The Fetus Nebula
Posted by Brian Ventrudo
Planetary nebulae are the snowflakes of the night sky. No two are alike, yet every one comes from the same process, in this case the death throes of a middleweight star. Compared to star clusters, many planetaries are small and faint, so stargazers must acquire a taste for these objects and a little patience and skill to see them well. But the rewards are worth it. Many planetaries have fascinating shapes and structures, each a testament to complex physical processes going on around the central star. One of the most beautiful and intriguing of nearby planetary nebulae is NGC 7008, the often-photographed Fetus Nebula in Cygnus. This mottled blue-green ellipse presents a small but lovely image to the patient observer armed with a small telescope.
Like many planetary nebulae, NGC 7008 was first cataloged by William Herschel in the late 18th century. Though Herschel knew many planetary nebulae—in fact he invented the term for these nebulae because they resembled blue-green planets—Herschel did not classify it as such because of its odd appearance. He lumped it in with shapely bright emission nebulae like the Orion and Lagoon Nebulae.
Later astronomers with better telescopes confirmed NGC 7008 was a planetary nebula, but were otherwise puzzled by this object. The nebula has at least two separate shells (see image below). The inner shell expands faster than the outer because of sporadic mass loss of the dying central star. The symmetry of the shells is further broken, possibly by interaction with the uneven interstellar medium. It all makes for a fascinating and richly-detailed object which strongly resembles the shape of an early-stage human fetus.
This 10th-magnitude nebula looks best in a 6-inch or larger scope at 70-100x, though it is visible in a smaller scope. Use enough magnification to see the mottled, elliptical shape of the nebula without spreading its light too thinly in your field of view. A UHC or OIII filter helps improve the contrast. Look for the pleasing double star h1606 on the south edge of the nebula. These gold and blue stars are 9th and 10th magnitude, respectively, and spaced by 18 arcseconds. The nebula itself, by contrast, spans about 95 arcseconds along its major axis.
Here’s what the nebula looks like in a small telescope…
NGC 7008 lies about 2,800 light years from Earth and spans about 1.3 light years.
The Fetus lies in the constellation Cygnus near the border with Cepheus, about halfway between between brilliant Deneb and Alderamin (alpha Cephei). Its immediate neighborhood is somewhat barren of bright stars to guide the way. Start at 2nd-magnitude Alderamin, then hop 3.5º southwest to η (eta) Cephei. From there, hop 4° due south to find a 5th-magnitude golden-yellow star. Then use your finder to hop 3.5º southeast. The nebula should be near the center of your field of view.
To learn more about planetary nebulae, have a look at this past article…