Despite the name, these objects are totally unrelated to "planets". Planetary Nebulae are formed by stars similar in size to our Sun during their final stage of existence. It is estimated that there are about 10,000 Planetary Nebulae in our galaxy. They are a short-lived phase of the stellar life cycle lasting only several thousand years.
When a star similar to our Sun, has converted all of its hydrogen to helium during its Main Sequence phase, the fusion reactions in its central regions begin to cease and the converted helium begins to burn in the shell outside the core. This process makes the star expand into what is called a Red Giant. As the helium burns off, the outer layers of the Red Giant become more and more unstable. Finally the star collapses, and in a violent rebound, a significant portion of its mass is ejected into an expanding shell. The stellar core remains as an extremely hot, small central star called a White Dwarf. Emitting high energy radiation, the White Dwarf, bombards the expanding gas shell. This radiation causes the various gasses in the shell to shine different colors in a process Similar to that of a neon light. The central White Dwarf will continue to cool until all thermal energy is radiated, and the star approaches an "end state" as a cold dark Black Dwarf.
Oun Sun – A Planetary Nebula?
95% of all stars that we see in our galaxy will ultimately become Planetary Nebulae. This includes our own Sun. In five billion years the hydrogen fuel will have been depleted… the outer layers will expand to form a huge Red Giant consuming the orbits of Mercury and Venus. On Earth, the oceans will boil and evaporate into space, along with the atmosphere. After its helium is burned, the Sun will collapse and shed its outer layer. The wind of the ejected gas will sweep past the Earth, slowly at first, a mere 5 miles per second, and then will pick up speed, to eventually reach 1000 miles per second. The remnant Sun will rise as a dot of intense blue-white light, no larger than Venus but more brilliant than 100 Suns. Light from this fiendish blue dot will braise the Earth and tear apart its surface into molecules and atoms. The sky will be aflame with the whispy, colorful shapes of the nebula of ejected solar material. This spectacular show will last a few thousand years as the ejected gasses merge into the interstellar medium from which new generations of stars will form.
The famous Ring Nebula is often regarded as the prototype of a planetary nebula, and a showpiece in the northern hemisphere summer sky. It lies at a distance of about 2,300 light years, and for observers, it is always a challenge to identify the faint central star of the Ring, a tiny white dwarf, which shines at about 15th magnitude.
The Ring Nebula is actually not a ring at all, but a torus shape of bright light-emitting material surrounding its central star. Viewed from a different plane, it would resemble the Dumbbell Nebula M27. It just so happens that from our position, we look straight down at its pole.
Did you know?
Planetary Nebulae are named after the planet Uranus.
The name "planetary nebula" is a misnomer. The name arose over two centuries ago when astronomer William Herschel, observing through telescopes which by today’s standards had very poor optics, saw these nebula as compact, round, green-colored objects whose appearance resembled the greenish disk of the planet Uranus which he had discovered earlier in 1781.