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Coma Berenices


Transit Date of principal star:
9 April


The constellation Coma Berenices refers to a classical story concerning the hair of Berenice, the wife of Ptolemy III of Egypt. While the story is an old one, the constellation is relatively new, being introduced by Tycho Brahe (1546-1601).

According to the story, Ptolemy had waged a long war on the Assyrians, since it was they who had killed his sister. As Ptolemy returned successfully from the war, his wife Berenice had her beautiful tresses ceremoniously clipped and given to Aphrodite, laid out on the temple altar.

As the evening's festivities continued, the shorn hair was discovered to be missing. The priests might be sacrificed, if the queen's hair couldn't be found. It was the astronomer Conon of Samos who came to their rescue - proclaiming that Aphrodite had accepted the gift of Berenice's hair, which now shown brightly in the heavens next to Leo.


The stars that form the constellation really aren't that remarkable to look at, only a handful of fourth-magnitude stars, including three Bayer stars. Yet there are several fine binaries, eight Messier objects and the Coma Star cluster, not included in Messier's list.

From Denebola (beta Leonis) draw a line to the bright star to the southeast, Arcturus (alpha Bootis). Alpha Comae is found on this line at about the midpoint.

Now proceed north from alpha Comae to beta Comae and then west about the same distance to gamma Comae. These three stars form half of a nearly perfect square. They aren't very prominent, and you will have to have a nice dark night in order to study them.

Alpha Comae, sometimes called Diadem, has the same diameter as our Sun, and is 62 light years away with a luminosity of nearly three. It's a rapid motion binary (see below) and in the same field is the globular cluster M53 (see below).

Beta Comae is actually the brightest star in the constellation, and certainly the closest at 27 light years. It too has a diameter equal to the Sun.

Gamma Comae is an orange star about 260 light years away. It is in the same region as the well-known Coma Star Cluster, but isn't a member of that group.


Double stars in Coma Berenices:

Alpha Comae is a rapid binary of two equal stars (5.05, 5.08). The companion orbits every 25.87 years and is presently decreasing; the current (2000) separation is less than 0.05". The orbit is an unusual one, seen perfectly edge-on.

Zeta Comae is a fixed binary: (6.0, 7.5; PA 237, separation 3.6").

17 Comae and 24 Comae are two binaries with contrasting companions.

17 Comae is one of the members of the Coma Star Cluster. The primary is white, the companion a soft blue: 5.3, 6.6; PA 251, separation 145.3".
     From gamma Comae follow the slight arc of stars south that bend to the east. First comes 14 Comae, then 15, and finally 17.

24 Comae is even more spectacular: a fixed binary with an orange primary and emerald component. (5.2, 6.5; PA 271, separation 20.3").

This binary is located eight degrees west of alpha Comae and one degree north.

35 Comae is a slow double with an orbit of over 300 years. However, unlike most long period binaries, this one is presently quite close. The companion is beginning to emerge from its close pass with the primary, gradually lengthening its separation, recently having achieved one arc second of separation. The present values are: 5.2, 7.2; PA 185 and separation 1.04".

35 Comae is in a fairly barren part of the sky, found five degrees northwest of alpha Comae.

Struve 1633 is a very pleasant fixed binary: 7.1, 7.2; PA 245, separation 9.0". To find it start from gamma Comae, then drop down exactly one degree south where you'll find 14 Comae. Struve 1633 is one degree to the west.

Struve 1639 is a closer binary: 6.8, 7.8; PA 327, 1.6". This is a slow moving binary with an orbit of 678 years.

This double star makes a small triangle with 12 Comae and 13 Comae. Start at 14 Comae and look south. The bright star to the east is 15 Comae, while below this and to the west is 13 Comae. Nearby, immediately southwest, is 12 Comae. Now look between these two stars to the southeast, where you'll find the third point in the triangle. This is Struve 1639. (Not shown on the chart due to crowding.)


Variable stars in Coma Berenices:

The constellation doesn't have a wealth of variable stars. We list the two variables that might be of some interest.

13 Comae is an alpha-CV type variable with very small range (5.15-5.18).

R Comae is a long-period variable with period of 362.82 days, and range of 7.1 to 14.6. Thus the maximums are nearly a year apart. In the year 2000 the maximum should occur in the first week of December.


Deep Sky Objects in Coma Berenices:

There are eight Messier objects (M53, M64, M85, M88, M91, M98, M99, and M100), as well as a number of other fine galaxies, with NGC 4565 being the best of the bunch.

However the best object is the unrivalled open cluster known as ‘The Coma Star Cluster’.

The Coma Star Cluster

Best seen in binoculars, the cluster fills the entire field of view: about 40 stars spread out over a five degree area.

The cluster was once known as the tuft of hair at the end of Leo's tail. It now constitutes Berenice's golden tresses.

The cluster extends south from gamma Com (which is not, however, a member). At about 270 light years away, the cluster is one of the closest to our solar system.

The brightest member of the cluster is 12 Comae. Other fourth-magnitude members are 13 and 14 Comae, and another thirty or so fainter stars go to make this one of the loveliest sight in the heavens.

The Messier Objects in Coma Berenices

M53 is a globular star cluster one degree northeast of alpha Comae. The brightest Messier in the constellation (7.7), it tends to be most impressive with larger telescopes, which are needed to resolve the individual stars. The cluster is thought to be 65,000 light years away.

M64, the Black Eye Galaxy, is a bright (8.5) compact spiral one degree east-northeast of 35 Comae. The "black eye" can only be seen under ideal conditions with large telescopes. The galaxy is over 20 million light years away.

M85 is a bright spiral galaxy and member of the Virgo Galaxy Cluster, most of which is found about five degrees further south. All the remaining deep sky objects discussed also belong to this cluster.

M88 is a many-armed spiral galaxy some forty million light years away. Quite bright (9.5), it's a favourite with many Messier observers.

M91 (NGC 4548) is another spiral galaxy, but is a rather confusing object, sometimes being labelled M58. It is a rather faint galaxy (10.2) and one wonders why, with so many galaxies in the region, spreading down through Virgo, that this one was chosen by Messier.

M98 is a faint (10.1) spiral seen practically edge-on, lying just half a degree west of 6 Comae.

M99 is roughly one and a half degrees east-southeast of M98. An open spiral seen face on, its several arms are visible in large scopes. It has a brightness of 9.8.

M100 is the largest of these spiral galaxies, although difficult to appreciate in small telescopes. It's seen face-on, and has a brightness of 9.4.


NGC 4565 is a well-known edge-on spiral with highly visible dust lane from end to end. It's the largest galaxy of its type and has a visual magnitude of 9.6. The galaxy is found one degree due east of 17 Comae.

Coma Berenices has many more deep sky objects, particularly the southern regions, where it borders Virgo. This is a fertile part of the sky to investigate, as the evenings grow a little warmer and more inviting.


For a closer appreciation of Coma Berenices, visit the Binocular Section.


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Richard Dibon-Smith.