The Pleiades, also known as Messier 45 (M45), is a type 'c' Open Cluster in the Taurus constellation. At least about 500 mostly faint stars belong to the Pleiades star cluster. Their density is pretty low, compared to other open clusters. This is one reason why the life expectation of the Pleiades cluster is also pretty low.

The Pleiades also carry the name "Seven Sisters"; according to Greek mythology, seven daughters and their parents. Their Japanese name is "Subaru", which was taken to christen the car of same name. The Persian name is "Soraya", after which the former Iranian empress was named.

On Stardate 41463.9 the USS Enterprise-D was on a mission to map new planets found in the Pleiades Cluster. In one of these planets, Velara III, the Enterprise crew discovered glowing lifeforms that declared 'war' on the 'bags of mostly water' – Humans (TNG: "Home Soil"). While on a date, on a holographic recreation of Mars, with Denara Pel, Voyager's Doctor pointed out the Pleiades Cluster as among the many star formations and astronomical highlights seen from Mars. (VOY: "Lifesigns")

In reality, the Pleiades are apparently imbedded in nebulous material. The Pleiades nebulae are blue-colored, which indicates that they are reflection nebulae, reflecting the light of the bright stars situated near (or within) them. Physically, the reflection nebula is probably part of the dust in a molecular cloud, unrelated to the Pleiades cluster, which happens to cross the cluster's way. It is not a remainder of the nebula from which the cluster once formed, as can be seen from the fact that the nebula and cluster have different radial velocities, crossing each other with a relative velocity of 6.8 mps, or 11 km/sec.

Pleiades contains several white dwarfs, which leads to a problem in stellar evolution. How can white dwarfs exist in such a young star cluster? As they are numerous, it's highly probable that the stars are original cluster members and not field stars that have been captured. Stellar evolutionary theory insists that the upper mass limit for white dwarfs – the so-called Chandrasekhar limit – is about 1.4 solar masses. Yet, stars with such a low mass evolve so slowly that it takes billions of years to attain that final state, not the mere 100 million-year age of the Pleiades cluster.

A possible explanation may be that these white dwarfs were formerly very massive and therefore evolved rapidly. Effects such as strong stellar winds, mass loss to close neighbors, and/or rapid rotation subsequently caused them to lose a high proportion part of their mass in planetary nebulae. Thus the residual stars – previously the stars' cores – have come below the Chandrasekhar limit, forming the observed stable white dwarfs.

New observations of the Pleiades since 1995 have revealed several candidates of an exotic type of stars, or starlike bodies, the so-called Brown Dwarfs. These hitherto hypothetical objects are thought to have a mass intermediate between that of giant planets (like Jupiter) and small stars (the theory of stellar structure indicates that the smallest stars, i.e. bodies that produce energy by fusion somewhen in their lifetime, must have at least about 6..7 percent of one solar mass, i.e. 60 to 70 Jupiter masses). So brown dwarfs should have 10 to about 60 times the mass of Jupiter. They are assumed to be visible in the infrared light, have a diameter of about or less that of Jupiter (143,000 km), and a density 10 to 100 times that of Jupiter, as their much stronger gravity presses them tougher together.