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 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 15th
magnitude star at the center, which is now a cooling, but still very
hot, white dwarf with a temperature of 145,000 Kelvin and a luminosity
about 600 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.
While there are many far more detailed images than this one, the photo
gives a good sense of how the Ring looks in a small telescope (minus the
central star, which is quite difficult to see).
University of Illinois Prairie Observatory.
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