The Ring Nebula in Lyra is perhaps the classic planetary nebula. Curtis calls it "well known and remarkably complex." The term, which simply means "disk-like," comes from William Herschel. The Ring Nebula was found in 1779 before Herschel announced his discovery of the first of his "planetary nebulae" (NGC 7009) in 1785, and was added to the class later.
Planetary nebulae are the compressed ejecta of dying stars as they turn from giants into white dwarfs. The photograph on the left gives a good sense of how the Ring looks in a small telescope (minus the central star, which is quite difficult to see). The image in the middle is Curtis's composite drawing made from several photographs, while that on the right is the Hubble view. The Ring is easily found between Beta and Gamma Lyrae.
The distance of the Ring Nebula is measured by direct parallax to be 2300 light years away (accurate to about 40 percent). The angular dimensions of this elliptical object of 86 X 62 seconds of arc (a "second" 1/3600 of a degree) translate to true dimensions of 0.95 X 0.7 light years. The long axis would therefore stretch 20 percent of the way from the Sun to Alpha Centauri. The nebula, expanding at a rate of about 30 kilometers per second, is illuminated by the ultraviolet light of the 16th magnitude (15.7) star at the center, which is now a cooling, but still very hot, nascent white dwarf with a temperature of about 150,000 Kelvin and a luminosity some 500 times that of the Sun. It looks faint only because most of its light is radiated in the ultraviolet. Outer shells produced by mass loss in the giant star that created the nebula extend out almost twice as far as seen here, making the whole system nearly two light years across.
On the right is the spectacular Hubble view. The layered colors reveal radiation from different chemical elements in different stages of ionization, blue from ionized helium, yellow-green from doubly-ionized oxygen, red from ionized nitrogen (see the spectrum below). The central star is barely visible at the center. We might be looking down the mouth of a barrel, or more likely the throat of an hour glass. The structure may be similar to that of the Dumbbell Nebula, just seen from a different perspective.
Spectre réalisé avec Alpy 600 temps de pose 30s au 350