NGC 40, mag. +10.7, is a planetary nebula located in the northern constellation of Cepheus. It was discovered by William Herschel on November 25, 1788. He described it as "a 9th magnitude star, surrounded with milky nebulosity". Herschel used his 475mm (18.7-inch) telescope to make the discovery, but today's amateur astronomers don't require such a large instrument and it can be glimpsed with just a 100mm (4-inch) refractor. NGC 40 is also known as the Bow Tie nebula, a nickname it shares with another planetary nebula, NGC 2440 in Puppis. It's listed as number 2 in the Caldwell catalogue.
NGC 40 is located just over 17 degrees from the North Celestial Pole and is therefore circumpolar from most northern latitudes. It's one of the finest examples of its type in the far northern section of sky. The best time to look for the nebula is during October, November and December when it appears high in the sky during early evening. The Bow Tie nebula is also visible, although lower down, from most tropical latitudes. However, from southern temperate locations it never rises above the horizon.
Finding NGC 40 can be tricky as it's positioned in a star poor region of eastern Cepheus. One method is to imagine a line connecting Errai (γ Cep - mag. +3.21) with γ Cassiopeiae (mag. +2.15). The planetary lies approximately one-third of the way along this line.
NGC 40 can be spotted with 100mm (4-inch) telescopes as an out of focus "star" tucked away between two 9th magnitude stars. It spans just 36 arc seconds in diameter and therefore requires at least a medium size aperture to see any significant detail. A 200mm (8-inch) scope at about 200x magnification, reveals a slightly oval hazy patch of light with a bright central star (mag. +11.6). The nebula is easily distinguishable from the surrounding stars and on closer inspection, appears bluish-green in colour. Like most planetaries, it appears to blink on and off when you look at. This is especially noticeable when switching between averted and direct vision. Large amateur scopes, of the order of 300mm (12-inch) aperture or more, show brightness variations, twists and knots across the complete nebula face.
NGC 40 is 3,500 light-years distance. It has an actual diameter of 0.6 light-years.
NGC 40 Data Table
|Name||Bow Tie Nebula|
|Object Type||Planetary Nebula|
|RA (J2000)||00h 13m 01s|
|DEC (J2000)||+72h 31m 20s|
|Apparent Size (arc mins)||0.6 x 0.4|