Open Cluster M45
03 : 47.0 (h:m)
+24 : 07 (deg:m)
110.0 (arc min)
Mentioned by Hesiod between 1000 and 700 B.C. 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
Atlas of Deep Sky Splendors, mentions that in 1579, well before
the invention of the telescope, astronomer Moestlin has
correctly drawn 11 Pleiades stars, while Kepler quotes
observations of up to 14).
Modern observing methods have revealed that at least about 500
mostly faint stars belong to the Pleiades star cluster, spread
over a 2 degree (four times the diameter of the Moon) field. Their
density is pretty low, compared to other open clusters. This is one
reason why the life expectation of the Pleiades cluster is also
pretty low (see below).
According to Kenneth Glyn Jones, the earliest known reference
of this cluster is a mention by Hesiod, about 1000 BC
(according to Burnham, they were seen in connection to the
agricultural seasons of that time). Homer mentions them in
Odyssee, and the Bible has three references to the
The Pleiades also carry the name "Seven Sisters";
according to Greek mythology, seven daughters and their parents.
Their Japanese name is "Subaru", which was taken to christen
the car of same name. The Persian name is "Soraya", after
which the former Iranian empress was named. Old European (e.g.,
English and German) names indicate they were once compared to a "Hen
with Chicks". Other cultures tell more and other lore of this
naked-eye star cluster. Ancient Greek astronomers Eudoxus of
Knidos (c. 403-350 BC) and Aratos of Phainomena (c. 270
BC) listed them as an own constellation: The Clusterers. This
is also referred to by Admiral Smyth in his Bedford
Burnham points out that the name "Pleiades" may be
derived from either the Greek word for "to sail", or the word
"pleios" meaning "full" or "many". The present author prefers the
view that the name may be derived from the mythological mother,
Pleione, which is also the name of one of the brighter stars.
According to Greek mythology, the main, visible stars are named for
the seven daughters of "father" Atlas and "mother" Pleione: Alcyone,
Asterope (a double star, also sometimes called Sterope), Electra,
Maia, Merope, Taygeta and Celaeno. Bill Arnett has created a map of
the Pleiades with the main star names. These stars are also labeled
in a labeled copy of the UKS image which appears in this page. Also
note our Pleiades map.
In 1767, Reverend John Michell used the Pleiades to calculate
the probability to find such a group of stars in any place in the
sky by chance alignment, and found the chance to be about 1/496,000.
Therefore, and because there are more similar clusters, he concluded
correctly that clusters should be physical groups (Michell 1767).
On March 4, 1769,
Charles Messier included the Pleiades as No. 45 in his first
list of nebulae and star clusters, published 1771.
About 1846, German
astronomer Mädler (1794-1874), working at Dorpat, noticed
that the stars of the Pleiades had no measurable proper motion
relative to each other; from this he boldly concluded that they form
a motionless center of a larger stellar system, with star
Alcyone in the center
This conclusion was to be, and was, rejected by other
astronomers, in particular Friedrich Georg Wilhelm Struve
(1793-1864). Nevertheless, the common proper motion of the
Pleiades was a proof that they move as a group in space, and a
further hint that they form a physical cluster.
photographs (and also short focal ratio, i.e. short focal length
compared to their aperture, "rich field" telescopes of considerably
good quality, especially good binoculars) have revealed that the
Pleiades are apparently imbedded in nebulous material, obvious in
our image, which was taken by David Malin with the UK
The Pleiades nebulae are blue-colored, which indicates that they
are reflection nebulae, reflecting the light of the bright stars
situated near (or within) them. The brightest of these nebulae, that
around Merope, was discovered on October 19, 1859 by Ernst Wilhelm
Leberecht (Wilhelm) Tempel at Venice (Italy) with a
4-inch refractor; it is included in the NGC as NGC 1435. Leos
has made the biography of Wilhelm Tempel available online
together with a drawing of the Merope Nebula, and has agreed
to include it in this database.
The extension to Maya was discovered
in 1875 (this is NGC 1432), the nebulae around Alcyone, Electra,
Celaeno and Taygeta in 1880. The full complexity of the Pleiades
nebulae was revealed by the first astro cameras, e.g. by that of the
brothers Henry in Paris and Isaac Roberts in England,
between 1885 and 1888. In 1890, E.E. Barnard discovered a
starlike concentration of nebulous matter very close to Merope,
which found its way into the IC as IC 349. The analysis of the
spectra of the Pleiades nebulae by Vesto M. Slipher in 1912
reveled their nature as reflection nebulae, as their spectra are
exact copies of the spectra of the stars illuminating them.
Physically, the reflection nebula is probably part of the dust in a
molecular cloud, unrelated to the Pleiades cluster, which happens to
cross the cluster's way. It is not a remainder of the nebula from
which the cluster once formed, as can be seen from the fact that the
nebula and cluster have different radial velocities, crossing each
other with a relative velocity of 6.8 mps, or 11 km/sec.
According to new calculations published by a team from Geneva, G.
Meynet, J.-C. Mermilliod, and A. Maeder in Astron.
Astrophys. Suppl. Ser. 98, 477-504, 1993, the age of the Pleiades
star cluster amounts 100 million years. This is considerably more
than the previously published "canonical" age of 60--80 million
years (e.g., the Sky Catalog 2000's 78 million). It has been
calculated that the Pleiades have an expected future lifetime as a
cluster of only about another 250 million years (Kenneth Glyn
Jones); after that time, they will have been spread as
individual (or multiple) stars along their orbital path.
The distance of the Pleiades cluster has been newly determined by
direct parallax measures by ESA's astrometric satellite
Hipparcos; according to these measurement, the Pleiades are at
a distance of 380 light years (previously, a value of 408
light years had been assumed). The new value requires an explanation
for the comparatively faint apparent magnitudes of the Pleiades
The Trumpler classification is given for the Pleiades as
II,3,r (Trumpler, according to Kenneth Glyn Jones) or I,3,r,n (Götz
and Sky Catalog 2000), meaning that this cluster appears detached
and strong or moderately concentrated toward its center, its stars
are spread in a large range of brightness, and it is rich (has more
than 100 members). Some of the Pleiades stars are rapidly rotating,
at velocities of 150 to 300 km/sec at their surfaces, which
is common among main sequence stars of a certain spectral type
(A-B). Due to this rotation, they must be (oblate) spheroids 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 for a rapidly rotating star in
this cluster is Pleione, which is also variable in brightness
between mag 4.77 and 5.50 (Kenneth Glyn Jones).
spectroscopically observed that between the years 1938 and 1952,
Pleione has ejected a gas shell because of this rotation, as had
been predicted by O. Struve.
Cecilia Payne-Gaposhkin mentions that the Pleiades
contain some white dwarf (WD) stars. These stars give rise to a
specific problem of stellar evolution: How can white dwarfs exist in
such a young star cluster ? As it is not only one, it is most
certain that these stars are original cluster members and not all
field stars which have been captured (a procedure which does not
work effectively in the rather loose open clusters anyway). From the
theory of stellar evolution, it follows that white dwarfs cannot
have masses above a limit of about 1.4 solar masses (the
Chandrasekhar limit), as they would collapse due to their own
gravitation if they were more massive. But stars with such a low
mass evolve so slow that it takes them
billions of years to evolve into that final state, not only the
100 million year age of the Pleiades cluster.
The only possible explanation seems to be that these WD stars
were once massive so that they evolved fast, but due to some
(such as strong stellar winds, mass loss to close neighbors, or fast
rotation) have lost the greatest part of their mass. Possibly they
have, in consequence, lost another considerable percentage of their
mass in a planetary nebula. Anyway, the final remaining stars (which
was previously the star's core) must have come below the
Chandrasekhar limit, so that they could go into the stable white
dwarf end state, in which they are now observed.
New observations of the Pleiades since 1995 have revealed several
candidates of an exotic type of stars, or starlike bodies, the
so-called Brown Dwarfs. These hitherto hypothetical objects
are thought to have a mass intermediate between that of giant
planets (like Jupiter) and small stars (the theory of stellar
structure indicates that the smallest stars, i.e. bodies that
produce energy by fusion sometime in their lifetime, must have at
least about 6..7 percent of one solar mass, i.e. 60 to 70 Jupiter
masses). So brown dwarfs should have 10 to about 60 times the mass
of Jupiter. They are assumed to be visible in the infrared light,
have a diameter of about or less that of Jupiter (143,000 km), and a
density 10 to 100 times that of Jupiter, as their much stronger
gravity presses them tougher together.
Even with the naked eye and under modest conditions, the Pleiades are
rather easily found, roughly 10 degrees north-west of the bright
red-giant star Aldebaran (87 Alpha Tauri, mag 0.9,
spectral type K5 III). Apparently surrounding Aldebaran is
another, equally famous open cluster, the Hyades;
Aldebaran is known to be a non-member foreground star (at 68
light years distance, compared to 150 ly for the Hyades).
The cluster is a great object in binoculars and rich-field telescopes,
showing more than 100 stars in a field about 1 1/5 degrees in
diameter. In telescopes, it is frequently even too large to be seen
in one lowest magnification field of view. A number of double and
multiple stars are contained in the cluster. The Merope Nebula
NGC 1435 requires a dark sky and is best visible in a rich-field
telescope (Tempel had discovered it with a 4-inch telescope).
As the Pleiades are situated close to the ecliptic (4 degrees off), occultations of the cluster by the Moon occur quite frequently: This
is a very appealing spectacle,
especially for amateurs with less
expensive equipment (actually, you can observe it with the naked
eye, but even the smallest binoculars or telescopes will increase
observing pleasure -- the March 1972 Pleiad occultation was one of
the first amateur astronomical experiences of the present author).
events demonstrate the relations of the apparent sizes of the
Moon and the cluster: Burnham points out that the Moon may be "inserted
into the quadrangle formed by" Alcyone, Electra,
Merope and Taygeta (Maia,
and possibly Asterope, is occulted in this
come close to the Pleiades cluster (Venus, Mars, and Mercury
even occasionally pass through) to give a conspicuous spectacle.
As mentioned in the
description for the Orion Nebula M42, it is a bit unusual that
Messier added the Pleiades (together with the Orion
Nebula M42/M43 and the Praesepe cluster M44) to his catalog, and
will perhaps stay subject to speculation.
• John Michell, 1767. An Inquiry into the probable Parallax, and
Magnitude, of the Fixed Stars, from the Quantity of Light which they
afford us, and the particular Circumstances of their Situation.
Philosophical Transactions, Vol. 57, p. 234-264 (1767).