OF THE ROGER N. CLARK

--
--
M45, THE PLEIADES OPEN CLUSTER IN
TAURUS
R.A. 03h 46.9m, Dec. 24° 07' (2000.0)
Technical. The Pleiades have been familiar
the world over since ancient times. The cluster
is very close, relatively speaking, at a distance
of only 400 light-years. It is also unusually
large, 30 light-years in diameter, so it
is easily resolved by the unaided eye.
This unique grouping has had special significance
in all cultures for which records can
be found. The earliest recorded observation is
found in Chinese annals from 2357 BC. The
cluster had particular meaning in agricultural
societies, since its rising and setting near
sunrise and sunset marked important times
in the growing season. Even Halloween is
tied to the Pleiades. In the Middle Ages the
cluster culminated around midnight on the
"Witch's Sabbath", which had its origin in
ancient Druids' rites. (Since then the midnight
culmination of the Pleiades has shifted
to November 21st owing to precession.)
References to the Pleiades are fo und
throughout literature, music, and religion.
The great pyramid at Teotihuacan, 28 miles
northwest of Mexico City, has its west face
directed to the setting of the Pleiades (14°
north of west there), and all the east-west
streets of the ancient city are oriented in the
same direction. Other cultures that gave the
Pleiades special significance include American
Indian, Maya, Aztec, Australian Aborigine,
Egyptian, Greek, Roman and Persian.
Robert Burnham devotes many pages to
Pleiades lore in his Bumham's Celestial Handbook.
The Pleiades are often called the Seven
Sisters, though nine of the stars now have
names. Seven of the names date back at least
as far as the Greek poet Aratus in the 3rd
century BC. (Aratus took his constellation
lore from writings by Eudoxus that were
already about a century old, but are now
lost.) Aratus spoke of a lost Pleiad, and indeed,
many people can see only six stars here.
Perhaps, it has been speculated, a star did
fade sometime within the historical memory
of the ancient Greeks - possibly Pleione, since
it is variable today. References to the lost
Pleiad are also found in Japanese literature
and legends from several other cultures.
M45 contains about 300 to 500 stars. Since
VISUAL ASTRONOMY OF THE DEEP SKY
all are at essentially the same distance,
apparent brightnesses reflect their in
luminosities. The brightest Pleiades are all
blue-white; fainter ones are yellow and red_
dish. When the stars' brightnesses are
graphed as a function of their color (the Socalled
color-magnitude diagram), they all
fall on the curve known as the main sequence.
This is where a star resides for most of its
active lifetime while it converts hydrogen
helium. None of the Pleiades visible today
has yet evolved off the main sequence to the
red giant stage.
All the brightest Pleiades except Maia
spinning very fast, with rotation periods
about two days. They are also quite young,
.
about 80 million years old, and are
consuming their hydrogen fuel. There is
known white dwarf in the cluster. Astronom_
er Alan Sandage has theorized that in
distant past, the Pleiades probably V"Ha"",,u
two stars even brighter than those we
now, which have since become white d
On photographs the brighter Pleiades
surrounded by a delicate web of
This is a fine example of a reflection ne
one visible because of starlight reflected
scattered by small dust grains. Color
graphy shows the Pleiades nebulosity to
quite blue. The reason is not just that
stars illuminating the dust are bluish.
grains tend to be smaller than the wa
of light and therefore scatter blue light
ferentially - a process similar to the
scattering that makes our sky blue.
the light is reflected starlight, its spectrum
continuous rather than concentrated
emission li ' nes as in emission nebulae
M42 (the Great Nebula in Orion) or M8
Lagoon Nebula).
Throughout the Pleiades, the ne
appears streaked in long filaments almost
cirrus clouds. Magnetic fields may be
sponsible for aligning this dust. Some of
filaments appear only a few arc-seconds
width and many arc-minutes long.
The brightest patch of nebulosity,
1435, appears to surround and extend
of the star Merope, and hence is k.nown as
Merope Nebula. It has also been called
Thumbprint from its appearance on an
drawing. In 1965 the astronomer F.
showed that the nebula is actually
90
Merope, not enveloping it or in front of it.
for visual observers. Dust- and dew-free opaided
eye, in binoculars, or in a telescope. Its
s a
O .
A VISUAL ATLAS OF DEEP-SKY OBJECTS
Visual. M45 is a beautiful sight to the un-
tics are essential for the pursuit, since light
t rs have a total magnitude of 1.4 and are scattered from the bright stars will hide the
spread across 10 arc-mInutes. Because the nebulosity. The easiest portion is NGC 1435
jeets in this book.
luster is easily resolved by the normal un-
ided eye, its mean surface brightness (20.0
magnitudes per square arc- econd) has little
meaning-except for companson to other ob-
south of Merope. Other nebulosity appears to
surround Alcyone, Maia, and Electra. There
is little or none around Celaeno, Taygeta,
and Asterope, and definitely none around
Atlas and Pleione.
Most people can see six or eight Pleiades The Merope Nebula was discovered by W.
with the naked eye, though people with good
visual acuity can often see 10 to 12, and as
Tempel in 1859 with a 4-inch refractor. In
1874 Lewis Swift fo und it detectable with a
many as 18 have been claimed. The number 2-inch refractor at 25x. Modern observers
visible to the naked eye is not only a test of
seem to be having more difficulty, and it is
how fa int one can see, but also a test of visual often stated that a 6- to 8-inch telescope is
acuity and the sky's freedom from haze.
The table of bright Pleiades on page 94
required. This discrepancy is no doubt due to
increased light pollution, and probably also
shows that 10 stars are brighter than magni- to increased atmospheric dust particles,
tude 5.65. Only because they are so close which scatter .starlight. The mean surface
together are the stars of this magnitude and brightness of NGC 1435 is 21.6 magnitudes
fainter hard to see. I have never been able to per square arc-second and the total magnisee
more than six Pleiades while wearing tude is 6.8, only slightly fainter than the
glasses, even under excellent skies, and with- Helix Nebula, NGC 7293.
out glasses the cluster appears as a blob. Modern observers have reported viewing
However, even though my nearsightedness is the nebulae around other stars. John Mallas
severe, the sensitivity of my retina to the fain- in The Messier Album reported nebulosity, intest
of light appears to be normal. cluding fine streaks, around Maia, Taygeta,
Detection of faint telescopic members of Alcyone and Celaeno with his 4-inch refracthe
Pleiades is also confused by the glare of tor. At least some of this must be spurious,
the brighter stars. Even under excellent skies, because there is little or no nebulosity around
stars of only about magnitude 14.4 can just Celaeno and Taygeta. Waiter Scott Houston
be seen with an 8-inch telescope in the saw an unusual sight through an 8-inch tele-
Pleiades. Elsewhere the limit is about 15.2. scope on an exceptional night in the southern
Th at is a loss of nearly a magnitude. Arizona mountains: "When I looked into the
The Pleiades are often confused with the eyepiece, expecting to see a few faint wisps,
Little Dipper (Little Bear) by beginning the field was laced from edge to edge with
amateurs and laypeople. The cluster does bright wreaths of delicately structured nebuhave
the shape of a small dipper about twice losity ... " .
the size of the Moon, and it is easier to see The brightest of the linear streaks are near
than the Little Dipper ofUrsa Minor because Electra. One about 20 arc-seconds wide exthe
main stars are brighter. tends from just south of that star about a
M45 is certainly a favorite object among third of the way to Alcyone. A second streak
amateur astronomers for its beauty in virtual- about half as long runs parallel to it one
ly any opical instrument with an adequate arc-minute farther south. They are easily
eld of VIew. Binoculars provide a pretty confused with artificial streaks caused by dif-
SIght, but a 3- to 8-inch telescope at about 30 fraction or reflections in the telescope.
pwer probably shows the most spectacular The drawing shows what was seen through
VIew. A wide-field eyepiece at 60x will still the 8-inch under good skies. The lowest mag-
:o .
all the bright stars at once, including nification with the 1.25-inch eyepiece holder
e
.
Ipper. At higher powers only smaller was 82 X, giving a field of view 40 archrIOnS
of the cluster are seen, though these minutes in diameter. This field encompassed
Ig er powers can be useful and are discus- the bowl of the dipper quite nicely. I had seen
sed below.
Th
the Merope Nebula many times before, at
e Pleiades nebulosity is a prized trophy low power under excellent skies. This
91