The Pleiades


The Pleiades in the constellation of Taurus, close to the Hyades are one of the most remarkable astronomical phenomena in the firmament. They are also called “the Seven Sisters” ( in German: Siebengestirn). Beside the Greek name  ‘Pleiades’, astronomers use their catalogue number M45 and NGC 1432. In Japan they are known as ‘Subaru’, like the Japanese car.

The Pleiades are approximately 405 light-years from the solar system. A light-year corresponds to the distance covered by the light in one year at a speed of about 300.000km/sec., i.e. about ten billion kilometres in one year. Sophisticated means of observation have shown that the Pleiades comprise about more or less 500 (mostly weak) stars. They spread over an extension of two degrees, which equals four times the diameter of the moon. The extension of the Pleiades in the space is approximately 5 light-years. However, the most important stars are these that can be recognised by the unaided eye.

Please, consult the following chart:

Denomination

Name

Brightness

Spectral type

Remarks

Μ Tau

Alcyone

2,8 mag

B7 III

The brightest star of the Pleiades, central position, also has 3 little companions.

16 Tau

Celaeno

5,5

B7 IV

 

17 Tau

Elektra

3.7

B6 III

 

18 Tau

-

5,7

B8 V

 

19 Tau

Taygeta

4,3

B6 IV

 

20 Tau

Maia

3,9

B7 III

 

21 Tau

Asterope

5,8

B8e V

Double star: Sterope I and Sterope II

22 Tau

-

6,4

B9 V

 

23 Tau

Merope

4,2

B6 IV

Considered as the 8 th star of the Pleiades

27 Tau

Atlas

3,6

B8 III

Considered as the 9 th star

28 Tau

Pleione

4,8-5,5

B8e V

Spheroid

 


                                                                                                                                                             

The Pleiades are one of the best-known clusters. The concept of ‘cluster’ needs defining. By cluster we understand a local gathering of stars, which are born at the same time and in the same place. That is why they are made of identical initial material. A distinction is made between ‘open cluster and, globular cluster’.

Open Cluster:

The different members can be distinguee as individual stars, which are easy to separate from each other. The Cluster has an irregular shape. This gathering is made of young stars which do not have a high density. They are situated in the arms of the Milky Way.

Globular Star Cluster:

Contrary to the Open Cluster, the Globular Cluster are very compact gathering of stars, which cannot be recognised as individual stars.

The Pleiades are still a very young constellation, only 100 million years old. The nebulae around the stars probably represents the remains of their origin. 

Stars emit light due to their temperature.. The Pleiades belong to the group of  blue stars. The colour gives indications about their nature. These blue stars are much hotter than our sun. Their temperature is about round 20 000°C, the temperature of our sun is only 6000°C at the surface..

The fact that the Pleiades are still located in a rather small area, shows that they are rather young. Indeed after their formation stars are normally drifting away. This can be explained by the fact that t most  Open Clusters only spend a little time as a flock. The galactical gravitation field and the approach of other stars may also  produce changes in the speed of the members of a cluster, which may lead to the loss of some of these members. The Pleiades are moving presently at 40km/sec. through space. So the Pleiades will dissolve after a few 100 million years, but  the stars will still continue turning around the Galaxy, but only as individual stars.

The Pleiades have an unusual high rotation speed. It varies from 150km/sec. to 300km/sec. This speed is sufficient to produce ‘Reflection Nebulae’. A certain dust is found between stars, which mainly consists of hydrogen. This dust masses because of a repulsion of gas envelope by the star’s rotation. The Reflection Nebula reflects the bright light of the young stars. Around the Pleiades this light shines blue. Reflection nebulae often have a connection with Emission Nebulae in which hydrogen is excited and emits light are often found together with Reflection Nebulae. However, the Pleiades belong to the ‘pure Reflection Nebulae’, therefore they posses less details. 

The Pleiades are not only formed by blue stars, they also contain some white dwarfs. This fact rises an important: How can a white dwarf exist within a young cluster? White clusters are sun-similar stars, which unload their gas envelop at the end of their live. Their nuclear fire is extinct. As there are more white dwarfs, they must belong to the cluster. So they aren’t flare stars. One explanation is that this stars once had a big mass, yet they developed very fast and they lost a part of their mass by the influence of the neighbour-stars or by a too rapid rotation. Anyhow, those are only vague theories.

In 1995, new observations have shown that other exotic objects exist beside the white dwarfs, such as ‘Brown Dwarfs’. Those aren’t stars, but hot planets.

Die Plejaden Beside this exotic objects, there also are some ‘Flare-stars’. Those are weak ‘Red Dwarfs’, which can light up 1 to 6 magnitudes and shrink again after a little time. The ‘Magnitude’ is an unit of measurement for the brightness. A difference of 5 mag. corresponds to an 100 times difference of luminosity. These stars can increase their luminosity within only a few minutes!

Nor some explanations about Pleione (from Chesley Bonesetll). For the reason that this star rotates very fast, it has the shape of a spheroid, so it isn’t a real sphere. Pleione has the typical white colour for its kind, it has a 45 times brighter luminosity than our sun and it also rotates 100 times faster. If our sun was replaced by Pleione, which posses a great radiation, our planet would be red ardent.

The Pleiades in comparison with Praesepe

Praesepe, also known as M44 and NGC 2632 in the constellation of Cancer, possesses some common characteristics with the Pleiades. Both may be recognised easily with the unaided eye and therefore they have been known since the antiquity. Different names are used for Praesepe such as Manager, or Beehive Cluster. Praesepe owns some white dwarfs, but also some double stars and variable ones.

However, there are a lot of differences between this two open clusters. The age is already different. The age of Praesepe is estimated 400 million years in contrast to the Pleiades, which perhaps will no longer exist as a cluster at this age. The beehive cluster has a distance of 577 light-years acc. from new observations, id est. about 177 light-years farer than the Pleiades. Praesepe owns 5 red giants at least, the Pleiades don’t posses. We can argue that Praesepe includes a lot of different stars in temperature, colour and magnitude.

An other point is important. Praesepe probably has the same origin as the Hyades (the Pleiades’ neighbours), because both are matched by the same age and course. Their origin is probably located in a great diffuse nebula, which existed 400 million years ago. Even though they lie hundreds of light-years apart.

Visual Observations

The constellation of Taurus lies in the North-West of Orion and it is easily to localise by means of the Hyades and the Pleiades. Taurus is one of the best-known winter-constellation.

Here some information about the exact position

Declination

Geographical latitude

+23° 58’

Rectascencion

Measured in time-units 24 hours = 360°

3 h 43.9 m

Magnitude

 

1.6 mag.

The name of ‘Seven Sisters’ may be misleading. Six stars of the Pleiades are recognised by the unaided eye even in bad weather conditions, however, when the visibility is good, nine stars and more are recognised by the unaided eye. The number ‘Seven’ is attributed by the Greek Mythology.

In the winter sky, the flocked place is immediately eye-catching, but the binoculars show the whole beauty of the Pleiades.

30 stars may be identified with simple binoculars and a amateur telescope shows already the reflection nebula.

For a telescope observation large field optics are recommended. The cluster looses otherwise a lot of its impression, as its extends above a large area. Of course the reflection nebula is interesting, but a nebula filter doesn’t help to give a better picture.

The Pleiades are an easy object for taking photographs. A quite short exposure is sufficient to take good picture.

A good optic an a high speed film even allow to get pictures of the nebulae.

Since the Pleiades lie in the ecliptic we often can see eclipses of this stars by the moon. This event shows the apparent size of the moon and of the Pleiades. Another interesting event happens when  planets such as Venus, Mars and Mercury approach the Pleiades.

Practical Exercise

The Pleiades offer a simple possibility to decide if you are in a good site for sky observations. If the unaided eye sees an unclear-flocked spot, you may say that the atmosphere is disturbed, the air is polluted by dust or the sky is polluted by artificial light sources.

6 stars at least are visible in a clear and unpolluted atmosphere. If you are able to see more than 6 stars, you have found an ideal site for observations without pollution, or you have got the right night without atmospheric disturbances. On a mountain, far away from city lights, you may see up to 11 stars of the Pleiades.


The Pleiades in the Antiquity

The Pleiades are known since the early antiquity. The constellation of Taurus has already been venerated 5000 years ago by the Babylonians and the. The rising and setting of the Pleiades indicated  the beginning and the end of the navigation season.

In the Roman World, the Pleiades were seen a bunch of grapes for the wine god ‘Bacchus’.

In the Middle Age, they were interpreted as hens and the farmers saw them as umbrella, because the Pleiades were a bad omen for rain.

Also the Hindus have known this astronomical object. The Pleiades represented the sacred flame of the fire god ‘Agni’.

But it were the Greeks who had much more regard for the Pleiades than all the other ancient civilisations, as well in their culture as in their mythology. Homer mentioned them in his ‘Odyssey’ and also Hesiod 1000 b. Chr. The Pleiades helped Odysseus to navigate and to find the way to Scheria, and Hephaistos represented them on the shield of Achilleus.

This constellation is also mentioned in the Bible.

Charles Messier, who was born the 26 th of June, 1730, had the job to record the observations of the observatory, where he worked. He attributed to the Pleiades the catalogue number M45 (M = Messier). Why he recorded the Pleiades and not their neighbour ‘The Hyades’, will remain his secret.


 

  Mythology

The Pleiades are a group of seven nymphs. They are the daughters of Atlas and Pleione.

                                          Atlas                    +                 Pleione

 

    

  Alcyone        Merope        Celaeno         Elektra          Sterope          Taygete        Maia          

They are also called ‘Atlandids’  because of their father ,Atlas’. The seven daughters weren’t only very pretty, but they also were very wise. The young Dyonisos and even Zeus was educated by this creatures. The Pleiades also were the virgin companions of the goddess Artemis.

Their half-sisters were the Hyades, they are descendants of Atlas and Aethra.

As the colossus ‘Orion’ wanted the seven nymphs, their cry for help was heard by the gods who put them as doves on the firmament. However, the pursuit continued, when Orion was also put to the sky. This could be a reason why the Pleiades continuously were continually crying.

An other version says, that the Pleiades and the Hyades have cried because of their brother’s Hyas death, who died during a hunt.    

Sources :

Books : Abenteuer Astronomie (David H. Levy), Astronomie für Einsteiger (Storm Dunlop), Atlas für Himmelsbeobachter (Erich Karkoschka).

CD-Rom: Au-delà de la  planète terre, Science Interactive

Internet : www.google.de (entry : Pleiades, Reflection Nebula, Emission Nebula, Greek Mythology, Messier, M44).

www.astronomie.de

www.astronomie.ch

www.wissen.de

www.sungaya.de