History

The planetary nebula Abell 31 was discovered in 1955 by the American astronomer George Ogden Abell on the photo plates of the Palomar Observatory Sky Survey (POSS). In 1955 he published a first list of 13 globular clusters and the positions of 73 planetary nebulae. The PN was then listed as number 20 (A55 20). In 1966 Abell published a completed list including the size and description of the 86 planetary nebulae discovered on the POSS photo plates. The PN was then listed as nebula 31 (A66 31). He added the note: «Appears as a disk with superposed filaments.» ^{[331, 332]}

In 1959, the American astronomer Stewart Sharpless listed this nebula in his «Catalogue of H II Regions» as Sh 2-290, but noted that it could be a planetary nebula. ^[310]

The designation PK 219+31.1 originates from the two Czechoslovak astronomers Luboš Perek and Luboš Kohoutek, who in 1967 compiled a catalog of all the planetary nebulae of the Milky Way known at the time. ^[146]

The lesser known designation ARO 135 originates from 1971 survey of microwave radiation from planetary nebulae conducted by Canadian radioastronomer Lloyd A. Higgs using the 46-metre Algonquin Radio Observatory in Ontario, Canada. ^{[136, 137]}

Physical Properties

Abell 31 is an ancient planetary nebula, created when a low-mass star blows off its outer layers at the end of its life. It is one of the largest known planetary nebulae, with a diameter of about 7 parsecs. The interior glows from energized oxygen atoms. The bright side of the nebula is due to its interaction with ambient interstellar gas. ^[182] Estimated distances range from 235 pc to 542 pc. ^[145]