Year opens, mighty Mirphak, at magnitude 1.8, stands high in the northeast at nightfall. Roughly 600 light-years distant, Mirphak is an ultraluminous, yellow-white supergiant 80 times larger than our Sun and thousands of times more brilliant. It dominates Melotte 20 (Mel 20), a group of stars once known as the Alpha Persei Association. (An association is a stellar family too loosely bound by gravity to stay together for very long in the astronomical timescale.) Mel 20 is now understood to be a true cluster inside a larger “association.” Either way, Mel 20 is so big and bright when viewed in a dark country sky that it catches my gaze as a sparkly patch in the Perseus Milky Way. (The large oval outlining Mel 20 on the guide chart above is nine degrees wide.)

A binocular is your best aid to appreciate Mel 20. Most of its two dozen brightest members, ranging from magnitude 4 to 7, form a sinuous pattern between Mirphak and third-magnitude delta (δ) Persei, 3.7 degrees to the southeast. The curve runs through reddish fourth-magnitude sigma (σ) Persei, then terminates at fourth magnitude psi (ψ) Persei, a star of slight variability near delta. On the other side of Mirphak are two outlying wide doubles. The first set, boasting fifth-magnitude twins nine arc minutes apart, I call the “even eyes.” Farther out are the “uneven eyes,” comprising a fifth-magnitude golden star and a sixth-magnitude white star under four arc minutes apart. All this glitter fits — just barely — in the 4.4-degree field of my 15×70 “big binos.”

Almost 10 degrees south-southwest of Mirphak is beta (β) Persei, or Algol, the famous Demon Star. Most of the time, Algol shines at magnitude 2.1, but every 2.9 days, it dips to magnitude 3.4. Algol is a type of variable star known as an eclipsing binary. Its closely spaced suns are of steady but unequal luminosity; their combined light begins to fade when the larger, dimmer secondary passes in front of the smaller, brighter primary. The entire event — maximum to minimum and back again — lasts about 10 hours, so occasionally, the full eclipse can be witnessed during a winter’s night. If you want to follow the Demon Star’s slow-motion wink, remember that Algol remains as bright as nearby 2.1-magnitude gamma (γ) Andromedae until minimum light, when it’s briefly over a full magnitude dimmer.

M34 sketch - Karlsson

Easily visible in binoculars, M34 is a scattering of several dozen stars of eighth magnitude and fainter, about 1,400 light-years distant. The cluster is known for its numerous double stars that are readily revealed in a small telescope.
Sketch by John Karlsson

Lying on the edge of the Milky Way, slightly more than five degrees west-northwest of Algol, is the 5.2-magnitude open cluster Messier 34 (M34). Even when viewed from town, M34 is a grainy patch in my 7×50 binoculars and a compact sparkle of pinpoints in my monster 15×70s. And no wonder: Although roughly 1,500 light-years away, M34 spans nearly half a degree and contains five dozen blue-white stars, 10 of them better than magnitude 9.0. A bright outlier, a 7.3-magnitude reddish yellow star, is a cindery sun that is a foreground star slightly to one side of the cluster. Viewed in binoculars, that star and M34 are close but entirely separate objects.

To me, M34 is the “Doubles Cluster.” My backyard 4.25-inch Newtonian reflector at 22× resolves an eighth-magnitude, 20-arc-second-wide binary called Herschel 1123, near M34’s middle. A couple of wider sets are visible among the individual cluster members, and three dimmer doubles show at 72x. In my 10-inch Newtonian at 58x, I count eight pairs at differing magnitudes, separations and position angles. One duo is very tight and uneven, while another resolves into a triple shaped like an isosceles triangle. The warm-hued lucida mentioned earlier seems part of the cluster’s southern outskirts. Two dimmer dots of similar colour adorn the north. To finish off, I like to look two degrees south of Algol at rho (ρ) Persei, a semiregular variable that fluctuates between magnitude 3.3 and 4.0 over a period of about seven weeks. Observing with binoculars one night, I noticed that Algol had dipped to minimum light when rho was near maximum. The stars were approximately equal in brightness but delightfully different in colour. Rho shone reddish orange, while Algol was pure white. Beautiful!

SkyNews contributing editor Ken Hewitt-White has observed deep-sky fuzzies over southern British Columbia for more than four decades.

Categories: Clusters, Nebulas, and Galaxies
One comment on “Across the Middle of Perseus
  1. Bill Scoffield says:

    A method of locating Perseus would be handy. Little children and old men have problems with such things. I’m the latter, and often forward Skynews stuff to young grands and other children, who need initiation, and don’t always remember lessons from hearing them only once.

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