Best Double Stars in the Pleiades Cluster

   

From -Sky & Telescope

By - Bob King

 Edited by- Amal Udawatta

       

The dipper-shaped Pleiades cluster (M45) is also called the Seven Sisters and named for the mythological seven daughters of Atlas and Pleione. The young cluster is between 75 and 150 million years and lies 444 light-years from Earth.
Jared-Bowens

The Pleiades star cluster is one of the night sky's best-known astronomical sights. Alluring to the naked eye, it's even more amazing through binoculars or a small telescope, both of which reveal dozens more stars. As the cluster plows through space at 6 kilometers per second (13,400 mph), its hot, youthful suns illuminate a happenstance interstellar cloud, turning it into a gossamer nebula that temporarily enshrouds the stellar bunch.

Additional treasures lie within its bounds: There are also about a half-dozen double and multiple stars within the Pleiades. You might already be familiar with 2.9-magnitude Alcyone, a choice triple star located near the cluster's core. Also known as Eta (η) Tauri, it's the brightest Pleiad and the only one with a Bayer designation.

On cold February nights, it helps if you can complete an observing goal before your fingers and toes freeze. Finding all half-dozen Pleiades pairs with plenty of time to admire them will take under an hour. You can even observe all of these targets without repointing. Once the cluster's in the field of view, it's easy to hop from one star to the next using the map provided below.

Star	Magnitudes	Separation(s)	Position angle of companion	Type
Taygeta (19 Tau)	4.3, 11.0	71.7″	329°	Not a physical double
Σ444 (HD 23410)	6.9, 10.1	3.4″	338°	True physical pair
S 437 (HD 23479)	8.1, 9.4	1.1″	178°	True physical pair
Alcyone (Eta Tau)	2.8, 6.3 (B) 8.2 (C), 8.7 (D)	117.6″ (A-B), 182.0″ (A-C), 191.8″ (A-D)	A-B = 291° A-C = 313° A-D = 297°	True physical triple; D is not a member
OΣΣ40 (HD 23873)	6.6, 7.5	86.8″	309°	True physical pair
Σ450 (HD 23631)	7.3, 9.4 (A-B)	5.7″	263°	Nature uncertain
OΣ64 (HD 23964)	6.8, 10.2 (B) 10.5 (C)	3.3″ (A-B) 10.1″ (A-C)	A-B = 234° A-C = 235°	A-B a true pair A-C is uncertain

These are the "vitals" for our featured double and multiple systems in the Pleiades star cluster, listed in order of increasing right ascension (R.A.). Separations in arc-seconds are shown between the primary star (A) and secondary (B) or secondaries. Position angle (PA) is the angle measured from due north to a line connecting the primary and secondary. A PA of 0° is north, 90° east, 180° south, and 270° west. Source: Stelle Doppie

Use this simplified map to track down all of our featured double and multiple stars in the Pleiades star cluster.
MegaStar, courtesy of Emil Bonanno, with additions by Bob King

Visual impressions

I used a beat-up 10-inch f/6 Dobsonian telescope with a primary mirror in serious need of recoating for the observations.

Taygeta — Bright primary strongly contrasts with the faint, pinpoint secondary to northwest. Sadly, not a true double.

Σ444 — Secondary star appears delicate and nearly overwhelmed by the much brighter primary. The stark contrast between the two, combined with their relatively close separation, made this one of my favorites. Note: The Cambridge Double Star Atlas incorrectly labels this pair as Σ450.

S 437 — One of the finest close pairs in the winter sky. When two stars nested in fluttering, concentric Airy rings barely touch, my heart goes pit-a-pat. This pair looked wonderful in calm seeing at 218×. Bonus! Just 40″ due west you'll see the only obviously red star in the Pleiades, HD 23463. It's nearly the same brightness as its neighbor and makes a splendid color contrast. HD 23463 is an orange dwarf, a main sequence star intermediate in size between red dwarfs and stars like our Sun.

I made these sketches of the S 437 pair (left) and triple star Alcyone (right) through a 10-inch Dob using Photoshop.
Bob King

Alcyone — Brightest star in the Pleiades. I've always seen it as a quadruple with three companions dangling from the primary like a triangle instrument on a thread. But based on its parallax, the D star (see sketch) is an interloper. The C companion is the pulsating variable star V647 Tauri, which varies by a small fraction of a magnitude every 68 minutes. All of Alcyone's companions are easy to see at 20x in a small scope. Alcyone itself is brilliant B-type giant star with a bluish tinge in the 10-inch.

OΣΣ40 — A wide, bright pair for any size telescope — even a pair of 10× binoculars will split it.

Σ450 — This attractive duo is located a short distance southeast of the "dipper bowl" and forms a right triangle with Merope and Alcyone. The two-magnitude difference between primary and secondary and their somewhat tight separation make seeing the companion a worthy hunt. Use 200× for a clean split.

OΣ64 — Pleasantly surprised when I tracked down this fine, close-set triple star. How did it elude me all these years? The three stars form a short, almost perfect straight line. The arrangement is very compact with the secondary star snugged up near the primary. Use a magnification of 150×-200×.

The Pleiades are also fair game from southern latitudes. This photo was taken Jan. 26, 2025 with a smartphone and 8x42 binoculars from Ezeiza, Argentina at latitude 35° south.
Piqui Diaz

All these doubles are fair game for scopes as small as 4-inches. I hope you'll enjoy getting to know another facet of this familiar and beautiful star cluster.

Unique event: Comet 29P occults a star

If you're in the right place at the right time, you might have a look at the eruptive comet 29P/Schwassmann-Wachmann, which will occult a 12th-magnitude star in Leo on February 5th at 23:06-23:11 UT. This event will be visible to observers along a path about 60 kilometers wide crossing Asia and Europe. Occultation details and maps are available at the British Astronomical Association's MISSION 29P News webpage and Lucky Star Predictions webpage. To observe and record this rare event, a six-inch (150mm) or larger telescope is recommended.

Earlier this month, the comet underwent back-to-back eruptions, spewing cryogenic liquids from its interior into space. Some of that material may still be orbiting the comet's nucleus and be detectable during the occultation. The expected maximum duration of the event will be about 3.5 seconds.