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Designations:
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M45 - Pleiades or Seven Sisters
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Object
Type:
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Open Cluster and Nebulosity
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Constellation:
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Taurus
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3 hr 47.5 min (Alcyone)
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+24° .3 min (Alcyone)
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1.5
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Size:
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2°
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Distance:
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407 light years
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Discoverer:
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Known Since Antiquity
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Just how many stars (Pleiads) in the Pleiades are truly visible with the
naked eye is the subject of some debate. Traditionally, the number has been seven. But that count stems
from ancient Greek mythology and refers to the seven doves that carried ambrosia to the infant Zeus, or
to the seven sisters who were placed in the heavens so that they might forget their grief over the fate
of their father, Atlas, condemned to support the sky on his shoulders. Although largely symbolic, the
age-old association of the Pleiades with the number seven remains fixed to this day - to the point that
some observers swear they cannot see more than seven members, even though the Pleiades contains 10 stars
brighter than 6th
magnitude.
Some observers question how it is possible to see 10 Pleiads in the Seven
Sisters a demonstration of the power of words or, as Clerke inferred, the power of the Pleiades over
human affairs. The fact is that almost three times that magic number of stars can be seen without
magnification by an astute observer under dark skies. Clerke notes that Kepler's tutor, Maestlin, saw 14,
and he mapped 11 before the invention of the telescope. Archinal reports routinely seeing 12 Pleiads and
sometimes 14 under good skies. Houston counted 18, and about 20 years ago O'Meara logged 17 from Cambridge,
Massachusetts! The trick is to spend a lot of time looking and plotting.
Another lingering debate concerns whether the nebulosity associated with the cluster can be seen with the naked eye. Some argue that the haze is just an illusion, a vision created by the tight gathering of stars whose unresolved light simply appears fuzzy - just as close doubles do at low power (like M40). Others, however, claim that the cluster appears just as Tennyson described it in Locksley Hall: "Many a night I saw the Pleiades, rising thro' the mellow shade. glitter like a swarm of fireflies tangled in a silver braid."Houston once scoffed, "Why does everyone fuss over the Pleiades nebulosity? Of course you can see it with the naked eye!" He went on to say something colorful about the naysayers. I did not reply, and he didn't expect a response; he knew he was preaching to the choir. However, Skiff points out that some of the perceived nebulosity may be scattered starlight. He suggests testing this hypothesis by screening Alcyone, the brightest Pleiad, behind the edge of a building. Try it. The 70-million-year-old cluster contains about 100 stars in a sphere 14 light years in diameter. It is sailing through space at about 25 miles per second, and it will take them about 30,000 years to move an apparent distance of one moon diameter. The brightest Pleiads are all rapidly rotating, with Pleione spinning about 100 times faster than our sun. And Alcyone, the centralmost and brightest Pleiad, is a thousand times more luminous that the sun. Through a telescope the Pleiades and its attendant nebulosity fill a low-power field, where they are best seen. The nine brightest members fit well in a 1° field (a true diameter of 7 light years). From dark skies, the cluster looks like a cobwebbed coffin filled with glistening jewels. The individual Pleiads are interconnected by gauzelike veils of nebulosity, the Brightest of which surrounds the star Merope (23 Tauri), and is called the Merope Nebula (IC 349). Just south of Merope is a condensation of nebulosity, NGC 1435, also known as "Tempel's Nebula," which was first noticed in 1859 by W. Tempel with a 4-inch refractor. He described it as a faint stain of fog, like a "breath on a mirror." In 1874 Lewis Swift saw it in a 2-inch refractor at 25 x. One century later Houston used an 8-inch reflector from southern Arizona to see the field laced from edge to edge with bright wreaths of delicately -structured nebulosity. Interestingly, the great double-star observer S. W. Burnham could not see this nebulosity with the great 18-inch refractor at Dearborn Observatory in Illinois. But faint, diffuse glows can evade large-aperture telescopes. At first glance the Merope Nebula network looks like a tapered comet tail. But if you take the time to sweep the telescope back and forth over its surroundings, you might notice that this patch is but part of a larger fan of material that sweeps westward. Using low magnification, just stare at the cluster for a while and occasionally tap the telescope tube. You should start to see wide dark lanes running amid the nebulosity, especially at the cluster's center, where it drapes around a bridal veil of nebulosity adorning Alcyone. Study the drawing and photograph and look for prominent Breaks of nebulosity, especially around Maia and Electra. |
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Telescope:
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Focal Length:
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384 mm (480 * 0.8x reducer)
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Mount:
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Camera
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Guider:
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Exposures:
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50 20-second exposures (Luminance) 20 30-second exposures for each color (Red, Green and Blue)
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Location:
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Cicero, IN
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Software:
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CCDSoft for image acquisition, processed with CCDStack and Photoshop CS2
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Notes:
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The Pleiades cluster is troublesome image target
for my particular setup. The ST10XME blooms very rapidly with the mag 1.5 stars involved in the cluster.
In addition to the normal blooms, the microlens architecture of the ST10XME imaging chip produces some
artifacts that resemble points on brighter stars. 20-second exposures were used to reduce the effects of the
blooms and microlens artifacts which also greatly reduces the signal to noise ratio of the image. Even with 20-second
exposures, several hours of effort was required to remove star blooms and artifacts. The diffractions spikes in
the images were artificially introduced in Photoshop. This was done to hide some of the microlens artifacts and
also because I like the affect.
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