The Pleiades, also known as the Seven Sisters and Messier 45, is an open star cluster constitutes of middle-aged, hot B-type stars in the of the constellation Taurus, the Bull. It is among the star clusters nearest to Earth and is the cluster most obvious to the naked eye in the night sky, often confused as Ursa Major by an inexperienced observer. Easily visible from a light-polluted sky.
The cluster is dominated by hot blue and luminous stars that have formed within the last 100 million years; all seven brightest stars are these. Reflection nebulae, around the brightest stars, were once thought to be leftover material from the formation of the cluster, but are now considered likely to be an unrelated dust cloud in the interstellar medium through which the stars are currently passing. Nebulosity can only be seen in long exposure shots or by a large aperture telescope, if you are in a dark sky location then you can also see a hint of haze from a pair of astronomical binoculars.
The Pleiades are mainly visible in winter nights – from September to February – in the Northern Hemisphere, and are easily visible out to mid-Southern latitudes. They have been known since antiquity to cultures all around the world. In Hinduism, the Pleiades are known as Kritika (one of the nakshatras) and are associated with the war-god Kartikeya. They are also mentioned three times in the Bible. The Chinese know M45 as the Hairy Head of the White Tiger of the West. Native Americans used to measure eyesight by the number of stars the observer could see it in.
Observations throughout the history
The earliest mention can be traced back to 2000 BC to the Babylonian star catalogs which have mentions of Pleiades. The earliest visual depiction of M45 is seen in a Northern German Bronze Age artifact known as the Nebra sky disk, dated to approximately 1600 BC. Some Greek astronomers considered them to be a distinct constellation while some scholars of Islam suggested that the Pleiades (ath-thurayya) are the “star” mentioned in Sura An-Najm (“The Star”) of the Quran.
Galileo Galilei was the first astronomer to view the Pleiades through a telescope. He thereby 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.
The Pleiades have long been known to be a physically related group of stars rather than any chance alignment, i.e. they are clustered together in space, and formation of stars is related. John Michell calculated in 1767 that the probability of a chance alignment of so many bright stars was only 1 in 500,000, and so surmised that the Pleiades and many other clusters of stars must be physically related. When studies were first made of the stars’ proper motions, it was found that they are all moving in the same direction across the sky, at the same rate, further demonstrating that they were related.
Charles Messier measured the position of the cluster and included it as M45 in his catalog of comet-like objects, published in 1771.
Edme-Sébastien Jeaurat then drew in 1782 a map of 64 stars of the Pleiades from his observations in 1779, which he published in 1786.
Under ideal observing conditions, some hint of nebulosity around the cluster may even be seen with small telescopes or average binoculars. It is a reflection nebula, caused by interstellar dust (unrelated to the stars) reflecting the blue light of the hot, young stars.
Nebulosity that is visible around the star cluster was earlier thought to be leftover dust and gas from the formation of stars of M45, but the studies have shown that this nebulosity may have been formed by deceleration due to radiation pressure as the dust has moved towards the stars. In conclusion, the interstellar dust although unrelated to the cluster has aligned in our line of sight due to the radiation pressure developed in that area of space while the stars were forming in M45.
Analyzing deep-infrared images obtained by the Spitzer Space Telescope and Gemini North telescope, astronomers discovered that one of the cluster’s stars is surrounded by an extraordinary number of hot dust particles. This could be evidence for a possible planet formation or a young solar system.
How to spot it in the night sky?
On a winter night, look out for the brightest star in the sky, it will be in the south-east. That’s Sirius, now look for three stars in a perfectly straight line in the southern sky (you can’t miss them). Join Sirius and these three with a straight line and extend it towards the north-west. Go for about the same distance as from Sirius to the middle star (in the group of those three stars) in north-west direction – there will be a small fuzzy group of stars, if you have great eyesight it might appear like a small Ursa major – that’s the Pleiades. The size will be slightly greater than a full moon.