IC 418


In Lepus

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From Jim Kaler's STARS; Return to Planetary Nebulae

IC 418 IC 418
"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).


IC 418

Historical blue to near-ultraviolet photographic spectra of IC 418 taken with the coude spectrograph of the 120-inch telescope at Lick Observatory. The scale is in Angstroms. The upper spectrum is that of the nebula plus the central star, while the lower one is that of the nebula only. Emission-line comparison spectra used for wavelength measure bracket those of the nebula. Note the broad ionized helium, carbon, and nitrogen lines in the top strip that come from the wind of the central star. The low excitation nebula, with its relatively cool central star, does not exhibit He II emission lines (those of ionized helium), just those of He I (neutral helium). Square brackets denote forbidden lines. At the right end of the top strip lie the famed "nebulium" lines at 5007 and 4959 Angstroms first found in the spectrum of NGC 6543 by Sir William Huggins. The bottom strip clearly shows the confluence of the hydrogen Balmer lines, which merge into the strong Balmer continuum, and the splitting of the forbidden [O II] line at 3727 Angstroms. For additional discussion see The Ring Nebula

From L.H. Aller and J.B. Kaler, Astrophysical Journal vol. 140, p. 936, 1964.

See the spectrum at full resolution.