The Helix Nebula

In the constellation Aquarius, we will find the largest planetary nebula in our sky, the The Helix Nebula. It’s the largest such nebula (in appearance) because it’s also physically the closest planetary nebula to us at a distance of only around 665 light years, give or take a little. When the light you see or record with your camera left the nebula, Edward the III was king of England in the mid 1300’s. Of course, if you were riding one of those photons, you would not have experienced any time at all according to special relativity -- thanks Einstein! From the light’s frame of reference, the photons were created, immediately transported to Earth, and then absorbed by your retina or camera sensor. They died so young… but I digress.

Most planetary nebula we see in the sky are quite small, but the Helix is a special treat because of its proximity to us. It’s quite bright really with a visual magnitude of 7.6 and can just be made out with a pair of binoculars under a dark sky. It was discovered visually with an 8.5-inch reflector by the German Karl Ludwig Harding in 1823 or 1824 (“incomplete records haunt me so”) but can be seen in smaller instruments with the aid of an OIII visual filter.

The Helix nebula is readily accessible to the camera however with any sized instrument. As mentioned, it’s bright and colorful with a color camera. It’s also very bright in the narrowband wavelengths of hydrogen alpha (Ha) and ionized oxygen (OIII). The photo shown here is actually a combination of images taken with a color camera, and images taken with a monochrome camera using Ha and OIII filters. The Ha wavelengths can enhance the red and to a smaller amount blue, and the OIII filters add cyan or blue/green to the image. The hydrogen alpha signal is a red wavelength of color, but Ha in emission nebulae is also accompanied by about 20% hydrogen beta. Hydrogen beta is blue, and many imagers including me, will use the Ha data to enhance both the red and blue color channels using about an 80/20 ratio of red to blue. Adding this narrowband data to your color data really brings out the red and blues in these kinds of emission nebula.

We are fortunate that the Helix nebula is face on towards us and we can see it in all its splendor. It’s actually shaped like a torus or donut/bagel. I think “Helix” is still a better name than “Space Donut” though. It’s also often called the “Eye of God”, and more recently some have given it the moniker “Eye of Sauron”. Planetary nebula such as this are formed when an intermediate sized star near the end of its lifespan blows off its outer layers of gas. The central star, which is visible in this nebula, then becomes a white dwarf and its energy causes the expelled gasses to glow. These gasses in the Helix now extend about 2.5 light years across, and the outer ring is moving away from the central star at about 40km/s (24 miles per second, or about 86,400 miles per hour!).

The Helix nebula also has the distinction of being the first nebula to be discovered to contain cometary knots. You can capture these knots today as well with sufficient focal length and image scale. You can see them along the inner ring with their heads oriented towards the central star, and their “tails” streaming behind them. The heads are about one arcsecond in size, and their tails extend by as much as 30 arcseconds. Astronomers estimate there may be as many as 40,000 of these knots in total. Each one is the size of our solar system!

Most planetary nebula have a life span of 10,000 to 50,000 years, and the Helix nebula is only about 11,000 years old since its formation. In time the white dwarf that powers it will continue to cool off, and the glowing gas will no longer be visible. You’d better get out and get it while you can!

• Name: The Helix Nebula / NGC 7292, Caldwell 93
• Constellation: Aquarius
• Classification: Planetary Nebula
• Distance: 655 +/- 13 light years
• Apparent Size: 25 Arc minutes
• Magnitude: 7.6