OJBs Web Site: NGC 3918

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NGC 3918

NGC 3918 is a tiny (10 arc seconds in diameter), bright planetary nebula near the Southern Cross, but actually in the constellation Centaurus. Exact position is right ascension 11h 47m 30s and declination -57.7182°. The magnitude is 8.4 and the central star is around 13 (quite dim). The distance is approximately 3000 light years. Its true diameter is about 0.3 light year.

It is easy to find but needs some magnification - I used a 36cm Newtonian reflector - although the disk can be seen in telescopes as small as 5 cm. A white central star with a blue-green, roughly spherical, ring of gas is obvious in larger telescopes. While this isn't a spectacular object it does have quite intense colour.

Above are some drawings which should give you an idea what the nebula looks like in common amateur telescopes. Note that this is what it looks like to me - these aren't photos and may not be too accurate. The sharpness and colour of the object varies with the type of telescope and seeing conditions, of course. Some people say its very similar to the planet Uranus - a small green disk.

This is how to find NGC3918. First locate the Southern Cross (if you are observing from a southerly point its circumpolar and very easy to find). You need to follow a line through the shorter axis of the "kite" shape and continue on a similar distance away from the Pointers. Start at Beta Crucis (which is a close binary with a combined brightness of over 20,000 Suns) and cross across to Delta Crucis (remember that Crux is the smallest constellation, so this isn't very far). Continue the same distance again and look for a blue-green object. Remember that you will need a fair amount of magnification to see the disk. While you are there why not have a look at the other amazing objects in the area!

This is another object in the same area. NGC 3532 is a large open cluster, which is easy to find without optical aid because its visual magnitude is 3. Another name occasionally used is "the Wishing Well Cluster" (because the twinkling stars resemble silver coins shimmering at the bottom of a wishing well). The cluster is apparently almost 1° across and is just 1300 light years away. Location is 11h 06.4m right ascension, and -58° 40m declination.

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