IC 418
A bright planetary nebula with a spirograph-like pattern.
IC 418, called the Spirograph Nebula, is a bright planetary nebula in the constellation Lepus, about 3,600 light-years away. It measures 0.3 light-years across. Its central star, HD 35914, is an O-type star (spectral type O7fp). The nebula formed a few thousand years ago as the star ended its red giant phase, ejecting its outer layers into space. The nebula glows because the star’s ultraviolet radiation interacts with the gas.
The colors come from different chemical elements. Red indicates nitrogen (the coldest gas), green is hydrogen, and blue traces are ionized oxygen (the hottest gas, closest to the star). Neutral regions contain large grains of amorphous carbon, while ionized regions have smaller graphite grains. Silicon carbide, magnesium, and iron sulfides are likely present, explaining the nebula’s emission features.
Structurally, IC 418 has multiple shells. The main nebula is slightly off-center within an outer elliptical halo. It contains radial filaments, rays, and three concentric rings, with about 630 years between each ring. Two detached haloes are separated by 10,000 to 50,000 years. Small blister-like bubbles appear on the outer shell walls. Extinction maps show higher extinction near the nebula’s rim.
The inner shell is bright and has higher relative excitation, typical for planetary nebula inner shells. Two slits cross the central star over this shell. Gas at the poles moves at 15.7 km/s, while equatorial gas moves at 12.6 km/s—these speeds apply only to the outer edges of the inner shell.
Low-surface-brightness radial filaments and rays emanate from the bright shell, mostly pointing toward the central star. They are broad and featureless, with uneven ring spacing.
The two haloes—inner and outer—are separated by 10,000 to 50,000 years. The faint inner halo is ionized but poorly understood. The outer halo is elliptical, five times fainter than the inner one, with a limb-brightened shape; its western tip is brighter than the eastern tip. No kinematic data exists for the haloes due to their low surface brightness.
The name comes from the nebula’s pattern, which resembles designs made with a Spirograph toy (producing hypotrochoids and epitrochoids). The pattern’s origin is unknown.
- Distance
- ~3,600 ly
- Size
- 0.3 ly across
- Central star
- HD 35914
- Spectral type
- O7fp
- Inner shell polar velocity
- 15.7 km/s
- Inner shell equatorial velocity
- 12.6 km/s
- Time lapse between rings
- ~630 years
- Time lapse between haloes
- 10,000–50,000 years
Lore & Background
IC 418 formed a few thousand years ago during the last stages of its central star's red giant phase, when material from the star's outer layers was ejected into space. The nebula's glow results from ultraviolet radiation from the central star interacting with the gas. Its colors come from different chemical elements: red from nitrogen (the coldest gas), green from hydrogen, and blue from ionized oxygen (the hottest gas, nearest the star). The neutral regions contain large grains of amorphous carbon, while ionized regions hold smaller graphite grains; silicon carbide, magnesium, and iron sulfides are also likely present, explaining observed emission features.
The nebula exhibits a multi-shelled structure with a slightly off-center main nebula inside an outer elliptical halo. It contains numerous radial filaments and rays, a system of three concentric rings with a time lapse of about 630 years between them, and two detached haloes separated by 10,000–50,000 years. Small bubble-like blisters appear on the exterior walls of the nebular shell. Extinction maps show higher extinction near the rim. The inner shell is particularly bright with higher relative excitation; its polar gas travels at 15.7 km/s and equatorial gas at 12.6 km/s, applicable only to the outer skirts of the inner shell. The radial filaments are low-surface-brightness, mostly radial toward the central star, broad and featureless, with non-uniform ring separation. The inner halo is faint and ionized but poorly understood; the outer halo is elliptical, five times fainter, with limb-brightened morphology and a brighter western tip. Kinematic data for the haloes are unavailable due to low surface brightness.
Reader's Guide
IC 418 is significant as a nearby, bright planetary nebula whose intricate structure—including concentric rings, radial filaments, and multiple shells—provides insight into the mass-loss history of its central star during the late red giant phase. The time lapse of about 630 years between rings and 10,000–50,000 years between haloes reveals episodic ejection events. The nebula's chemical composition, with amorphous carbon grains in neutral regions and graphite in ionized zones, along with silicon carbide and metal sulfides, helps explain its emission features and the processing of dust in such environments. Its spirograph-like appearance, though of unknown origin, has made it a popular subject for astronomical imagery and public outreach, as reflected in its inclusion in press releases and Astronomy Picture of the Day features. The nebula's multi-shelled structure and velocity measurements of its inner shell contribute to understanding the dynamics and shaping mechanisms of planetary nebulae.
Did You Know?
- IC 418 is about 3,600 light-years away and spans 0.3 light-years across.
- The central star, HD 35914, is an O-type star with spectral type O7fp.
- The time lapse between the three concentric rings is about 630 years, and between the two haloes is 10,000–50,000 years.
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