The Pleiades, also known as the Seven Sisters and Messier 45, are an open cluster containing several middle-aged B-type stars located in the constellation Taurus. It is one of the closest star groups to Earth and is the most obvious group in the night sky.
It was once thought that the reflection nebulae around the brightest stars were remnants of the cluster’s formation, but it is now believed that they are probably unrelated dust clouds in the interstellar medium through which the stars are currently passing.
Origin of the name
The name Pleiades comes from ancient Greek. It probably derives from plein (“to sail”) due to the importance of the cluster in marking the sailing season in the Mediterranean Sea: “the sailing season began with its heliacal rising.”
However, in mythology, the name was used for the Pleiades, seven divine sisters, the name supposedly derived from their mother Pleione and effectively meaning “daughters of Pleione.” In reality, it is almost certain that the name of the star cluster came first, and Pleione was invented to explain it.
Folklore and mythology
The Pleiades are a prominent feature in the winter sky in the northern hemisphere and are easily visible in southern latitudes. They have been known since ancient times to cultures around the world, including the Celts, Hawaiians (who call them Makalii), Maori (who call them Matariki), Australian Aborigines (from various traditions), Persians, Arabs (who called them Thurayya), Chinese (who called them mao), Quechua, Japanese, Mayan, Aztec, Sioux, Kiowa, and Cherokee.
In Hinduism, the Pleiades are known as Krittika and are associated with the god of war Kartikeya. They are also mentioned three times in the Bible.
The oldest known representation of the Pleiades is probably a Bronze Age artifact in northern Germany known as the Nebra Sky Disk, dating from approximately 1600 BC. Babylonian star catalogs name the Pleiades MUL, meaning “stars” (literally “star”), and they head the list of stars along the ecliptic, reflecting the fact that they were near the vernal equinox point around the 23rd century BC.
The ancient Egyptians may have used the names “Followers” and “Ennead” in the forecast texts of the Calendar of Lucky and Unlucky Days from the Cairo papyrus. Some Greek astronomers considered them to be a distinct constellation, and they are mentioned by Hesiod, Homer’s Iliad, the Odyssey, and the Geoponica. Some Islamic scholars suggested that the Pleiades (ath-thurayya) are the “star” mentioned in Sura An-Najm (“The Star”) of the Quran.
Subaru
In Japan, the constellation is mentioned under the name Mutsuraboshi (“six stars”) in the 8th-century Kojiki. The constellation is now known in Japan as Subaru (“to join together”). It was chosen as the Subaru car brand to reflect the firm’s origins as a union of five companies, and is shown in the firm’s six-star logo.
Observational history
Galileo Galilei was the first astronomer to see the Pleiades through a telescope. He thus 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.
It has long been known that the Pleiades are a group of physically related stars, rather than any random alignment. John Michell calculated in 1767 that the probability of a random alignment of so many bright stars was only 1 in 500,000, and he surmised that the Pleiades and many other groups of stars must be physically related.
When the first studies of the stars’ proper motions were made, it was found that they are all moving in the same direction across the sky, at the same rate, further proving that they were related.
Charles Messier measured the position of the group and included it as M45 in his catalog of comet-like objects, published in 1771. Along with the Orion Nebula and the Praesepe group, the inclusion of the Pleiades in Messier’s catalog has been noted as curious, as most of Messier’s objects were much fainter and easier to confuse with comets, something that seems unlikely for the Pleiades.
One possibility is that Messier simply wanted to have a larger catalog than his scientific rival Lacaille, whose 1755 catalog contained 42 objects, so he added some bright and well-known objects to improve his list.
Distance
The distance to the Pleiades can be used as an important first step in calibrating the scale of cosmic distances. Because the cluster is so close to Earth, its distance is relatively easy to measure and has been estimated by many methods.
Precise knowledge of the distance allows astronomers to plot a Hertzsprung-Russell diagram for the cluster, which, when compared to plots for clusters whose distance is not known, allows their distances to be estimated.
Ultimately, astronomers’ understanding of the age and future evolution of the universe is influenced by their knowledge of the distance to the Pleiades.
However, some authors argue that the controversy over the distance to the Pleiades discussed below is a red herring, since the cosmic distance scale may (currently) depend on a number of other nearby groups where there is consensus on the distances established by the Hipparcos satellite and independent means (e.g., the Hyades, Coma Berenices cluster, etc.).
Measurements
Measurements of the distance have caused much controversy. Hipparcos data yielded a surprising result, namely a distance of only 118 pc when measuring the parallax of stars in the cluster, a technique that should produce the most direct and accurate results. Subsequent work consistently argued that Hipparcos’ distance measurement for the Pleiades was erroneous.
In particular, distances derived from the group using the Hubble Space Telescope and the infrared color magnitude diagram (called “spectroscopic parallax”) favor a distance between 135 and 140 pc; while a dynamic distance from optical interferometric observations of the Double Pleiades Atlas favors a distance of 133 to 137 pc.
However, the author of the 2007-2009 catalog of revised Hipparcos parallaxes reaffirmed that the distance to the Pleiades is approximately 120 pc and questioned the dissenting evidence.
Discoveries
Recently, Francis and Anderson proposed that a systematic effect on Hipparcos parallax errors for stars in clusters biases the calculation using the weighted average and gave a Hipparcos parallax distance of 126 pc and a photometric distance of 132 pc based on stars in the moving groups AB Doradus, Tucana-Horologium, and Beta Pictoris, which are similar in age and composition to the Pleiades. Those authors point out that the difference between these results can be attributed to a random error.
The most recent results using very long baseline interferometry (VLBI) (August 2014) and preliminary solutions using Gaia Data Release 1 (September 2016) and Gaia Data Release 2 (August 2018) determine distances of 136.2 ± 1.2 pc, 134 ± 6 pc, and 136.2 ± 5.0 pc, respectively. Although the Gaia Data Release 1 team was cautious about the result, the VLBI authors state “that the Hipparcos distance to the Pleiades cluster is erroneous.”
Selected distance estimates to the Pleiades.
Years Distance (pc)
- 1999 125 Hipparcos
- 2004 6 ± 3.1 Hubble Fine Guidance Sensor
- 2009 2 ± 1.9 Revised Hipparcos
- 2014 2 ± 1.2 Very long baseline interferometry
- 2016 134 ± 6 Gaia Data Release 1
- 2018 2 ± 5 Gaia Data Release 2
For another debate on the distance, also with a different measurement from Hipparcos, although this time it suggested a greater distance.
Composition
The radius of the cluster’s core is approximately 8 light-years, and the tidal radius is approximately 43 light-years. The group contains more than 1,000 statistically confirmed members, although this figure excludes unresolved binary stars.
Its light is dominated by young, hot, blue stars, 14 of which can be seen with the naked eye depending on local observing conditions. The total mass contained in the cluster is estimated to be approximately 800 solar masses and is dominated by fainter, redder stars.
The cluster contains many brown dwarfs, which are objects with less than 8% of the Sun’s mass, which are not heavy enough for nuclear fusion reactions to begin in their cores and become stars themselves. They may constitute up to 25% of the total population of the group, although they contribute less than 2% of the total mass.
Astronomers have made great efforts to find and analyze brown dwarfs in the Pleiades and other young groups because they are still relatively bright and observable, while brown dwarfs in older groups have faded and are much more difficult to study.
Brightest stars
The nine brightest stars in the Pleiades are named after the Seven Sisters of Greek mythology: Sterope, Merope, Electra, Maia, Taygeta, Celaeno, and Alcyone, along with their parents Atlas and Pleione. As daughters of Atlas, the Hyades were sisters of the Pleiades. The English name for the group itself is of Greek origin, although its etymology is uncertain.
Suggested derivations include: from “to navigate,” making the Pleiades the “navigators,” from “full of many,” or from “flock of doves.”
Age and future evolution
The ages of star clusters can be estimated by comparing the Hertzsprung-Russell diagram for the cluster with theoretical models of stellar evolution. Using this technique, ages for the Pleiades have been estimated to be between 75 and 150 million years.
The wide range of estimated ages is the result of uncertainties in stellar evolution models, which include factors such as convective over-impulse, in which a convective zone within a star penetrates a non-convective zone, resulting in older apparent ages.
Another way to estimate the age of the group is to observe the lowest-mass objects. In normal main-sequence stars, lithium is rapidly destroyed in nuclear fusion reactions.
However, brown dwarfs can retain their lithium. Because the ignition temperature of lithium is very low, at 2.5 × 10 6 K, higher-mass brown dwarfs will eventually burn it out, and thus determining the highest mass of brown dwarfs still containing lithium in the group can give an idea of its age. Applying this technique to the Pleiades gives an age of around 115 million years.
The group is slowly moving in the direction of the feet of what is now the constellation Orion. Like most open groups, the Pleiades will not remain bound together by gravity forever. Some component stars will be ejected after close encounters with other stars; others will be stripped away by tidal gravitational fields.
Calculations suggest that the cluster will take around 250 million years to disperse, with gravitational interactions with giant molecular clouds and the spiral arms of our galaxy also accelerating its demise.
Reflection nebula
With larger amateur telescopes, the nebulosity around some of the stars can be easily seen, especially when taking long exposure photographs. Under ideal observing conditions, some hints of nebulosity around the cluster can even be seen with small telescopes or average binoculars.
It was previously thought that the dust was left over from the formation of the group, but at the generally accepted age of about 100 million years for the group, almost all of the originally present dust would have been dispersed by radiation pressure.
Possible planets
By analyzing deep infrared images obtained by the Spitzer Space Telescope and the Gemini North telescope, astronomers discovered that one of the stars in the cluster, HD 23514, which has a mass and luminosity slightly greater than that of the Sun, is surrounded by an extraordinary number of hot dust particles.
