2020. 8. 26. 14:53ㆍSky observation
The Pleiades star cluster is also known as the Seven Sisters, and as Messier 45. It is a prominent object in the night sky with a conspicuous place in ancient mythology. The cluster is among the nearest to Earth, and the most obvious to the naked eye. Although only a handful are visible to the unaided eye, it contains hundreds of stars, some of them surrounded by swirls of nebulosity.
The Pleiades are a prominent winter sight in the Northern Hemisphere, and in summer in the Southern Hemisphere. They have been known since antiquity to cultures all around the world, including the Maori (who call them "Matariki") and Australian Aborigines, the Chinese, the Mayans (who called them "Tzab-ek"), the Aztecs ("Tianquiztli"), and the Sioux and Cherokee of North America.
The earliest Greek references to the Pleiades were by Homer in the Iliad and Odyssey, around 750 and 720 B.C., and by Hesiod, about 700 B.C. Some Greek astronomers considered the Pleiades to be a distinct constellation. They are called "Kiymah" in Hebrew, and the Bible refers to them three times - in Job 9:7-9, Job 38:31-33, and Amos 5:8.
The Pleiades are revered in Hindu mythology as Krittika, the six mothers of the war god Skanda, who developed six faces. Some scholars of Islam suggest that the Pleiades ("Al thuraiya" in Arabic) are the Star in Najm which is mentioned in the Quran. Their Persian name is "Soraya", after which the former Iranian empress was named.
In Japan, the Pleiades are named "Subaru". The Japanese automaker derives its name from the cluster, which is represented in its corporate logo.
Old English and German names indicate that the Pleiades were once compared to a "Hen with Chicks". Their modern English name is of Greek origin, though of uncertain etymology. The name "Pleiades" may be derived from the Greek word "plain" for "to sail", or the word "pleios" meaning "full" or "many"; or from "peleiades", meaning "flock of doves".
The Pleiades' name may also be derived from their mythological mother, Pleione, which is also one of its brighter stars. The nine brightest stars of the Pleiades are named for the Seven Sisters of Greek mythology: Sterope, Merope, Electra, Maia, Taygeta, Celaeno, and Alcyone, along with their father Atlas and mother Pleione. As daughters of Atlas, the Hyades were sisters of the Pleiades.
Historical Observations
In 1767, Reverend John Michell calculated the probability of finding a group of bright stars such as the Pleiades by chance alignment as 1 in 496,000. Therefore, he concluded correctly that the Pleiades, and many other star clusters, must be physically related. Around 1846, German astronomer Madler noticed that the stars of the Pleiades had no measurable proper motion relative to each other. The common proper motion of the Pleiades was a further hint that they formed a physical group.
In 1769, Charles Messier included the Pleiades as number 45 in his first list of comet-like objects, published in 1771. Messier's inclusion of the Pleiades is curious, as most of Messier's objects are much fainter and more easily confused with comets - something which seems scarcely possible for the Pleiades. Messier may simply have wanted a larger catalogue than his scientific rival Lacaille, whose 1755 catalogue contained 42 objects.
Appearance and Occultations
The Pleiades are one of the most beautiful objects in the sky. Even with the naked eye, under modest conditions, they are easily found, 10° NW of the bright orange star Aldebaran (Alpha Tauri). Surrounding Aldebaran is another, equally famous open cluster, the Hyades; Aldebaran is a foreground star at 68 light years' distance, rather than the 150 ly of the Hyades.
At least 6 of the Pleiades' member stars are visible to the naked eye, forming a pattern reminiscent of the Big Dipper. Under moderate conditions, their number increases to 9; and under clear dark skies it jumps to more than a dozen. In 1579, well before the invention of the telescope, astronomer Moestlin correctly drew 11 Pleiades stars, while Kepler quoted observations of up to 14.
The Pleiades photographed in 1893 from the Lick Observatory.
The nine brightest stars of the Pleiades are concentrated in a field just over 1° in diameter. The cluster is a great object in binoculars, showing more than 100 stars in the field. For telescopic viewing, a wide-angle, very low power eyepiece is needed to view the entire group. A number of double and multiple stars are contained in the cluster. Modern observations reveal at least 500 stars belonging to the Pleiades, spread over a 2° field - four times the diameter of the Moon!
As the Pleiades are situated only 4 degrees off the Ecliptic, occultations of the cluster by the Moon occur quite frequently. Also, planets come close to the Pleiades cluster; Venus, Mars, and Mercury occasionally pass through it. This is a very appealing spectacle, especially for amateurs with less expensive equipment, and even can be observed with the naked eye!
Reflection Nebulosity
On clear, dark nights, when the moon is absent, swirls of nebulosity are noticeable around a some of the brighter stars, especially near Merope. Longer exposure photographs, and "rich field" telescopes, reveal that the Pleiades are embedded in nebulous material. The Pleiades nebulae are blue-colored, which indicates that they reflect the light of the bright stars situated near (or within) them. The brightest of these nebulae, around Merope, was discovered in 1859 by Wilhelm Tempel at Venice, Italy with a 4" refractor. The Merope Nebula (NGC 1435) requires a dark sky and is best visible in a rich-field telescope.
The nebulae around Maia (NGC 1432), Alcyone, Electra, Celaeno and Taygeta were found on photographs between 1885 and 1888 by the Henry brothers in Paris and by Isaac Roberts in England. In 1890, E. E. Barnard discovered a concentration of nebulous matter very close to Merope (now catalogued as IC 349). Analysis of the spectra of the Pleiades nebulae by Vesto Slipher in 1912 revealed their nature as reflection nebulae, as their spectra are exact copies of the spectra of the stars illuminating them.
Barnard's Merope Nebula, IC 349, imaged by the Hubble Space Telescope.
Physically, the reflection nebulae are unrelated to the Pleiades cluster, as can be seen from the fact their radial velocity differs from the cluster's by 11 km/sec. Instead, the cluster is simply passing through a particularly dusty region of the interstellar medium. The dust is not a remainder of the nebula from which the Pleiades formed, as was formerly thought. At the cluster's age of 100 million years, almost all the dust originally present when it formed would have been dispersed by radiation pressure.
Properties and Evolution
Before the launch of the Hipparcos satellite in 1991, the Pleiades' distance was thought to be about 440 light years. Hipparcos caused consternation among astronomers by measuring a distance of only 380 light years. This would have required the Pleiades stars to be comparatively fainter, without explanation. However, subsequent investigations by the Hubble Space Telescope, and the Mount Wilson and Palomar Observatories, found that Hipparcos's measurement of the Pleiades' parallax was too small. Their distance is currently thought to be 440 light years, and it is not yet known why the Hipparcos parallax error occurred.
The Pleiades' core cluster radius is about 8 light years, and its tidal radius is about 43 light years. The cluster contains over 1,000 confirmed members, with a total mass estimated at about 800 Suns. The stars in the Pleiades are thought to have formed together around 100 million years ago; its brightest members all hot, young, blue-white class B giants and subgiants, with absolute magnitudes from about -1.5 to -2.5.
Some of the Pleiades stars are rotating rapidly, with velocities of 150 to 300 km/sec - common among young class B main sequence stars. This rotation gives them oblate spheroid bodies. The rotation can be detected because it broadens their spectral lines as parts of the stellar surface approach us on one side, while those on the opposite side recede. Pleione is the most rapidly rotating star in the cluster, and has ejected a gas shell because of this rotation. It is also variable, between magnitudes 4.77 and 5.50.
The Pleiades cluster contains some white dwarf stars. White dwarves cannot have masses above 1.4 Suns (the Chandrasekhar limit). Such low-mass stars take billions of years to evolve into that state, not just the 100-million-year age of the Pleiades cluster. How, then, can white dwarves exist in such a young star cluster? The only possible explanation seems to be that these white dwarf stars were once more massive and evolved faster, but have since lost mass due to strong stellar winds, close neighbors, or fast rotation.
The cluster also contains many brown dwarves, which are objects containing less than 8% of the Sun's mass, and are not heavy enough to start nuclear fusion reactions in their cores and thus become proper stars. Brown dwarves may constitute up to 25% of the total population of the cluster, although they contribute less than 2% of its total mass.
Eventually, the Pleiades' space motion will carry them below the feet of Orion, as seen from Earth. After that, they will take about 250 million years to disperse, due to gravitational interaction with the galactic neighborhood.