The Solar System

Slow stellar wind

from a red giant

Visual + Infrared

The gases of the slow wind

are not easily detectable.

You see a planetary nebula

where the fast wind

compresses the slow wind.

Fast wind from

exposed interior

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2 The process that produces planetary nebulae involves two stellar winds.
First, as an aging giant, the star gradually blows away its outer layers in a
slow breeze of low-excitation gas that is not easily visible. Once the hot interior of
the star is exposed, it ejects a high-speed wind that overtakes and compresses
the gas of the slow wind like a snowplow, while ultraviolet radiation from the hot
remains of the central star excites the gases to glow like a giant neon sign.

The process that produces planetary nebulae involves two stellar winds.
First, as an aging giant, the star gradually blows away its outer layers in a
slow breeze of low-excitation gas that is not easily visible. Once the hot interior of
the star is exposed, it ejects a high-speed wind that overtakes and compresses
the gas of the slow wind like a snowplow, while ultraviolet radiation from the hot
remains of the central star excites the gases to glow like a giant neon sign.

Roman Corradi/Nordic Optical Telescope

NASA/JPL-Caltech/ESA

The Cat’s Eye Nebula

Visual

Simple observations tell astronomers what

planetary nebulae are like. Their angular size

and their distances indicate that their radii range

from 0.2 to 3 ly. The presence of emission lines in

their spectra assures us that they are excited,

low-density gas. Doppler shifts show they are

expanding at 10 to 20 km/s. If you divide radius by

velocity, you find that planetary nebulae are no

more than about 10,000 years old. Older

nebulae evidently become mixed into the

interstellar medium.

Astronomers find about 3000 planetary

nebulae in the sky. Because planetary

nebulae are short-lived formations,

you can conclude that they must be

a common part of stellar evolution.

Medium-mass stars up to a mass

of about 8 solar masses are

destined to die by forming

planetary nebulae.

The Helix Nebula is 2.5 ly in

diameter, and the radial texture

shows how light and winds from the

central star are pushing outward.

The Cat’s Eye, below, lies at the center of

an extended nebula that must have been

exhaled from the star long before the fast wind

began forming the visible planetary nebula. See

other images of the nebula on opposite page.

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