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 - Planetary Nebula (credit:- Steve and Paul Mandel/Adam Block/NOAO/AURA/NSF)

Finder Chart for NGC 40 (credit:- freestarcharts)

Finder Chart for NGC 40 - pdf format (credit:- freestarcharts)

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

NGC40
Caldwell2
NameBow Tie Nebula
Object TypePlanetary Nebula
ConstellationCepheus
Distance (light-years)3,500
Apparent Mag.+10.7
RA (J2000)00h 13m 01s
DEC (J2000)+72h 31m 20s
Apparent Size (arc mins)0.6 x 0.4
Radius (light-years)0.30
Age (years)4,500

Sky Highlights - July 2017

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for July

Meteor Shower
Southern Delta Aquariids (Aquarids) meteor shower peaks on July 29

Northern Hemisphere
Evening
West:- Mercury (mag. -0.5 to +0.3) (second half of month)
Southwest:- Jupiter (mag. -2.0)
South:- Saturn (mag. +0.2)
Midnight
West:- Jupiter
South:- Saturn
East:- Neptune (mag. +7.8)
Morning
Southwest:- Saturn
South:- Neptune
Southeast:- Uranus (mag. +5.8)
East:- Venus (mag. -4.1)

Southern Hemisphere
Evening
West:- Mercury (second half of month)
Northwest:- Jupiter
East:- Saturn
Midnight
West:- Jupiter
North:- Saturn
East:- Neptune
Morning
West:- Saturn
North:- Neptune
Northeast:- Venus, Uranus

Deep Sky

Small telescopes:-
Messier 13 - M13 - Great Hercules Globular Cluster
Messier 92 - M92 - Globular Cluster
Messier 11 - M11 - The Wild Duck Cluster (Open Cluster)
Messier 7 - M7 - The Ptolemy Cluster (Open Cluster)
Messier 6 - M6 - The Butterfly Cluster (Open Cluster)
Messier 4 - M4 - Globular Cluster
Messier 8 - M8 - Lagoon Nebula (Emission Nebula)
Messier 16 - M16 - Eagle Nebula (Emission Nebula with Open Cluster)
Messier 20 - M20 - Trifid Nebula (Emission and Reflection Nebula)

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