NGC 6720
THE RING NEBULA IN LYRA
MESSIER 57
The Ring Nebula in Lyra is a classic
"planetary nebula," the name from
William Herschel simply meaning "disk-like." Planetary nebulae
are the ejecta 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). It's easily found
between Beta and Gamma Lyrae
The distance of the Ring 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 ultralviolet light of
the 16th magnitude (15.7) star at the center, which is now a
cooling, but still very hot, white dwarf with a temperature of
about 150,000 Kelvin and a luminosity some 500 times that of the
Sun. 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. The central star is barely visible at the center. While
NGC 7009 is considered discovery object
among planetary nebulae, the Ring was already known. It was placed
into the class later on after more were discovered.
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.
Left: University of Illinois Prairie Observatory. Right: The
Hubble Heritage Team (AURA/STScI/NASA).
