OPEN STAR CLUSTERS : 10 of 11

(2) The PLEIADES : The Seven Sisters
Another Important and Well-Studied Cluster

The Pleiades or Seven Sisters / M45 / Mel 22 (03470+2406) is likely the most famous and brightest of all the open star clusters in the sky. It is perhaps one among very few deep-sky objects that does not need any real description to be able to find in the sky. Indeed some have considered it placed as a constellation in its own right! Placed in the zodiac constellation of Taurus, the Pleiades possesses about one hundred bright stars that are mostly visible in amateur telescopes. Total combined magnitude is given as 1.2, ranking as if it were among the brightest of naked-eye stars.

Known and recorded since antiquity, we find in Greek mythology that the name is closely associated with the legendary Atlas and Pleione and their goddess daughters. These stars form the small asterism popularly known as the Seven Sisters and are each named; Alcyone, Asterope, Celaeno, Electra, Maia, Merope and Taygete. All of these stars are the given names of the Pleiades stars, that presumably in legend, were changed by Zeus into doves, ultimately to defended their chastity from the amorous advances of Orion the Hunter. Once they were set free, all flew up to heavens and formed the Pleiades star cluster.

Central to most legends surviving today highlight stories about little Pleione, which is said to be a faded star — that mimics her presumed fall from grace. Debate about this apparent loss of brightness in fact goes back to antiquity. Even the ancient author Aratos attributed of this ‘lost’ Pleiad as a blunder. It was in reality during the 19th Century that Professor Edward Pickering first suggested that faint Pleione is really this lost star.

Webb writes on this mystery;

”Pickering found bright lines in Pleine which were confirmed by Keeler, but since 1905 the lines have been dark. In Alcyone Campbell sees red hydrogen line bright while the others are dark. Merrill finds a similar combination in Electra, while Merope, F is also bright. Spec[troscopic] Obs[ervations] suggests that the nebula is not gaseous.[??]”

The Pleiades also appears several times in the Bible, being placed in the Old Testament in both Job 9:7—9 and 38:31—33, and in Amos 5:8.

The first telescopic observation was made by Galileo, whose revelation of many extra stars invisible to the naked-eye — as clearly depicted in Galileo’s book “Starry Messenger” (1610).

Nearer the centre, the brightest star is 2.9 magnitude, Alcyone, while the faintest of the seven principal stars is 5th magnitude Pleione. With average eyes six or seven stars are visible, but those with exceptional sight are known to be able to see at least ten to twelve Pleiades stars. There has been some debate over the years about how many stars can be seen, and this is usually considered as testing the observer’s visual acuity.

One of the first general quotes I read on this was by T.W. Webb, in his “Celestial Objects for Common Telescopes” (4th-6th Edition) who said;

”The 6 principle stars of the Pleiades are evident on any clear night; but glimpses of more are easily attainable. Möstlin is said by Kepler to have distinctly made out 14, the relative positions of 11 being estimated with surprising accuracy in the absence of a telescope. Carrington and Denning have seen 14; even 16 are spoken of by Carl von Littrow, who says 11 are not infrequently perceived. A beautiful triangle of small stars will be found near the lucida [Latin meaning: bright], Alcyone.… I have noticed the remarkable absence of colour in the group, except in one minute ruby star, and or outlier. Wolf has charted 499 stars here down to 14 m[agnitude]. The photographic plate shows over 2000.”

The Pleiades also features as M45, being the last object in the first half of the original Messier star catalogue.

Figure. 2. The Pleiades : Brightest Stars This shows the brightest star visible down to 8.5 magnitude. Stars are labelled with Bayer Letter and Flamsteed Number or just the Flamsteed Number. Most of these stars appear in the Table below.

BRIGHTEST VISIBLE PLEIADES STARS

NOTES
1. Electra is variable V971 Tau.
2. Pleione is the variable BU Tau, being the supposed missing Pleiad.

Recent work has found that the outer regions are elliptical. Unlike most of the physically parameters for open clusters, but found in globulars, core radius r_c of 1.4pc., a tidal radius r_t of 16pc). Perhaps as many as 1000 stars are associated with the cluster.

The Pleiades, like the southern Jewel Box / NGC 4755 is relatively young compared to the much older Hyades. All of the stars lie on the Main Sequence except possibly Pleione which may be just evolving away. Distance presently is estimated as 135 pc. (420 ly.) whose inner-core spans some 2.5pc. Age is estimated as 110 million years old.

The Pleiades : Colour Magnitude Diagram Fig. 4. This is the colour-magnitude diagram for 270 stars which has been adopted from the data from the paper “Investigation of the Pleiades cluster. IV. The radial structure.”, Raboud, D., Mermilliod, J.-C. A&A., 329, 101 (1998).

Associated Nebulae

IC 349 (03467+2358) is a bright nebula surrounding much of the Pleiades. 4.19p magnitude 26×26 arcmin.

NGC 1435 / Merope Nebula or Temple Nebula (03465+2347) is the brightest part of the whole Pleiades nebula, and surrounds the 4.2v magnitude star, Merope. This whole nebula is listed as 4.19p magnitude, subtending roughly 30×30 arcmin.

In the autumn of 1864, the English observer Mr. W. Matthews, reported in the Astronomical Register about the visibility of the Pleiades’ nebulosity. (AReg., 4, 168 (1866) Using an 8-inch speculum mirror he says;

“…the nebula has something the shape of a fan, branching out from one of the bright stars in the Pleiades, and gradually increasing in breadth as it leaves the star. In 1864 I found one boundary of the nebula very well defined, and I could trace it to considerable distance. What, however, I was struck with was the colour of the nebula, which is a dull dingy appearance, very unlike the bright white mist of Orion. I was much interested in this nebula, for it is very unlike any other I have seen ; its extent is immense, and its colour remarkable. The nebula must, I think, be variable; for during the last autumn I have obtained no good views, although having the advantage of an additional inch and a quarter aperture. This nebula is a very interesting object when well seen.”

Another report of the nebulosity of the Pleiades by Mr. Charles Grover appeared in the Astronomical Register (AReg., 10, 173 (1872))

“Reading [of the nebulosity] being unsuccessfully searched for with an 8-inch achromatic, and a query as to whether the nebula has dispersed itself among the stars of this cluster. …since December, 1869, this object has always been seen without any difficulty in the 12¼ speculum,… Among them is a large, oval, ill-defined object, with the star Merope a little within the northern and smaller end, the faint haze extending sufficiently far north of this star to include a pair of very faint stars, making a neat double with a low power ; from Merope it extends a long way south, a little preceding, and can certainly be traced from the pair of stars just mentioned a distance of 25 minutes southwards.”

By far, the most detailed of the popular accounts of this nebulosity is written by Thomas W. Webb, who in the 5th Edition of “Celestial Objects for Common Telescopes” (1907) says;

“Nebula in Pleiades, discovered by Tempel, 1859; a faint, extended, somewhat triangular haze, involving, at its north extremity, Merope, the bright star sp Alcyone, the lucida of this group. Suspected var[iable], but evidence conflicting. Has been seen with less than 2-inch, but invisible in the 11-inch achromatic, at Copenhagen, possibly from want of contrast in diminished fields; D’Arrest says there are nebulæ invisible or barely seen in great telescopes, which can be perceived easily in their finders. Wolf is certain that it has not changed since 1864. Found readily with 5½−inch, 1863, October 6. Very feeble, 1865, September 25; a mere glow when the star out of field, 9-inch speculum, 1881, January 31. Goldschmidt saw it as a projection from a diffused nebulosity encompassing Pleiades; so Wolf. Temple thinks this an illusion, but it has been confirmed by photography. The photograph of the brothers Henry shows a remarkable nebulous wisp attached to Maia, and a curious narrow ray running through 6 stars for 2½′ f.”

Other Important Star Clusters

Many other clusters exist at much further distances, but the resultant distances or age remain fairly unreliable compared to close ones like the Hyades or Pleiades. These assumptions, especially of cluster ages, can still be used to derive the evolutionary trends from the CMD. These often appear in dedicate open star cluster catalogues, sometimes tabulated or shown graphically. Examples are like the very useful, but now slightly out of date, 5th Lynga Open Star Catalogue., (1987), as created by Gosta Lyngå of the Lynga Observatory in Sweden.

Another more modern and better suited catalogue is the on-line database WEBDA that lists all stellar clusters in the Galaxy and the Magellanic Clouds, and is produced by Jean-Claude Mermilliod at the Institute for Astronomy of the University of Vienna. This particular database is very useful, especially as it is continuously updated. It continues to be a useful tool for astronomers, containing relevant papers, information on double and variable stars, plots of the components of the cluster, and basic astrophysical data.

Last Update : 19th April 2017

Southern Astronomical Delights © (2017)

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