NGC 2818

Lore & Background

NGC 2818 consists largely of glowing gases from the star's outer layers ejected during the final stages of its life when it had run out of the fuel necessary to sustain its core fusion processes. The remnants of its core will remain as a white dwarf. The nebula presents a complex morphology, with an overall bipolar structure; its two lobes are somewhat broken and irregular. Filamentary structures radiate from the center, and near the center several cometary knots are present. The nebula's mass is estimated at 0.6 solar masses, and it is about 11,000 years old. The progenitor star was likely about 2.3 times the mass of the Sun, and the central star is very hot, with an effective temperature of 130 kK.

NGC 2818 appears very close to the open cluster NGC 2818A. Whether it is a member of the open cluster has been controversial: earlier papers found the radial velocity differences between the planetary nebula and open cluster to a chance alignment. However, more recent studies suggest that NGC 2818 is indeed a member of NGC 2818A. Partly because of their small total mass, open clusters have relatively poor gravitational cohesion, and they tend to disperse after a relatively short time, typically some 10 million years, though under exceptional conditions they can remain intact for up to 100 million years. Theoretical models predict that planetary nebulae can form from main-sequence stars of between 8 and 1 solar masses, which puts their age at 40 million years and older. Although there are a few hundred known open clusters within that age range, a variety of reasons limit the chances of finding a member of an open cluster in a planetary nebula phase. One such reason is that the planetary nebula phase for more massive stars belonging to younger clusters is on the order of thousands of years. Only one association has been established between open clusters and nearby nebulae, the extremely distant nebula PHR 1315-6555.

Reader's Guide

NGC 2818 is significant as a planetary nebula with a complex bipolar morphology, featuring broken lobes, filamentary structures, and cometary knots. Its estimated mass of 0.6 solar masses and age of about 11,000 years provide context for the late stages of stellar evolution. The central star's high effective temperature of 130 kK and the progenitor star's mass of 2.3 solar masses align with theoretical models of planetary nebula formation from stars between 1 and 8 solar masses. The nebula's legacy is tied to its controversial membership in the open cluster NGC 2818A. While earlier studies suggested a chance alignment based on radial velocity differences, more recent studies indicate that NGC 2818 may indeed be a member. This is notable because open clusters have poor gravitational cohesion and typically disperse within tens of millions of years, and the planetary nebula phase lasts only thousands of years, making such associations extremely rare. Only one other association between an open cluster and a planetary nebula has been established, the distant nebula PHR 1315-6555. Thus, NGC 2818 contributes to the understanding of stellar evolution within clusters and the rarity of such co-located objects.

Did You Know?

• NGC 2818 is located in the southern constellation Pyxis (The Compass).
• The nebula has a bipolar structure with broken and irregular lobes.
• Its central star has an effective temperature of 130 kK.
• Membership in the open cluster NGC 2818A remains controversial, with recent studies suggesting it is a member.