The “Blue Planetary”

June 7, 2009

So enough talk about planetary nebulae and white dwarfs.  Let’s see one.  Deep in the southern sky in Centaurus lies the “Blue Planetary”, cataloged as NGC 3918, and one of the brightest planetaries in the southern sky.  Its blue-green color makes it look like the planet Uranus or Neptune.


The Basics

• The Blue Planetary lies in a rich star field in the constellation Centaurus, just 2.5 degrees NW of delta Crucis in the Southern Cross (RA 11h50m, Dec -57d11m)

• It was first recorded in 1834 by John Herschel, whose father William, coined the term “planetary nebula” because of the resemblance of some of these objects to Uranus, which he discovered.

• The hot core of the central star of the nebula, which is about to become a white dwarf, is too faint to see with a small scope.

A Deeper Look

• Like most small planetaries, NGC 3918 can take a lot of magnification… try at least 150x or so to bring out the bright disk.  It’s about 10″ across; magnitude 8.0  The trick is to use magnification low enough to find it, and high enough to resolve it into a disk.  Try an OIII filter too.

•The nebula lies about 3,000 ly away from us and spans 0.2 light years… roughly 12x the diameter of Pluto’s orbit!

• NGC 7662 in Andromeda is another blue-green planetary.  It’s well positioned for northern observers in autumn.  Most planetaries emit blue-green light, but some are particularly bright at this wavelength.

Good To Know

The blue-green color of this planetary nebula comes from doubly-ionized oxygen atoms that emit light at a wavelength of 500 nm, which is almost the color light reflected from of sea water in a shallow tropical bay.

Bonus Object

NGC 3960, a faint open cluster just 1.5 degrees north of the Blue Planetary at RA  11h 51 min Dec  – 55° 41′.  It’s a pleasant cluster, not terribly bright at magnitude 8.3, but easily visible in a small telescope.  The cluster formed about 0.8 billion years ago.  That’s old for a cluster; it’s about the same age at M44.  NGC 3960 lies in a rich background of stars along this part of the Milky Way, but only about 45 stars belong to the cluster.

Posted in Celestial Objects