This post describes the Ring Nebula as part of our series of interesting objects to observe with binoculars or a small telescopes. For information about this series please see our feature, “Summer Observing List“.
The Ring Nebula
Also known as M57 in the Messier Catalog and as NGC6720 in the New General Catalog, the Ring Nebula is a Planetary Nebula. It is located slightly less than halfway between the Beta (Sheliak) and Gamma (Sulafat) stars in the constellation Lyra at the edge of the Milky Way (closer to Beta).
Using 8 PM as a reference time, it is visible to observers in the Northern Hemisphere between late April and mid-December. (It is visible all year round if you include observing in the pre-dawn hours!)
The Ring Nebula has a visual magnitude of about 8.8 which makes it possible to locate with binoculars from a dark location. To find it, use a planisphere to locate the constellation Lyra (which is itself is fairly easy to find because it contains the fifth-brightest star in the sky, Vega). Simply search between the Beta and Gamma stars which are at the far end of the asterism from Vega. In binoculars it appears as a small glowing fuzzy silvery-grey dot but, because of its location, is unmistakable when viewed from a dark site. With a telescope of 4″-6″ you will be able to resolve it as a silvery oval, and at higher powers you will be treated to a view of it as a ring or “tiny ghostly doughnut” (Burnham). With even larger scopes you will be able to distinguish its soft bluish-green hue.
The actual nebula is approximately 20,000 light years away from us and 0.4 light years across and covers an area of sky of about 1 arcminute by 1.5 arcminutes so even at high magnifications it will still fit easily within the eyepiece.
A nebula is a simply a cloud of gasses in interstellar space. Generally, nebula are visible because they either glow (emission nebula) or reflect light from a nearby strong light source (reflection nebula). The Ring Nebula is an emission nebula glowing because radiation from it central star is ionizing the gas cloud.
A Planetary Nebula actually has nothing to do with planets. When a medium sized star exhausts its nuclear fuel it starts to expand and become more and more unstable until it ultimately ejects a large portion of its mass in strong stellar winds that can reach more than 600,000 mph. The expanding cloud of gas takes on unique shapes as it interacts with itself and other stellar winds. The central star left behind would become a White Dwarf with very high density and high surface temperatures creating a lot of UV radiation. This radiation ionizes the ejected gas cloud causing it to glow. It is this ejected gas that makes up the Ring Nebula that we can see. (The central star in the Ring Nebula is magnitude 14.8 so it will not be visible in most amateur equipment from most locations.)