The Pleiades

by Canoro Immacolata, Giordano Serena, Carusone Manuela

teacher Ernesta De Masi

Liceo Scientifico "A. Genoino" Cava de' Tirreni (SA) - ITALY

	Description
	Brightest Pleiads
	Reflection Nebula
	T he age of the Pleiades
	A brief history of observation
	Mythology
	Images
	Sources

Description

The Pleiades star cluster, also known as the Seven Sisters and Messier 45, is a conspicuous object in the night sky with a prominent place in mythology. The cluster contains thousands of stars, of which only a handful are commonly visible to the unaided eye. The stars in the Pleiades are thought to have formed together around 100 million years ago, making them 1/50th the age of our sun, and they lie some 130 parsecs (425 light years) away. From our perspective they appear in the constellation of Taurus, with approximate celestial coordinates of 3 hours 47 minutes right ascension and +24 degrees declination. For northern hemisphere viewers, the cluster is above and to the right of Orion the Hunter as one faces south, and it transits -- reaches its highest point in the sky, midway between rising and setting -- around 4am in September, midnight in November, and 8pm in January.

XEphem charts showing the cluster's position. Left: a 90-degree star field centered on the Pleiades;
north is up and east is to the left. Right: same field with the cluster marked (as M45), as well as
constellations and the celestial and Galactic equators (red solid and dashed lines, respectively).

The image at the top of the page shows the central part of the cluster, where the brightest stars are found. This color photograph was taken in three filters, each exposed for about a half-hour, by David Malin with the UK Schmidt Telescope. The image is roughly 1.5 degrees wide, or three times the angular diameter of the moon. North is up and east is to the left. The cluster distance of 130 parsecs makes the physical width of the picture about 3.4 parsecs (11 light years); the cluster itself has a width perhaps 10 times greater, but most of the bright stars are found within one or two degrees of the core.

Brightest Pleiads

The figure at right, covering a somewhat larger area than the AAO photograph, shows the brightest stars in the vicinity, most of which are members of the Pleiades cluster (some appear in the same part of the sky but actually lie at a different distance). Data for these stars are listed in a table below. All stars selected have magnitudes of 6.5 or brighter, which is about the limit of human vision under ideal conditions -- how many you actually see depends on your own eyesight, local atmospheric transparency, and light pollution levels. While few people can see stars as faint as magnitude 6.5, this limit is interesting for a different reason: it includes all stars in the vicinity that are listed in the 1725 British Catalogue of John Flamsteed, the first British Astronomer Royal. In a crude fashion, Flamsteed's stars correspond to the best the eye can do. The Greek-letter designations in Johann Bayer's 1603 Uranometria are also commonly used to label visible stars, but Bayer's catalog doesn't go as faint as Flamsteed's, and consequently only the brightest Pleiad has a Bayer designation.

The following table lists the bright stars in order from west to east (right to left in the figures above), giving the name, Bayer and Flamsteed designations where applicable. More modern Henry Draper catalog numbers are also given, in addition to apparent visual magnitude, spectral type, and cluster membership status.

Name

Bayer / Flamsteed

V mag

Spectral Type

Mem?

Cel�no

16 Tauri

23288

5.46

B7 IV

variable

Yes

Electra

17 Tauri

23302

3.70

B6 IIIe

emiss. line

Yes

18 Tauri

23324

5.64

B8 V

Yes

Taygeta

19 Tauri

23338

4.30

B6 IV

variable

Yes

Maia

20 Tauri

23408

3.87

B8 III

variable

Yes

Asterope 1

21 Tauri

23432

5.80

B8 V

variable

Yes

Asterope 2

22 Tauri

23441

6.43

A0 Vn

Yes

Merope

23 Tauri

23480

4.18

B6 IVe

emiss. line

Yes

24 Tauri

23629

6.29

Yes

Alcyone

Eta / 25 Tauri

23630

2.90

B7 III

emiss. line

Yes

23712

6.49

variable

23753

5.44

B8 V

variable

Yes

26 Tauri

23822

6.47

Atlas

27 Tauri

23850

3.62

B8 III

spect. binary

Yes

Pleione

28 Tauri

23862

5.09

B8 IVevar

irreg. var

Yes

23923

6.17

B8 V

Yes

23950

6.07

B8 III

Yes

23985

5.23

A2 V

variable

24368

6.34

A2 V

variable

33 Tauri

24769

6.05

B9.5 IV

ellips. var.

24802

6.19

As a matter of perspective, the faintest stars listed above are still 40 times brighter than our own sun would appear at a similar distance, and the brightest Pleiad, Alcyone, is 1000 times more luminous! Stars like our sun, of which there are a few in the cluster, appear as faint flecks of light in the AAO photograph at the top of this page, and are well below the sensitivity of the human eye. They are easy to confuse with the numerous stars behind the cluster that also appear in the picture and look very similar. Careful observation and analysis is required to determine which of these fainter stars are cluster members.

Reflection Nebula

Several Pleiads appear surrounded by intricate blue filaments of light. This nebulosity is the result of starlight scattering (reflecting) off minute grains of interstellar dust in the vicinity. The dust particles are inside a cloud of mostly hydrogen gas that the cluster seems to be plowing into.

Sensitive instruments show the nebula extending several degrees from the cluster center in optical, infrared, and radio emission. Much of it can also be seen in ulraviolet light. I studied the interstellar matter around the Pleiades for my PhD thesis .

The age of the Pleiades

The relatively tight grouping is a sign of the youthfulness of the Pleiades cluster, although it is drifting apart and will gradually disperse. Its youth is also indicated by the absence of red giant stars in the group. None of the stars has yet had time to reach that stage of maturity, although the brightest member stars are hot, B-type blue-white giants.

According to a recent calculation [G. Meynet, J.-C. Mermilliod, and A. Maeder in Astron. Astrophys. Suppl. Ser. 98, 477-504, 1993], the age of the Pleiades cluster is about 100 million years, whereas the "canonical" age is considered to be 60 to 80 million years (for instance, the Sky Catalog 2000 gives 78 million). The Pleiades may have an anticipated lifetime as a cluster of only some 250 million years � after then they will have been dispersed as single (or multiple) isolated stars along their orbital path.

A brief history of observation

The Pleiades are among those objects which are known since the earliest times. At least 6 member stars are visible to the naked eye, while under moderate conditions this number increases to 9, and under clear dark skies jumps up to more than a dozen (Vehrenberg, in his 'Atlas of Deep Sky Splendors', mentions that in 1579, well before the invention of the telescope, the astronomer Moestlin had correctly drawn 11 Pleiades stars, while Kepler quotes observations of up to 14).

The cluster was first examined telescopically by Galileo, who recorded more than 40 member stars. It was an early subject for astronomical photography, being first photographed by Paul and Prosper Henry in 1885.

The cluster was the final entry, as M45, in an early modern astronomical catalogue by Charles Messier, Tables des Nebuleuses, ainsi que des amas d'Etoiles, que l'on decouvre parmi les Etoiles fixes sur l'horizon de Paris; observes a l'Observatoire de la Marine; Memoires de l'Academie des Sciences for 1771, Paris (Table of nebulae and star clusters, which have been discovered between the fixed stars over the horizon of Paris; observed at the Marine Observatory).

In 1846 Johann von Maedler of the Estonian Dorpat Observatory, an eminent lunar cartographer, had been measuring the motions of the various Pleiades stars. Finding that they showed no relative motion within the cluster, he that they were at the centre of the Galaxy, and Alcyone was the star at the centre of the known universe. For a brief period, until this erroneous argument was exposed, the Pleiades were the subject of intense public debate and much baseless speculation.

Recent observations

Recent advances in telescopy are revealing some of the fine structure of our galactic neighbours � such as the Pleiades cluster. Eerie though it appears, the picture below is not the product of a fertile imagination. This is a photograph of an interstellar cloud in the process of disintegration by intense radiation from an adjacent hot star. The cloud � IC 349 or Barnard's Merope Nebula � is illuminated by Merope in the Pleiades star cluster.

The left hand photograph was produced by the Space Telescope Science Institute and is yet another informative and beautiful product of the Hubble Space Telescope. The adjoining figure, prepared by David Malin of the Anglo-Australian Observatory, shows the location of Merope and its nebula within the cluster.

The cloud, which is part of Tempel's Nebula or NGC 1432, is drifting through the Pleiades star cluster. Since the cluster is itself dispersing and moving through space, the combined velocity of Merope and the nebula is some 11 kilometres per second. As a result of its close proximity of the star � in astronomical terms � of about 0.06 light-years (say 550 billion kilometres) the cloud has been extensively deformed. The phenomenon of 'radiation pressure' due to the intense stellar radiation emitted by the nearby star acts on the dust particles which make up the cloud. It selectively decelerates the particles: less massive dust particles are subject to greater deceleration than larger particles. This radiation pressure thus acts as a sieve, sifting the particles by size. The clearly formed linear structures directed toward the star are streams of larger particles, whereas the smaller and thus more decelerated particles are, for the moment, retained within the main body of the cloud. If the nebula is not entirely dispersed or absorbed by the star during its close passage, it will pass Merope and continue on into interstellar space.

Note: In the upper right of the picture, the apparent rays of light focused on the star are not present in reality, they are an optical phenomenon produced in the apparatus.

Mythology

The Pleiades are the daughters of Atlas seven in number: Electra, Maia, Taygete, Alcyone, Merope, Celaeno, and Sterope. They were always persued by Orion but, they always fled him successfully. Zeus took pity on them and placed them in heaven as stars, to keep them out of Orion's reach. Maia, was the mother of Hermes. Electra, was the mother of Dardanus, the founder of Troy.

Images

Short exposure of the Pleiades, just showing the stars.

The Pleiades in color. A very nice photo showing the reflection nebulae. Captured from Usenet. Also in Patrick Murphy's collection .

Image of the Pleiades, M45, taken with the 0.9-m Telescope of Kitt Peak National Observatory, in 1975. At the Pleiades' distance of about 400 lightyears, the image field of view corresponds to a linear diameter of 20 lightyears.

More information on this image (N.A. Sharp/NOAO)

Image of the Pleiades, M45, taken with the 4-meter Mayall Telescope of Kitt Peak National Observatory.

More information on this image (N.A. Sharp/NOAO)

More NOAO images

Wider field image of the Pleiades star cluster, M45. It was created by David Malin from photographic plates obtained by Malcolm Hartley with the 1.2-meter UK Schmidt Telescope.

SOURCES

http://www.pleiade.org/pleiades_03.html

http://www.ras.ucalgary.ca/~gibson/pleiades/

http://www.seds.org/messier/more/m045_more.html

Catch a Star 2002

The Pleiades