M57: The Ring Nebula
Planetary Nebulae are fascinating objects, out of the ordinary from collections of stars, and excellent beginner targets. M57, the Ring Nebula, is an excellent representative of this class of objects, showing a distinct, very round ring with a hollow centre. It is harder to find than M27 because it is quite small and may not be visible in your finder. However, it is placed between two close-spaced stars, and should be within reach of a beginner.
Planetary Nebulae are shells of gas ejected by stars in the late stages of their life. When an ordinary mass star (such as our own Sun) expends all its Hydrogen fuel, internal fusion stops. This stops the outward radiation pressure, which causes the gas to begin to collapse under its own gravity. The collapse raises the internal pressure and temperature enough to start a new fusion reaction, with the Helium gas that was the byproduct of the earlier Hydrogen fusion. The radiation from this fusion expands the star, which lowers the pressure, which stops the fusion. The gas then collapses, and fusion restarts, and so on.
This on-off cycling is called Thermal Pulsing and, each time the direction of collapse or expansion reverses, an outer layer of gas is “puffed off” into space. Successive puffs build up a structured shell of gas at some distance from the star, and this gas glows in the radiation of the inner stellar core. When we look at this shell of gas from a distance, we tend to see the edges more than the near face, since at the edges we are looking through more gas; this is why we tend to see a shape with a distinct outline (often a circle) and not a globe of gas.
Finding M57
Switch to your magnifying finder.
Unfortunately, M57 is so small that it probably still won’t be visible in the field of the 6×30 finder that is common on beginner-level telescopes.
You can, however, confirm that you are centred between the two stars on the side of the trapezoid.
Now we will start using the main telescope, gradually increasing the magnification. Start with a low-power eyepiece.
This image simulates the view in a 650mm focal-length refractor with a 17mm eyepiece, giving a magnification of about 40x.
The ring is just visible. Carefully centre it, then switch to a shorter eyepiece.
Experiment with different eyepieces to get the best view.
In this telescope I find a 4mm eyepiece (magnification 150x) gives a very pleasing view of the ring. If I try to go higher than that, the view becomes dimmer and is not improved.
All the above images were generated with Starry Night Pro.