IC 418
THE SPIROGRAPH NEBULA
"Exceedingly bright" (Curtis): both the planetary nebula and the 10th
magnitude (10.2) central star. As NGC
7027 and NGC 2440 are to high-
excitation planetary nebulae and ultra-hot stars, IC 418 (the
Spirograph, in Lepus) is to low-
excitation nebulae and cool stars (cool at least in the context of
the planetary nebulae). Calculations of temperature from three
different methods (nebular emission
spectrum compared to the star, the nebula alone, the star's
spectrum alone) cluster nicely around 35,000 Kelvin, not that much
above the 26,000 Kelvin lower limit, below which the star cannot ionize the nebula. Even at 35,000 K,
the star is so "cool" that only about 75 percent of the nebular
helium is ionized. A stellar wind is evident from the star's
spectrum, which contains broad emission lines of highly ionized
carbon and helium.
The distance is uncertain. The best two estimates give 2500 light
years, though the range goes all the way to 6500. The shorter
distance gives a stellar luminosity of around 1500 times that of
the Sun. Curtis, whose composite drawing is
at the left, measures an angular diameter of 13 seconds of arc,
Hubble scientists (on the right) 18 seconds, though Curtis does
state that a "long exposure...shows the object as somewhat
larger..." At 18 seconds and 2000 light years, the ionized shell
of IC 418 is then 0.2 light years across. (There is almost
certainly un-ionized matter around the visible ring.) The small
size is consistent with the low star temperature of early
evolution, as the star is still heating and the nebula growing,
albeit at a low expansion rate of 12 kilometers per second (rather
typical of low excitation objects).
Rotate the Hubble image about 45 degrees to the right to match the
two. Curtis sees the interior filled with a light glow, which the
powerful resolution of the Hubble reveals to consist of a filigree
pattern and an inner ring. Curtis sees the star off-center, while
in the Hubble image, the star is off-center in the faint inner
ring. The observers, however, are not seeing quite the same
things, as the older image is a visual impression, the Hubble made
from a set of images that reflect specific emission lines. The
outer ring is also subtly bi-polar, rather reminiscent of the Ring Nebula in Lyra, perhaps showing us what
the Ring used to look like.
Left: Image and quotes by H. D. Curtis from Publications of the
Lick Observatory, Volume 13, Part III, 1918. Right: NASA and the
Hubble Heritage Team (STScI/AURA).
