Cecilia Payne-Gaposhkin mentions that the Pleiades contain some white dwarf stars. These stars give rise to a specific problem in the question of the cluster's stellar evolution. How can white dwarfs exist in such a young stellar system? As there is not only one it is most certain that they are original cluster members and not field stars which have been captured (a procedure which doesn't work effectively in the rather loose open clusters anyway). From the theory of stellar evolution it follows that white dwarfs can not have masses above a limit of about 1.4 solar masses (the Chandrasekhar limit) as they would collapse due to their own gravitation if they were more massive. But stars with such a low mass evolve so slow that it takes them billions of years to evolve into that final state, a lot longer than the 100 million year age of the Pleiades cluster.

The only possible explanation seems to be that these white dwarf stars were once massive so that they evolved fast, but due to some reason (such as strong stellar winds, mass loss to close neighbours or fast rotation) has removed the greatest part of their mass. It is also possible that they have lost another considerable percentage of their mass in a planetary nebula.

New observations of the Pleiades since 1995 have revealed several candidates of exotic star types, or star-like bodies, the so-called brown dwarfs. These hitherto hypothetical object are thought to have a mass intermediate between that of a giant planet (like Jupiter) and small stars (the theory of stellar structure indicates that the smallest stars, bodies that produce energy by fusion at some time during their life-cycle, must have at least about 6-7 percent of one solar mass, i.e. 60 to 70 Jupiter masses). They are assumed to be visible in the infrared light, have a diameter of about, or less than, that of Jupiter (143,000 km), and density 10 to 100 times that of Jupiter, as their much stronger gravity compresses them.