The Helix Nebula

NGC 7293, the Helix Nebula, is a planetary nebula in the constellation Aquarius. Image credit: Warren Keller at Billionsand Billions.com. Additional credit: Piermario Gualdoni and Alessandro Cipolat Bares.

NGC 7293, the Helix Nebula, is a planetary nebula in the constellation Aquarius. Image credit: Warren Keller at Billionsand Billions.com. Additional credit: Piermario Gualdoni and Alessandro Cipolat Bares.

The showpiece of the constellation Aquarius is the grand and elusive Helix Nebula, NGC 7293. One of the closest and apparently largest of all planetary nebulae, the Helix is one of the few sights that’s easier to see in a small telescope than in a large one. In images, like the excellent collaboration above led by Warren Keller at BillionsandBillions.com, the nebula looks like an expansive eye in deep sky. Some refer to the Helix as the “Eye of God”.

Like all planetary nebulae, the Helix is a region of rarefied gas thrown off by a dying star of moderate mass. The star is essentially ejecting its outer atmosphere and exposing its blazing hot core to full view. The Helix is distinguished by its proximity to Earth, about 700 light years, and its consequent very large apparent size. By comparison, the Ring Nebula in Lyra is some 2,000 light years away. The Helix gets its name from its ring-like appearance which resembles the two coils of a spring seen on axis.

When you first see the Helix Nebula, you may be astonished to discover how large it appears. Most planetaries are so small as to appear star-like, even at moderate power, so happening upon a planetary nebula that’s half the size of the full Moon can be a little unsettling, even for experienced observers. The apparent surface area of the nebula is about four times larger than M27, the Dumbbell Nebula, and more than 100x the area of the Ring Nebula (M57)! Charles Messier missed this nebula, likely because his telescopes had a narrow field of view and did not enable him to notice a large object with low surface brightness.

Location of the Helix Nebula, about 10 degrees NW of the bright star Fomalhaut.

Location of the Helix Nebula, about 10 degrees NW of the bright star Fomalhaut.

The Helix Nebula is located about 10º NW of the bright star Fomalhaut. It looks like a big Moon-sized grey circle in a pair of binoculars. At low-to-medium power in a telescope, the nebula appears as a smokey oval set in a triangle of 10th-magnitude stars. At first it appears well defined and somewhat featureless. But keep looking. With averted vision, you’ll begin to see the ring-like structure of this ghostly object in a 3″ to 4″ scope. The central region appears transparent at low power, and slightly brighter than the background sky at medium power. The 13th-magnitude central star is a challenge with a 4″ scope but a snap with a 6″ or 8″ reflector, especially at 80x. Notice how the bright outer ring appears better defined at its inner edge. A UHC or OIII filter is usually a big help with this nebula.

Some observers at high altitudes and under dry sky claim to have seen the Helix without optics. If you have exceptionally clear and dry skies, give it a try. This will be much easier at southern latitudes where the nebula can be observed much higher in the sky.

Share This:
Facebooktwittergoogle_plusredditpinterestlinkedinmail