Starship Asterisk*

http://apod.nasa.gov/apod/ap090122.html

Hubble Snaps Images of a Nebula Within a Cluster

The unique planetary nebula NGC 2818 is nested inside the open star cluster NGC 2818A. Both the cluster and the nebula reside over 10,000 light-years away, in the southern constellation Pyxis (the Compass).

NGC 2818 is one of very few planetary nebulae in our galaxy located within an open cluster. Open clusters, in general, are loosely bound and they disperse over hundreds of millions of years. Stars that form planetary nebulae typically live for billions of years. Hence, it is rare that an open cluster survives long enough for one of its members to form a planetary nebula. This open cluster is particularly ancient, estimated to be nearly one billion years old.

The spectacular structure of NGC 2818 (also known as PLN 261+8.1) contains the outer layers of a sun-like star that were sent off into interstellar space during the star's final stages of life. These glowing gaseous shrouds were shed by the star after it ran out of fuel to sustain the nuclear reactions in its core.

Planetary nebulae can have extremely varied structures. NGC 2818 has a complex shape that is difficult to interpret. However, because of its location within the cluster, astronomers have access to information about the nebula, such as its age and distance, that might not otherwise be known.

Planetary nebulae fade away gradually over tens of thousands of years. The hot, remnant stellar core of NGC 2818 will eventually cool off for billions of years as a white dwarf. Our own sun will undergo a similar process, but not for another 5 billion years or so.

This Hubble image was taken in November 2008 with the Wide Field Planetary Camera 2. The colors in the image represent a range of emissions coming from the clouds of the nebula: red represents nitrogen, green represents hydrogen, and blue represents oxygen

I was looking at the three stars that are aligned in the upper right hand corner;and wondering if the distance is the same or further than the nebula. if they are the same distance away as the nebula; than they must be less than a light year apart as the nebula is 4 light years across. That would also make them part of the open cluster. At any rate it is a very beautiful picture and worthy of saving for a wallpaper.

Orin

NGC 2818 and NGC 2818A in Pyxis (the Compass)

Deep Sky Object of the Month: July 1999: NGC 2818A

----------------------------------------------
<<Pyxis is a small and faint constellation in the southern sky. Its name is Latin for a mariner's compass (it should not be confused with Circinus, which represents a draftsman's compass). Pyxis is completely visible in latitudes south of 53 degrees north from January through March. Pyxis (Latin: box) was introduced by Nicolas Louis de Lacaille in the eighteenth century; he called it Pyxis Nautica, but the name was shortened. The constellation is located close to those forming the old constellation of Argo Navis (the ship Argo), and in the 19th century astronomer John Herschel suggested renaming Pyxis to 'Malus, the mast', but the suggestion was not followed.>>

http://www.astrosurf.com/antilhue/ngc_2818_in_pyxis.htm
----------------------------------------------
_The unique planetary nebula NGC 2818_ by Dufour, R. J.
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 287, Dec. 1, 1984, p. 341-352.

<<The planetary nebula NGC 2818, or PK 261 + 8 deg 1, is of special interest due to its apparent association with a Population I open cluster (of the same designation). The results of new observations of the nebula are presented, including interference-filter imagery, photoelectric photometry, and ground-based and IUE spectroscopy. Analysis of its composition based on new IUE and ground-based spectroscopy and model calculations of the very similar planetary nebula NGC 2440 suggests that NGC 2818 is a prototype of the He-rich and N-rich Type I group.

NGC 2440 :
http://antwrp.gsfc.nasa.gov/apod/ap070215.html
http://antwrp.gsfc.nasa.gov/apod/ap060507.html

Compared to the composition of H II regions in the solar neighborhood, He is 60 percent overabundant, C is 0.3 dex lower, N is 1.0 dex higher, and O, Ne, Si, Cl, and Ar are essentially at comparable values. Analysis of kinematical and morphological data suggests that the nebula is probably associated with the star cluster (revised distance = 3.5 kpc). If so, then it is of exceptional size (radius approximately 1.1 pc), mass (greater than or equal to 0.6 solar mass), expansion velocity (52 + or - 3 km/s), and age (possibly as old as 22,000 yr). Some implications of the results regarding the masses of planetary-nebula progenitors, the nucleosynthesis processes in stars of intermediate mass, and the formation of planetary nebulae are also discussed.>>
------------------------------------------

http://www.skyhound.com/sh/archive/mar/NGC_2818.html

<<Where the heck have I been? That's what I found myself thinking as I looked at this wonderful planetary nebula embedded in an open cluster. These two objects both share an NGC number and some references use the designation 2818A to describe one or the other of the two. Planetary nebulae in open clusters make for a rare visual treat and the planetary that appears to lie in M46 is one of my all-time favorite objects.

M46:
http://antwrp.gsfc.nasa.gov/apod/ap060910.html
http://antwrp.gsfc.nasa.gov/apod/ap990305.html

The planetary nebula NGC 2818 is relatively large and bright with interesting structure. This planetary is similar in size and brightness to the much better known "Bug" (NGC 6302) in Scorpius.

NGC 6302:
http://apod.nasa.gov/apod/ap070429.html
http://antwrp.gsfc.nasa.gov/apod/ap980602.html

It's relatively high surface brightness takes magnification well. Look for the appearance of two separate, parallel, lobes that arc toward one another, nearly touching at the ends. Also look for a fainter outer envelope, particularly off each end, using averted vision. I didn't detect any color, but Steve Coe notes that "A few dark lanes are seen in the planetary at 165X, it is light green at all powers." Although NGC 2818 has a lower surface brightness than NGC 2438 (the PN in M46) it does compare favorably and its intriguing structure will surely bring me back again and again.

The NGC 2818 cluster is fairly faint, mostly made up of 11th to 14th magnitude stars. It is scattered and irregular and does not immediately jump out at you as a cluster of stars. It doesn't begin to compare to M46. Regardless, the many stars do make a nice backdrop for the planetary. Interestingly, NGC 2818/A is the only cluster-planetary combination that is currently believed to be truly associated. Although the status of NGC 2438 as a member of M46 is not known for certain, most indications are that the planetary is a foreground object. The relatively young age of M46 makes the possibility of one of it's member stars being the progenitor of the planetary nebula fairly remote. The NGC 2818 cluster, on the other hand, is older (perhaps as old as Billion years).>>

I'm sure glad we didn't have to wait for our sun to explode before seeing something like this.

"NGC 2818 is one of very few planetary nebulae in our galaxy located within an open cluster. Open clusters, in general, are loosely bound and they disperse over hundreds of millions of years. Stars that form planetary nebulae typically live for billions of years. Hence, it is rare that an open cluster survives long enough for one of its members to form a planetary nebula. This open cluster is particularly ancient, estimated to be nearly one billion years old."

Is anything known about the ages of the other stars in the cluster"? If they are young, maybe they were triggered by the planetary nebulae...

sundog wrote:"NGC 2818 is one of very few planetary nebulae in our galaxy located within an open cluster. Open clusters, in general, are loosely bound and they disperse over hundreds of millions of years. Stars that form planetary nebulae typically live for billions of years. Hence, it is rare that an open cluster survives long enough for one of its members to form a planetary nebula. This open cluster is particularly ancient, estimated to be nearly one billion years old."

Is anything known about the ages of the other stars in the cluster"?
If they are young, maybe they were triggered by the planetary nebulae...

It's hard to believe that NGC 2818 had any effect at all on the cluster:
http://www.astrosurf.com/antilhue/ngc_2818_in_pyxis.htm

Can someone please clarify the use of the term "planetary" in this context... it has always confused me. If it is a nebula caused by an exploding star, why isn't it called a "stellar nebula"?

Annihilannic wrote:Can someone please clarify the use of the term "planetary" in this context... it has always confused me. If it is a nebula caused by an exploding star, why isn't it called a "stellar nebula"?

[b] http://en.wikipedia.org/wiki/Planetary_nebula [/b] wrote:
<<The first planetary nebula discovered was the Dumbbell Nebula in the constellation of Vulpecula, observed by Charles Messier in 1764 and listed as M27 in his catalogue of nebulous objects. To early observers with low-resolution telescopes, M27 and subsequently discovered planetary nebulae somewhat resembled the gas giants, and William Herschel, discoverer of Uranus, eventually coined the term 'planetary nebula' for them, although, as we now know, they are very different from planets.>>

Thanks, it all makes perfect nonsense now!