Observing the Fetus Nebula

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This NGC 7008 image was created via color synthesis of Red & Blue Digital Sky Survey images. North is down and east is to the right, as typically seen in a telescope.

NGC 7008 is one of my all-time favorite objects in the eyepiece. Offering something interesting to see in all but the smallest telescopes, this planetary nebula is a must see. If you have not seen it for yourself, now is a good time to have a look. It will be an evening object through October.

Called the Fetus nebula because of its appearance in photographs, NGC 7008 is a planetary nebula located in the constellation of Cygnus. Its integrated magnitude is ~12, making it detectable in all but the smallest telescopes.

NGC 7008 was discovered by by William Herschel in 1787 using his 18.7-inch telescope. It was originally given the designation H I-192.

My first view of this planetary came way back in 1998 in a six-inch (15 cm) Newtonian. I had a peek before reading about it or even looking at any pictures. I was very surprised at what I saw. Rather than the small, oval disk that typifies these objects, it was more of a dash, emanating to the north from the nearby pair of bright stars. At 50x it reminded me of a tiny comet with tail. The best view was at 135x, where it looked like a hazy dash of light. At 270x it looked like an edge-on spiral galaxy. I was so struck by this that I went inside to verify that I was indeed looking at a planetary nebula! I couldn't wait to have a look in a larger instrument.

Several years later I completed my 18-inch (46 cm) Dob, and NGC 7008 was on my list of objects to see right away. Much more detail was apparent, and even at 94x the full circle of the nebula appeared, ring-like, symmetrical about a bright central star. I noted that the north end had a bright knot in it, as seen in photographs. The oval ring stretched around to the west from there, meeting a similar, but less prominent knot on the southwest end. The eastern part was almost dark, which is what leads to the strange question-mark or dash-like appearance in smaller instruments. The OIII filter improved the contrast with the nebula in general and brightened the appearance of the knots, but it didn't help much in discerning the dimmer portions of the ring. I am told that in larger instruments even more detail is apparent.

The colorful wide double star at the south end of the nebula is also very pleasing. The primary is HD 235422, a cool red 9.5-magnitude K7 star. It forms a double, known as HJ 1606, with a 10 to 11th-magnitude star some 18.5 arc seconds away. This star is somewhat bluish in color, making a nice contrast with the primary.

Observing the Fetus Nebula for Yourself

One of the reasons that NGC 7008 isn't observed more often is that it lies way out in the middle of nowhere. It's not an easy find like the Ring Nebula. It lies on a rough line between Deneb (Alpha Cyg) in Cygnus, and Alderamin (Alpha Cep) in Cepheus, and there are few bright stars in the region. If you use traditional star hopping to find your objects, it may be difficult and time consuming.

The link below is for a SkyTools finder chart that may help. It is a pdf that you can view on your mobile device or print and take into the field. The view on the left shows what you would see with your unaided eye near Tucson, AZ. The three concentric circles are for a typical unmagnified finder such as a Telrad. Start by pointing your own finder at the same location in the naked eye sky in relation to the bright stars. This will work even in a light polluted sky.

The idea is that even if you aren't exactly in the right spot you will be pretty close. Insert your widest field eyepiece. Look for a faint fuzzy spot. If you are lucky, it will be apparent.

The view in an 8-inch telescope at 150x.

If not, you will need to match the star patterns in the eyepiece to find the objects. Note that the chart is for an 8-inch telescope with little light pollution. Normally you would make a chart to match your own telescope and conditions. If you use an alt/az scope in the evening, the orientation should be about the same as what you see in the eyepiece. Otherwise you may need to rotate the view to match. Look for a pattern of bright stars that you see in your eyepiece. It's ok to move the scope around a bit to see what is in the area. Once you find a memorable pattern in the eyepiece, look for it on the chart. Once you have identified where you are looking, it should be apparent which direction you need to move the telescope to find the nebula.

Once found, center it, and insert a higher power eyepiece.

Finder Chart (pdf)

The Science of NGC 7008

Approximately 2,800 light years distant, the nebula itself is about a light year in diameter. A planetary nebula is the result of a sun-like star that has reached a sort of old age. The Hydrogen that was fused into Helium to power the star in a stable manner for billions of years becomes scarce in the interior. As a result the outer atmosphere expands and cools, and the star becomes a red giant. Eventually helium begins to fuse in the core and the star swells a second time. This is called the Asymptotic Giant Branch (ABG) phase, named for its position on the H-R diagram. The star begins to pulse, expelling the loosely held material in its outer envelope. This material expands away from the star, creating the so-called planetary nebula.

NGC 7008 appears to be two different shells of gas, one within the other. It has been suggested that this might be the result of a binary pair of sun-like stars at the center of the nebula, with first one passing into the AGB phase, followed by the other. But it is also possible that we are seeing the result of complex interactions of the ejected material with the surrounding interstellar medium, possibly even influenced by the presence of planets and other minor objects orbiting the star.