All About The Pleiades

1. The mythological Pleiades

" Pleiades " is one of the many names of this star cluster. It is supposed to be derived from Greek " pleio ", " to sail ". The rising of the Pleiades was the sign of the opening of the Mediterranean world to sailors. An alternate suggestion is that the name is just a form of the Greek " peleia ", " full " or " many ", which is also the meaning of the Arabic " Al Thurayya ", and probably the Biblical " Kimah ". Seneca calls them " Pleiadum greges " - " The group of the Pleiades ". In various Greek and Roman writings they are referred to as " The starry seven "," The net of stars ", " The seven virgins ", " The seven Atlantic sisters ", " The daughters of Pleione " or " The children of Atlas "

In Greek mythology the oceanid Pleione was a wife of the titan Atlas. She gave birth to seven children: Alcyone (whose name has been given to the brightest star in the cluster), Cel?no, Electra, Taygeta, Maya, Asterope (which can be seen as a double star through a telescope) and Merope.
The starry Pleiades have always been more famous than their mythological counterparts. For this reason the people of Antiquity sometimes called a group of wandering actors and teachers a "pleiad".
The life conditions and different traditions of the peoples had found a reflection in popular beliefs concerning the Pleiades. Thus the names of the star clusters often arise as a result of the life, living standards, agricultural activities and the striving to understand the nature of the things they saw in the night sky, of the people who observed them.
In an early translation of the Bible, dating from 1535, a marginal note identifies " The seven stars " as " The clucking Hen with her chicks ". Some researches consider that the name of Pleiades is wide-spread not only in Europe but in Asia and Africa. It is interesting to find them associated with more mundane matters among rural peoples every where, usually as a " Hen and her chicks ", as in the " Massa Gallinae " of medieval times. The French " La Poussinniere " , for example, means " The Hen ". Farmers in Germany saw them as " Aften Hoene " or the " Evening Hen ". Another popular name in Germany some 200 years ago according to Christian Ideler, was " Schiffahrts Gestirn ", the " Sailor"s stars ". The Anglo-Saxon name " Sifunsterri ", simply means " The Seven stars ".


The seven daughters of the titan Atlas
and the oceanid Pleione
(Ladenscript 9th Century).

The Welsh had a similar traditions with their " Y twr tewdws ", " The closed bag ". J.R.R. Tolkien in his epic fantasy " The Hobbit ", tells us that the Pleiades were known in the ancient days of Middle-earth as " Remmirath " or the " Netted stars ". R.H. Allen tells us that various European peoples, such as the Finns and the Lithuanians, saw the Pleiades as a " sieve " or a " net " with the heavens light shining through them. The curious Swedish title " Suttjenes Rauko " has been translated as " Fur in frost ". The name " Kullukturset ", in use in Greenland at the time of the arrival of the first Norse missionaries, seems to mean " The dog pack " or possibly " The dogs attacking the bear ". The Russian peasants saw in them " Nasedha ", translated as " The sitting Hen " or " The nest of the duck ". In their folklore this star cluster is also compared with a hair tuft, " The Hair " or " Baba ", which means " The old wife ", possibly referring to the legendary old witch Baba Yaga. The Pleiades are also mentioned by the Russians with the specific common title of " Stogers ". In Antiquity the Eastern Slavs called wooden stakes " stogers ". The Slavs fixed these stakes into their hay-stacks. The peasant farmer rationalised the cluster as a group of "stogers", sticking in the night sky.

In the different traditions the names of the Pleiades reflect the local fauna, agricultural activities, living standards and interests of the people. They can be swallows, pigeons, sparrows, parrots, kids, bees, ants or as in an one Arabian text-a " Heard of camels ".
This extraordinary star cluster attracted the attention of the Bulgarian people, too. They referred to them with the classical title " The Hen and her chicks " or just " The Hen ". " The Hen " is the brightest star in the cluster (Alcyone) and the weaker ones are the "chickens". They continuously twinkle as if they are pecking millet grains. Alcyone has been the peasant's orientation for the weather during the night and the beginning of the sowing-time. When the "Hen" is low in the sky early, the wheat can be sown without fear.
The connection of the Pleiades with agricultural activities is immortalised in such titles as " Virgins of Spring ", " The stars of Abundance ", " The stars of the season of Blossoms ", etc. All these names refer to the fact that the Pleiades reaches conjunction with the Sun in mid-May (currently about May 20), the time of blossoming. In Buddhist scriptures, the birth of the holy child was prophesied to occur " when the Flower star shines in the East ", very possibly a reference to the Pleiades. In actual fact his birth (in a garden) in mid-May, about 563BC, would coincide closely with the yearly Sun-Pleiades conjunction.
In some American Indian legends the Pleiades are connected with the " Mateo Tepe " or " Devil' tower ", that curious and wonderfully impressive rock formation which rises like a colossal petrified tree-stump to a height of 1300 feet above the plains of north-eastern Wyoming. According to the lore of the Kiowa, the Tower was raised up by the Great Spirit to protect seven Indian maidens who were pursued by giant bears' claws may be seen (in the vertical striations) on the sides of the Tower unto this day.

In ancient Aztec and Mayan traditions , the midnight culmination of the Pleiades was an event of great and ominous significance, particularly at intervals of 52 years when the calendars of the two peoples (the Sacred and the Secular calendars) came into conjunction. The world, it was believed, would come to an end on one of these dates; it had already been destroyed and re-created on four such occasions the past.
The star cluster of Pleiades (and some members of it) have been honoured with a variety of titles. The Arabians called the brightest star Alcyone " Al Wasat " - the " Central one ", and " Al Nair " - the " Bright one ". Other names have been translated as - " The light of the Pleiades " and " The leading one ". The Hindu name was " Amba ", the mother. But Y. Hewitt relates that an earlier name was " Arundhati ", identified as the wife of the " Chief of the seven sages ". " Arundhati " was worshipped particularly by newly-married couples.

2. Historical information

According to R. H. Allen, one of the earliest mentions of the Pleiades in Chinese annals appears to refer to an observation made in 2357 BC. Kenneth Glyn Jones however considers that the earliest known reference of this cluster is a mention by Hesiod, an ancient Greek poet who lived 8-7 century B.C. He extols the rural labour and tells us about the connection of this star cluster with the agricultural seasons in his time:
"When Atlas - born, the Pleiad stars arise Before the Sun above the dawning skies, `Tis time to reap; and when they sink below The morn-illuminated west,`tis time to sow..."
The Starry Seven was not missed and by another ancient Greek poet either, namely Omir, who mentions them when he makes description of the buckler of Ahil: "He portrayed in the middle the earth, the sky and the sea, the tireless Sun and of course the unforgettable full moon all constellations, shining high in the dark sky the Pleiades, Hyades and Orion, the powerful archer, also the Bear, which often is called the cart, circling always in the north, watching forever the archer..."

In his lines in the "Mu Allakat" , composed in the 7th century, the Arabian poet Amr al Kais evokes for us "The hour when the Pleiades appeared in the firmament like the folds of a silken sash variously decked with gems..." In the 13th century in Gulistan or "Rose- garden" of the Persian poet Sadi we read: "The ground was as if strewn with coloured enamel, and necklace of Pleiades seemed to hang upon the branches of the trees..." Later, in the works of Middle-Eastern poets, the Pleiades often occur in glittering phrases which compare them to celestial jewellery. Hafiz of Persia , in the 14th century, wrote to a poet-friend: "To thy poems Heaven affixes the Pearl Rosette of the Pleiades as a seal of immortality..."
In the first half of the 19th century the famous American writer-romanticist Edgar Allan Poe, in his poem "Israfel" inspirited by a verse in the Koran , pictures the Pleiades as seven maidens of a starry choir, who have paused in heaven to hear the voice of the angel Israfel, "whose heart-strings are a lute", and who sings with the sweetest voice of all God`s creatures.

In the 20th century the English writer J.R. R. Tolkien, in his book "The lord of the rings" describes the situation of this unforgettable star cluster in the sky: "High in the eastern sky Remirat or the netted stars, was trembling. Above the mist slowly rose the red Borgil (Aldebaran), shining like a fiery ruby. After that the wind changed its way, moved like the mists a veil and behind the borders of the world rose the sky swordsman Menelvagor (Orion), with its radiant belt."

The beauty of the Pleiades did not remain unnoticed by the ancient explorers either. Greek astronomers Eudox of Knidos (c. 403-350 B. C.) and Aratos of Phainomena (c.270 B.C.) listed them as a single constellation - "The Cluster". This is also referred to by Admiral Smith in his Bedford Catalogue.
At the beginning of the era of telescopes the Italian astronomer Richoli (1598-1671) named 2 stars of the Pleiades with the names of his parents. Now these two are part of the constellation Taurus and are called Atlas and Pleiona (the stars 27"f and 28"h" in Taurus).

In the Uramometria of the German astronomer Johan Bayer dating from 1603, there is included only one star of the Pleiades - Alcyona. John Flamsteed, the first English Royal astronomer, includes 12 stars of this cluster and two from its field in the British catalogue for 1725. In 1767, Reverend John Michell used the Pleiades to calculate the probability of finding such group of stars in any part of the sky by chance alignment, and found the chance to be about 1\ 496000. Therefore, and because there are other similar clusters he concluded correctly that clusters must be a physical group.
Four years later in 1771 Charles Messier includes the Pleiades as no. 45 in his first list of nebulae and star clusters. Now this number is almost as widely used as the name Pleiades.
About 1846, German astronomer Madler (1794-1874) working at Dorpat, noticed that the measurable proper motion of the stars of the Pleiades were relative to each other. From this he concluded that they formed a motionless centre of larger stellar system with the star Alcyona in the centre.

In the 19th century, with the manufacture of better telescopes, people started discovering nebulae within the Pleiades. The brightest of these nebulae, that which is around Meropa, was discovered on October the 19 th 1859 by Ernst Wilchem Leberecht Tempel at Venice (Italy) with a 4-inch refractor. It is included in the New General Catalogue as NGC 1435 or Tempel`s nebulae.
The Pleiades cluster remains a group which commands attention and admiration even in the days of modern astronomy. Otto Struve has stated that the cluster has been target of more astronomical cameras than any other object in the heavens beyond the Solar system.

3. Visibility of the Pleiades

Star cluster Pleiades has a special charm for all astronomers - amateur and professional. In astronomy it`s the custom part for constellations, or a group of stars in their borders, which have their own names to be called asterisms. Some examples of asterisms are "The belt of Orion", "The head of Dragon", "The chain in Andromeda". The Pleiades are an asterism, too, but the difference is that they are a star group whose name has no connection with that of the constellation proper. Pleiades are a clearly seen group of weak stars in the constellation Taurus, 12 degrees north- west of the Hyades cluster with equatorial co-ordinates: Right Ascension=3h 47 min and Declination=24:07 (deg:min). To the average eye the Pleiades cluster appears as a tight knot of six or seven stars with visible diameter about 1 degree (the diameter of the Moon is 0.5 degrees). People with very good vision can generally differentiate up to 10 stars without any optics. But if the weather conditions are really perfect and your vision is excellent you can possibly differentiate 12 stars. The German astronomer Maestlin, tutor of Kepler, perceived 14 and mapped 11 Pleiades previously to the invention of the telescope. Galileo with his imperfect telescope counted 36 stars. Now we know that there are at least 20 stars in the group which might be glimpsed under the finest conditions. The crowded massing of the stars however, normally makes this impossible.
To an observer from the Northern Hemisphere this star cluster is situated to the right of and above Orion which we can see in south. After the junction with the sun in May the Pleiades reach their highest position in the night sky at a progressively earlier times; about 4 a.m. in September, midnight in November and 8 p.m. in January.
The cluster is most visible in the winter then we can enjoy them after sunset, when we look high in the south-east. In the spring we can find them on the night sky in west. At the beginning of the summer the Pleiades can not be seen, because they are too close to the Sun. In August they rise after midnight. In the autumn this cluster is easily seen late in the night.

"The seven sisters" are superb when you observe them in 7x50 binoculars. And if we use 50-mm (2 inch) telescope refractor with extension 15x we'll enjoy one of the most beautiful spectacles in the sky. With larger instruments this cluster is not as impressive because we will see just one part of it. With 100-mm (4 inch) telescope reflector with extension 60x the sight may be rather disappointing.
Through a telescope this cluster consist of several hundred stars which are spread over an area of no more of one degree around the brightest star Alcyone (2.9 magnitude). As almost all of the stars are at the same distance of us (400 l.y.) the visibly brightest one is really the brightest. The integral illumination of the Pleiades is equivalent to that of a star with a magnitude of 1.38, but a short-sighted observer will probably see it as a star with a magnitude of only 1.8.

Because of its lower perfunctory brightness the nebulae in which the cluster is plunged can be difficult to see with the naked eye. Photographing it is pretty complicated too. The correlation of the Pleiades to the nearby cluster Hyades and the star Aldebaran (Alpha Tauri) is shown in the upper photograph.

4. How do we investigate the stars?

Nowadays astronomical literature gives us full information on the distance, chemistry composition and the physical conditions of the stars. Meanwhile the most remote of modern space ships – “Pioneer 10” and “Pioneer 11” have barely left the solar system. How do the astronomers learn about these important characteristics of the stars? The brightness of a star is a measure of the amount of the stars illumination that it reaches the Earth. The measure for the visible brightness is visible star magnitude (m). Of course the fact that some stars are brighter than others doesn’t give us any information about the real light characteristics of the stars. The full brightness of a star depends on the total light energy radiated by the star, the distance we are from it, and the level of diffusion and absorption of the light in the intervening space. Considering these three factors allows us to compare the stars.
When we observe the sky we think that the distance to the stars is the same for all of them. The ancient people even imagined that a sky glass hemisphere exists with holes in it through which the light of the stars shone. Of course some scientists thought that the stars are very distant suns. Copernicus himself reasoned that if the Earth is spinning round the Sun, we can see how the nearer stars are moving over the background of the far off stars, drawing almost invisible ellipses – a reflection of the Earth’s orbit. The more distant star, the smaller ellipse it will draw. If we measure the angle under which this ellipse is seen from the Earth and we know the radius of the Earth’s orbit. We can easily calculate the distance to the star. This angle is called the parallax angle and the phenomenon is called trigonometric parallax . Trigonometric parallax is now the accepted method of directly measuring the distance to the stars. How we use this method is shown in the figure to the left. One parsec (pc) is the distance from which the radius of the Earth’s orbit is seen under 1 second of a degree. The word is made by relating the words "parallax" and "second". The parallax of the nearest star, for example, is 3/4 seconds of arc. When the distance (r) is in parsecs: r = 1/(parallax)". Then the star with a distance of 10 pc has a parallax of 0.1 of a second of arc. The distances to the stars are different, however astronomers compare the stars as they would be seen if they were all the same distance from the Sun, for convenience they use the distance of 10pc (1 pc = 3,26 l.y.).
So if we know the real distance and the visible star magnitude (m), we shall get the absolute star magnitude (M), which characterizes the real brightness. Òhe subordination: M=m+5-5.lg(r) allows us to get 'M" and "r". To define a parallax, even for the nearest stars is very hard work which requires a lot of observation time. Apart from this, the method is limited – it only allows us to calculate the distances of the nearest 100,000 stars. Modern sky catalogs include about 10,000 stars with well-measured parallaxes. Because of
those problems , other methods are being sought for by astronomers, even if they are less accurate.
Everything, which we know about the stars’ nature, is learnt from their light. As we know from the physics, light is a photon and a wave at the same time. Every photon corresponds to a wave with a defined length and it has got a defined energy quantity. Our eyes receive the photon’s energy as a colour. Photons with high energy are seen as violet and photons with low energy are seen as red colour. There are photons with higher energy, they belong to the ultraviolet, x-ray and gama-radiating. The photons that have lower energy than these to the red light belong to the infrared light and to the radio waves.

        A stars’ spectrum is its passport. From it we get a variety of information about the physical conditions and chemical composition of the star.

    From the spectrum of a star we can get:
- The chemical composition of the star – encoding the spectrum shows us the elements in the atmosphere of the star.
- Their temperature - dependent on the temperature, the atoms of one element may show different uninterrupted spectrums. Comparing spectrums where the temperature is known shows the temperature of the star at its surface.
- Their spatial velocity. Undoubtedly the stars are moving. But we with our telescopes see only the movement over the Sky. We cannot understand if they are coming toward us or if they are going away. But this can be known by the spectrograph. Astronomers see that the absorption lines become closer to the blue or red end of the spectrum depending which way the star is moving. Dopler discovered that the blue end equates with a star approaching us and the red end with a star departing with a defined velocity called "radial velocity". For the Pleiades it is +5 km/s.

   

The spectrums of most stars are linear with dark absorption lines. The lines in the spectra are different in their position and intensity – one line in the spectrum of one star is very well expressed but in another star it is almost invisible. The spectrums of the stars are varied but careful comparison shows that they can be divided into some spectral classes. We now use the spectral classification of the stars made in the end of 19 century and the beginning of 20 century at the Harvard observatory under the guidance of Edward Pickering. Depending on the presence of different lines, 7 spectral classes have been created, designated by capital letters from the Latin alphabet. From 1890 the Harvard observatory started to make a catalog of the stars’ spectra called Henry Draper’s catalog, or HD-catalog. The seven main spectral classes are: O – B – A – F – G – K – M
The theory of the spectra, developed in the beginning of 20 century shows that the arrangement of stars by their spectral classes from O to M is arrangement by the
temperature of their surfaces.
In the beginning of 20 century the Danish astronomer E. Hertzsprung noticed that stars with similar temperatures can have different luminosity. This means stars with similar temperature of the surface can produce different quantity of energy. To clarify this fact Hertzsprung investigated in detail the Pleiades and the Hyades. The investigations confirmed the observations. The stars with low luminosity have been called dwarfs and these with high luminosity giants.
In 1913 the American astronomer H. Russell made a list of all the stars for which the distance and the spectrum were known. With this information he determined the absolute star magnitude and built a graph. On the abscissa he put the spectral class and on the ordinate – the absolute star magnitude. The diagram shows that between the two characteristics exists a defined association. This diagram is like Hertzsprung’s. The stars àre arranged generally on the diagonal from the upper left to the lower angle. Stars that fall into this strip are called “Main sequence” stars. There is one other parallel strip called “The Giants’ sequence”. In this way the diagram Hertzsprung-Russell (H-R) was been created
(figure) .
To make the H–R diagram of an open star clusters it is necessary to know which stars are real members of the cluster and which are not. One of the criteria of belonging to the cluster is this diagram - if the star is considerably apart from the line made by the other stars, it probably isn’t member of that cluster. Another more promising method is by the independent movement of the stars.
So we say that the spectrum of a star is its passport in which is written the more important data concerning it. However unlike our passports where everything is clear for everybody, the passports of the stars are comprehensible only to those who know physics and its rules.

5. Information about the Pleiades
( their past, present and future )

The Pleiades are loose open star cluster. These are groups of hundreds of stars that form gravitationally connected systems. Open star clusters do not have a definite form; their size varies from 1.5 to 12 pc, and their average spatial density is 1 to 100 stars/pc 2. Around the Sun the density is about 0.12 stars/pc 2. Almost all loose star clusters are situated in the Milky way or close to it.
The Pleiades, although they are one of the nearest star clusters of this type to our Solar system are still quite distant from us at 250 pc. They have a visible arc diameter of 110 minutes. Only the star cluster Hyades is closer to us. And the light from its stars is travelling to us for only 380 l.y. and not for 408 l.y as was thought until recently. This information is from the astronomy satellite HIPPARCOS of European space agency.

In the Pleiades cluster the number of stars with a magnitude of less than 17 is 250. Modern observing methods have revealed that about 500 mostly faint stars belong to this cluster and they are spread over a 2 degrees field (four times the diameter of the full Moon). The brightest stars of the cluster are spread in space in a sphere with diameter of about 7 l.y. The actual diameter of the cluster is 21 l.y. The spatial model shows that the stars are grouped in an irregular shape and we see only the side with smaller area.

The Pleiades cluster was born in the Galactic plane about 70 million years ago. And it consists several hundred mostly young hot blue stars from spectral class B5-B8, with peripheral temperature over 15,000K. All of them are from the main sequence. Even the faintest visible stars of the Pleiades would be 40 times brighter than our Sun if they were as close to us as it is. Then Alcyone would be 1000 times brighter. If our Sun was as far away as they are it would have a magnitude of 10. Facts about the brightest stars of the cluster are shown in the table to the right.

THE BRIGHT MEMBERS OF THE PLEIADES
STAR NAME MAGNITUDE SPECTRUM
16 Tauri Cel?no 3.87 B7 IV
17 Tauri Electra 3.70 B6 IIIe
18 Tauri - 5.64 B8 V
19 Tauri Taygeta 4.30 B6 IV
20 Tauri Maia 3.87 B8 III
21 Tauri Asterope 1 5.80 B8 V
22 Tauri Asterope 2 6.43 A0 Vn
23 Tauri Merope 4.18 B6 Ive
24 Tauri - 6.29 A0
25 Tauri Alcyone 2.90 B7 IIIe
27 Tauri Atlas 3.62 B8 III sp.binary
28 Tauri Pleione 5.09 B8 IVe variable
The Pleiades are going away from us with radial velocity 5 km/s. The particular velocity of the Pleiades stars is 5 seconds of arc per century in direction towards the star Lambda Tau. It will take them at least 30,000 years to move a distance equivalent to the visible diameter of the full Moon (0.5 degrees) on the celestial sphere.
The brightest stars of the Pleiades seem to be surrounded by a blue veil which is actually a reflection nebulae. This cosmic cloud is elusive when you observe the cluster visually.
But long exposure shows interesting details. The cold hydrogen gas in which the stars are soaked is mixed with great amount of star light which gives the blue colour of the nebulae. The brightest part of the nebula includes the star Merope and territory of about 20 minutes of arc south of it. The reason for this is the strong magnetic field of the star. Today this nebula is known as NGC 1435. No less impressive is the nebula around the faint star Maya (NGC 1432).
Merope's Nebula

Physically the reflected nebula is probably part of the dust in a molecular cloud unrelated to the Pleiades cluster, which happens to cross the cluster's path. It's not a remainder of the nebula from which the cluster once formed as can be seen from the fact that the nebula and the cluster have different radial velocities, crossing each other with a relative velocity of 11 km/s. This nebula will probably just pass through the cluster and afterwards it will fall apart in space. The nebulae of the Pleiades are catalogued in Sky catalogue 2000.0.

Some of the Pleiades stars are rapidly rotating at velocities of 150 to 500 km/s at their surface, which is common among main sequence stars of a certain type B-A. Due to this rotation they must be spheroid rather than spherical bodies. The rotation can be detected because it leads to broadened and diffuse spectral absorption lines, as parts of the stellar surface approach us on the one side while those on the opposite side recede from us relative to the star's mean radial velocity.

The most prominent example of a rapidly rotating star in this cluster is Pleione, which is also variable in brightness with a magnitude of between 4.77 and 5.50 (Kenneth Glyn Jones). It was spectroscopically observed that between the years 1983 and 1952 Pleione ejected gas shell because of this rotation, as had been predicted by Otto Struve.

As well as the white and brown dwarfs the Pleiades group also contains some red dwarfs, and 22 double stars and 3 triple stars.The close connection of the Pleiades stars is a proof of their young age as is the absence of red giants in the cluster. The theory of stellar evolution states that the brightest stars take about 80 million years to pass through the main sequence, after this they can become red giants. Kenneth Glyn Jones calculated that the Pleiades have an expected future lifetime as a cluster of only about another 250 million years, by then they will have been become spread out as individual or multiply stars along their orbital path.

6. Pleiades and Hyades
(similarities and differences)

One constellation, Taurus possesses two remarkable star clusters - Pleiades and Hyades. The group of Hyades is fainter than the Pleiades one. The Greek title " hyades " means the " rainy stars ". Their appearance on the night sky was the sign of the beginning rainy season. According to Greek mythology, the Hyades are "The seven nymphs of the Rain" ; daughters of Atlas and half-sisters of the Pleiades. They have as many names as the Pleiades. In the famous Alphon's tables, finished in 1248 (Spain), the Hyades are called " The icon-lamps ". The Estonian title " The old sieve " reflects their wide connection with the Pleiades cluster - " The young sieve ". The Bulgarians describe them as " The horo ", a national round dance accompanied by a " bagpipe player " or Alpha Tauri. In the whole of the night sky the Hyades star cluster is the most similar object to the Pleiades group. The comparison between them is based on many similarities and differences. The Hyades is group of around 200 stars, surrounding Taurus. Its average equatorial co-ordinate is Alpha=4h 27m, Delta=+16 degrees of arc. The brightest members of the cluster form a triangle, possessing a visible diameter 3.5 degrees of arc (8 l.y). The visual magnitude is 0.5. Hyades spin is approximately 20 degrees and has a real diameter of 41 l.y. Their distance from us is 118 l.y. The motion of the stars is directed to one spot in the night sky, close to Alpha Orionis. All the members of the group are moving on parallel paths and the visible combined trajectory is caused by the effect of the perspective. The stars of Hyades are as varied as those of the Pleiades, but smaller and cooler than them. (Stars, counting from 9 magnitude are not more than 100.) Some of the stars are red giants or similar to the Sun. The age of the Hyades is thought to be between 0.4-1.1 billion years. The cluster is relatively close to our star system at a distance of only 36 pc.

It's calculated that 80 000 years ago, Hyades were twice as close to our Sun as they are now (59 l.y.). If they are moving away from us with a velocity of 49 km/s, in 65 million of years, they will have an apparent diameter of 20 minutes of arc. The occupied area of the night sky will be less than the full Moon. The brightest stars, seen with the naked eye now, will become stars of 12 magnitude. They will be visible only through a telescope. Everything in the world is changeable and this applies to night sky move.


The Pleiades at night from an ice-cave on a hypothetical nearby planet. Because the Pleiades star cluster was formed only recently, this world is very young. Painting by David Egge.

In a table are shown the similarities and differences of the two open star clusters in details, according to Sky Catalogue.
The diagrammatic "spectrum - magnitude" of open star clusters contains much valuable astrophysical information. The charts of the clusters Pleiades and Hyades are presented in a
figure .
The most famous open star cluster of our Galaxy are collected in the disc of Milky way with some small exceptions. The absorbing substance from Galaxy is concentrated in this disc, too. Nearly 1200 open star clusters can be seen in the night sky. They are an insignificant part of the total of the open star clusters in he Milky Way, whose actual number is considered to be 20 000 or 30 000.

The Pleiades
(a practical lesson)

Problem 1: A calculator is required.
Use the table of the visual magnitudes “m” of the brightnes stars of the Pleiades to determine their absolute magnitudes “M”. For this purpouse implement the following equation: M=m+5-5.lg(r), where “r” is the distance to the cluster in parsecs. Assume the distance is 125 pc. Using the same equation, calculate the absolute magnitude of the Sun “M”(Sun), where for the Sun: “r”(Sun) = 1 AU = 1/206265 pc, and “m”(Sun) = -26,8. Solve the reverse problem: using the obtained absolute magnitudes “M” of the Pleiades’members, calculate their visual magnitudes at a distance 1 AU. Order the results in a table and compare them to the initial table and to the visual magnitude of the Sun.
Problem 2: Requirements: stellar map of the Pleiades, small telescope or binocular, drawing book, soft pencil, red flashlight. The best choise of an instrument for this exercise will be a 2” refractor with 15x magnification and a guider, but any other small instrument with similar magnification can be used.
Get acquainted with the Pleiades map and memorize their configuration. Choose a moment when the Pleiades are good for an observation (not very high above the horizon). The autumn evenings are most suitable. Observe them with the instrument you have chosen. Carefully draw the Pleiades on the drawing book using the pencil and the red flashlight. First plot the brightest stars with circles proportional to their brightness. Then plot the fainter ones. Once you have drawn a star, please do not make corrections! Mark East and West on the plot. To do this, note the direction in which the stars are moving in the eyepiece – this is West (if the telescope has guiding, switch it off to determine the direction). East is in the opposite direction. Write down the date and the specifications of the instrument you have used. Compare your plot to the original map of the Pleiades, determine which are the brightest stars on your plot and write down their names.

BIBLIOGRAPHY:

Astronomia , N. Nikolov & M. Kalinkov, 1998.
Burnham’s celestial handbook , R. Burnham, v.3, 1978.
Nazvania zvezdnovo neba
, J. Karpenko, 1981.

Sky Catalogue 2000.0 , v.1&2, W. Tirion, 1985.
The Cambridge Atlas of Astronomy , 1985.
The Messier Album , J. Mallas &E. Kreimer, 1978.

http://www.pleiade.org/pleiades_03.html
http://www.MessierObject45.html
http://www.ras.ucalgary.ca/~gibson/pleiades/

The Project was prepared by:
Julia Uzunova – student, Nikolaj Kacharov – student, Vesselina Kalinova – student,
Ivanka Getsova - teacher
Sliven - Bulgaria