What is a Planetary Nebula?

There are several Messier and Caldwell objects classified as a planetary nebula.  Even though the name suggests they are in reference to a planetary phenomenon, the truth is quite different.  A planetary nebula is a giant ball of gas that has been ejected from a very old star.  The ejecta thrown off by the star is intensely heated by radiation from the core of the star causing the gas to glow and emit different wavelengths of light -- different colours.  The ejection from a spherically symmetric star causes the nebula to be basically round in shape.  This is how it received the name 'planetary'.  Astronomers thought they looked planet shaped and that they were condensing into planets.

A great thing about the ejection of matter from stars is that it seeds the galaxy with heavier elements formed by nuclear fusion in stars.  If you look closely through a larger backyard telescope you may see the core of the original star that caused the nebula to form.  At one time or another I've witnessed the magnitude 14.8 white dwarf stars at the center of the Ring Nebula in photographs I have taken at high exposure times.  These white dwarfs are shining brightly to illuminate the surrounding gas.

Atoms and ions emit radiation when they are energized by incoming radiation.  They emit only certain frequencies of radiation corresponding to different colours of the electromagnetic spectrum.  A doubly ionized oxygen ion is found in abundance in planetary nebula and emits a certain spectra of light consisting of three emission lines around 500nm.  This ion is also known as the (O III).  Telescope stores stock O III filters that you can use on your eyepiece to allow the transmission of these lines.  This allows an amazing amount of contrast of the nebula from the surrounding light pollution and stellar phenomena.

One of my favourite planetary nebulae is the Ring Nebula.  It is located approximately halfway between Beta and Gamma Lyrae (the bottom of the Lyre).  It is an extremely easy telescopic target to find and view.  In dark skies you may even see it through a good set of binoculars.  An image I took in Fall 2016 from my backyard is shown below.

M57 - Ring Nebula

As stated previously, the Ring Nebula is very easy to find in a telescope.  A star chart is provided below of the constellation Lyra with M57 highlighed.

M57 in Lyra

The next time you are at a public star night, ask to see a planetary nebula and see what you think it looks like!