A third category was added in 1912 when Vesto Slipher showed that the spectrum of the nebula that surrounded the star Merope matched the spectra of the Pleiades open cluster.
Nje kategori e trete u shtua ne 1912, kur Vesto Slipher tregoi se spektri i nje Mjegullnaje te rrethuar nga Yjet perputhej me spektret e grupeve tjera.