Set high in the winter sky as part of Orion’s glittering
retinue, the Pleiades have certainly been known to Man ever since prehistoric
times, ever since the first glimmerings of human intelligence compelled our
ancestors to look upwards to the night sky in awe and wonder.
At a distance of about 440 light years, the Pleiades is one
of the nearest star clusters to Earth. It is certainly the nearest Messier
object to Earth, and is the most obvious star cluster to the naked eye in the night
sky.
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| Location of image field of view in the night sky... |
In Greek mythology, the Pleiades were the seven daughters of
the Titan, Atlas, who was condemned by Zeus to forever bear the weight of the
heavens as punishment for rebelling against the Olympian gods. Without their
father to protect them, the Pleiades attracted the amorous attentions of the
nearby hunter, Orion, and so to save them, Zeus transformed them into the stars
we see today. Atlas and his wife, Pleione, also became part of the starry
retinue of the Pleiades: “Pleiades” means “the daughters of Pleione”, and Mum
and Dad keep watch over their daughters as the two stars closely adjacent stars
on the eastern side of the cluster.
The Pleiades have been traditionally known as a group of
seven stars. In various Greek and Roman writings, they are referred to as The
Starry Seven, The Seven Virgins or the Seven Atlantic Sisters. These ancient
descriptions come from days long before the advent of telescopes or binoculars,
yet today, only six Pleiads are easily visible to the unaided eye. This state
of affairs is supported by modern measurements of the brightness of the stars
of the Pleiades. Only six of them are above “fifth magnitude”, the
accepted threshold for naked eye visibility.
Magnitudes of the nine brightest
members of the Pleiades are shown in the greyscale image below:
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| Magnitudes of the nine brightest Pleiads... |
With an apparent magnitude of +2.85, the brightest star in
the cluster is Alcyone, a blue-white B-type giant, similar in type to the other
bright B-type stars in the Pleiades cluster. Alcyone has a mass of 6 times that
of the Sun, and an effective radius of almost ten times that of the Sun, but
the actual radius is lesser at poles and greater at the equator due to its high
rotational velocity, which causes it to have an ellipsoidal shape. Its
temperature is approximately 12,300 K (over twice as hot as our Sun) with the
actual temperature being greater at the poles and lesser at the equator. Its
luminosity is 2,030 times that of the Sun.
The light of the cluster is dominated by hot blue luminous
stars that have formed comparatively recently in astronomical terms. Ages for
star clusters can be estimated by comparing the Hertzsprung–Russell diagram for the
cluster with theoretical models of stellar evolution and using this technique,
ages for the Pleiades of between 75 and 150 million years have been estimated.
One of the oldest traditions concerning the cluster is the
persistent myth of a “lost Pleiad”. The Greeks identified her as Electra, who
is said to have veiled her face at the burning of Troy. Another story casts Merope in the role,
as she reputedly hid her face in shame at having married a mortal, the King of
Corinth, while all her other sisters were wedded to gods.
The Greek poet Aratus (310-240 BC) refers to the tradition
of the “Lost Pleiad” when he wrote:
“…Their number seven, though the myths oft say
And poets feign, that one has passed away…”
This tradition is not confined to Greek mythology. The story
of a lost Pleiad also appears in Japanese lore: the cluster is mentioned under
the name Mutsuraboshi ("six
stars") in the 8th-century Kojiki
(an early Japanese chronicle dating from the early 8th century), and is now known
as Subaru. A similar theme figures in
the legends of Australian aborigines, natives of the Gold Coast of Africa and
the head-hunters of Borneo:
“Their Sister Stars that were once seven
Mourn for their missing mate in Heaven…”
It seems that the legend of the lost Pleiad may have a basis
in fact, however. Those modern brightness measurements have revealed that the
mythical mother of the Pleiades, Pleione, is variable in brightness by at least
half a magnitude, and may well once have been above the threshold of naked eye
visibility.
Galileo was the first astronomer to view the Pleiades
through a telescope. He discovered that the cluster contains many stars too dim
to be seen with the naked eye. He published his observations, including a
sketch of the Pleiades showing 36 stars, in his treatise Sidereus Nuncius in March 1610.
Today, we know that the cluster is about 80 light-years across
and contains over 1,000 statistically confirmed members, a figure that excludes
an unresolved number of binary stars, which make up over 50% of the total stars
in the cluster.
Curiously, the 11th century Talmud (the central
text of Rabbinic Judaism) uses the term kimah to describe the number of
stars in the Pleiades, a word which means “over one hundred”.
Another thing revealed by long exposure photography is that
the Pleiades are veiled in fine, dense threads of nebulosity, like cirrus
clouds or interstellar cobwebs. It is questionable whether these are truly visible
to the naked eye today, yet some translations of the biblical Book of Job refer
to the “Chains of the Pleiades”. The seventh century Arabian poet Amr
al Kais also hints at the nebulosity:
“The hour when the Pleiades appeared in the firmament
Like the folds of a silken sash variously decked with gems…
Perhaps the most famous reference
to the Pleiades nebulosity in English literature occurs in Tennyson’s Locksley Hall…
“Many a night from yonder ivied casement, ere I went to rest
Did I look on great Orion, sloping slowly to the west
Many a night I saw the Pleiads, rising thro’ the mellow shade
Glitter like a swarm of fireflies tangled in a silver braid…”
Charles Messier measured the position of the cluster and
included it as M45 in his catalogue of comet-like objects, published in 1771.
Messier's inclusion of the Pleiades has been noted as curious, as most of
Messier's objects were much fainter and more easily confused with comets. The
brightest area of nebulosity (called “Tempel’s Nebula) lies to the south of
Merope does indeed look comet-like in appearance, however, so perhaps that is why
Messier included the Pleiades in his famous catalogue.
These reflection nebulae can clearly be seen in the main
image above, and more detailed photographs of the area show a mass of fine
filaments that glitter with reflected starlight.
They are included in the 1966 catalogue of
reflection nebulae compiled by Sidney van den Bergh, and are shown marked
on the grey scale image below:
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| van den Burgh reflection nebulae in the Pleiades... |
This dust was once thought to be left over material from
the cluster star formation, but is now considered likely to be an unrelated dust cloud in
the interstellar medium through which the stars are currently passing. The dust
cloud is estimated to be moving at a speed of approximately 18 km/s relative to
the stars in the cluster.
The long exposures used in this image show up the interstellar dust around the Pleiades as a brownish cloudy background.
The total mass contained in the cluster is estimated to be
about 800 solar masses and is dominated by fainter and redder stars, although its
visible light is dominated by young, hot blue stars. The cluster contains many brown
dwarfs, which are objects with less than about 8% of the Sun's mass, not heavy
enough for nuclear fusion reactions to start in their cores and become proper
stars. They may constitute up to 25% of the total population of the cluster,
although they contribute less than 2% of the total mass.
The Pleiades have long been known to be a physically related
group of stars rather than just a chance alignment. When studies were first made
of the stars' proper motions, it was found that they are all moving in the same
direction across the sky, at the same rate, further demonstrating that they
were related. The cluster itself is moving at the leisurely pace of 32 km/sec
towards the south of what is currently the constellation of Orion. Like most
open clusters, the Pleiades will not stay gravitationally bound forever. Astronomers
estimate that the cluster will survive for about another 250 million years,
after which it will gradually disperse due to gravitational interactions with its
galactic neighbourhood.
