NGC 2818
Encyclopedia
NGC 2818 is a planetary nebula
located in the southern constellation
Pyxis
(The Compass). This spectacular nebula 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
.
It is often cited as a member of the open cluster
NGC 2818A framing it, however, radial velocity
differences between the planetary nebula
and open cluster suggest a chance alignment.
Red giants in open clusters. IX. NGC 2324, 2818, 3960 and 6259, A&A
and M46
.
Partly because of their small total mass, open clusters have relatively poor gravitational cohesion. Consequently, open clusters tend to disperse after a relatively short time, typically some 10 million years, because of external gravitational influences amid other factors. Under exceptional conditions, open clusters 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 - a blink of the eye in cosmic terms. To date, no true association has been established between open clusters and planetary nebulae.
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...
located in the southern constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Pyxis
Pyxis
Pyxis is a small and faint constellation in the southern sky. Its name is Latin for a mariner's compass...
(The Compass). This spectacular nebula 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
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
.
It is often cited as a member of the open cluster
Open cluster
An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist...
NGC 2818A framing it, however, radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
differences between the planetary nebula
Planetary nebula
A planetary nebula is an emission nebula consisting of an expanding glowing shell of ionized gas ejected during the asymptotic giant branch phase of certain types of stars late in their life...
and open cluster suggest a chance alignment.
Red giants in open clusters. IX. NGC 2324, 2818, 3960 and 6259, A&A
NGC 2438
NGC 2438 is a planetary nebula about 3,000 light years away in the constellation Puppis. It was discovered by William Herschel on March 19, 1786....
and M46
Messier 46
Messier 46 is an open cluster in the constellation of Puppis. It was discovered by Charles Messier in 1771...
.
Partly because of their small total mass, open clusters have relatively poor gravitational cohesion. Consequently, open clusters tend to disperse after a relatively short time, typically some 10 million years, because of external gravitational influences amid other factors. Under exceptional conditions, open clusters 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 - a blink of the eye in cosmic terms. To date, no true association has been established between open clusters and planetary nebulae.