NGC 40 is a planetary nebula located in the northern constellation of Cepheus. It was discovered by William Herschel on November 25, 1788 who described it as "a 9th magnitude star, surrounded with milky nebulosity". Herschel used his 475mm (18.7) inch telescope to make the discovery but for today's amateur astronomers such a large scope isn't required, it can be glimpsed with just a 100mm (4-inch) instrument. 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 therefore circumpolar from most northern latitudes. It's one of the finest examples of its type in the far northern part of the sky. The best time to look for the nebula is during October, November and December when it appears highest in the sky during early evening. The Bow Tie nebula is also visible from most tropical locations although lower down. However, from southern temperate latitudes it's not visible at all.
Locating NGC 40 can be precarious as it's positioned in a star poor region of eastern Cepheus. One method to find it is by imagining a line connecting Errai (γ Cep - mag. +3.21) and γ Cas (mag. +2.15). The planetary lies approximately one-third of the way along this line.
Under dark skies, NGC 40 can be spotted with 100mm (4-inch) telescopes appearing as an out of focus star tucked away between two 9th magnitude stars. With an apparent magnitude of +10.7 and spanning just 36 arc seconds in diameter, this planetary requires larger apertures to see any significant detail. A 200mm (8-inch) scope at 200x magnification reveals a slightly oval hazy patch of light with the bright central star (mag. +11.6) easily visible. NGC 40 is easily distinguishable from surrounding stars and on closer inspection appears bluish-green in colour. Like many planetaries it appears to blink on and off as you look at. This is especially noticeable when focusing on and off the central star and/or switching between averted and normal vision. Large amateur scopes of the order of 300mm (12-inch) aperture or greater show brightness variations, knots and twists across the face of the nebula.
In total, NGC 40 is 3500 light-years distance and has an actual diameter of 0.6 light-years. It's a fine object for owners of medium and large amateur telescopes.
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|