Diffuse nebulae are stellar nurseries, huge clouds of gases inside which stars are born. Seen through the telescope or binoculars they appear as small luminous clouds on the starry background. When searching for them choose your timing, the most diffuse nebulae are visible in the winter and summer sky.
At first you may have a little trouble in finding them, with few exceptions diffuse nebulae are faint and difficult to observe.
When you’ve found the spot in which the nebula should be, use a low power eyepiece and slowly sweep the field, in the same time looking carefully for any area that is brighter than the background sky.
Now look at the nebula and try to see as many details as you can. Are there any stars inside the nebula? Is the light distributed uniformly or are there brighter and darker parts? If you own a large telescope you could also look if there is any hint of color inside the nebula.
Diffuse nebulae observed with a 114-mm telescope
With my 114-mm telescope, on a dark country sky, M42 barley fits in my eyepiece field at 36x and the details are incredible. The nebula is divided into three areas, according to brightness.
The brightest part surrounds the four young stars formed inside the nebula, then continues in the nebula’s “wings”.
The medium brightness part is located just southeast of the Trapezium Cluster, and finishes in the point where the eastern wing begins.
The faintest area is located in the southern part of the nebula, and continues in the wings. The eastern wing is the brightest and longest, and the western one is shorter but wider. In the northern part of the nebula, just above the Trapezium, you will see a dark intrusion that gives the characteristic aspect of the nebula.
North of M42, surrounding a 7.6 magnitude star, you will find M43. Its light is pretty dim, uniformly distributed, without brighter or darker parts. The nebula is elongated towards the south.
NGC 1977 surrounds five stars arranged in an “U” pattern, and is elongated towards the south. It is split into three areas, according to brightness.
The brightest part surrounds the star 42 Orionis. The medium bright part is located to the west. The faintest part is visible with averted vision towards the south.
NGC 1973 surrounds an 8th-magnitude star. It is easily seen without averted vision, round shape, the northern part is the brighter.
M78 is faint and elongated, it surrounds two 10th-magnitude stars.
M8 it is very bright, with lots of subtle details. Some parts are brighter and I’ve easily seen the dark lane that traverses it.
M20 is bright and very impressive. The nebula surrounds the triple star HN 40. Towards the edges of the nebula some darker areas are visible, but I’ve didn’t manage to see the three dark lanes that cross the nebula.
M17 is located in a very rich star field, it is very bright and it has an elongated shape, with the northern part a little brighter. A fainter part springs from one side of the nebula and becomes gradually fainter.
NGC 2071 is extremely faint, I’ve only managed to see it by using averted vision. It has a round shape, surrounds a 8th-magnitude star.
M16, the Eagle Nebula, is faint and difficult to see even with averted vision.
Diffuse nebulae observed with a 60-mm telescope
I can easily see M42 with my telescope, in the northern part of the nebula I’ve seen a dark intrusion, using averted vision. The “wings” of the nebula are not visible even on a dark sky. The Trapezium Cluster is easily seen, composed of three stars.
M43 is difficult even with averted vision. Diffuse aspect, it surrounds a star.
M17 – Very bright nebula, elongated shape. The northern part is almost straight, in the south I can see a small prominence.
M8 – Diffuse light surrounding a grouping of stars. Easily seen with averted vision.
M20 – Very difficult. Diffuse light surrounding a star.