Cat's Eye Nebula (NGC 6543): Hubble's Eleven Rings Around a Dying Star

Every 1,500 years or so, the star at the center of the Cat's Eye Nebula seems to have given a cough. Each pulse pushed out a shell of gas and dust carrying about as much mass as all the planets of the solar system combined, and Hubble's 2002 image records at least eleven of them as faint concentric rings around the bright core. Only around a thousand years ago did the star change its habits and begin building the knotted structure that gives the nebula its name.

The Cat's Eye, catalogued NGC 6543, lies about 3,000 light-years away in Draco. It was the first planetary nebula ever discovered and remains one of the most complex: nested bubbles, jets of fast gas and knots formed by shock waves, all packed into a region about a light-year across.

A bull's-eye of ancient shells

A planetary nebula forms when a star of roughly the Sun's mass reaches the end of its life and sheds its outer layers, which then glow under the ultraviolet light of the exposed core. The rings around the Cat's Eye are older than the nebula proper. Each is the edge of a spherical bubble seen in projection, so it looks brightest along its rim, where the line of sight passes through the most material. The view resembles an onion cut in half, with every layer visible.

The regular spacing was the surprise. Nothing in the standard picture of a dying star predicted mass loss repeating on a 1,500-year cycle, and several explanations have been proposed: a magnetic activity cycle somewhat like the Sun's sunspot cycle, the influence of a companion star in orbit around the dying one, or pulsations of the star itself. Another idea is that the star shed material smoothly and the rings formed later, as waves developed in the outflowing gas.

Rings like these were once thought rare. In April 2004 Romano Corradi of the Isaac Newton Group of Telescopes and collaborators published a study in Astronomy and Astrophysics showing that they are probably the rule around planetary nebulae, not the exception.

The change a thousand years ago

About 1,000 years ago the pattern broke. The star stopped puffing out regular shells and began ejecting the material that now forms the bright, twisted core of the nebula. That inner structure has been expanding ever since, and its growth can be measured by comparing Hubble pictures from 1994, 1997, 2000 and 2002. What triggered the change is not known. Planetary nebulae are one of the few records of the last few thousand years of a Sun-like star's life, which makes the question worth answering.

Hubble's 1994 and 2002 views

Hubble first revealed the inner detail of NGC 6543 in 1994, with the Wide Field and Planetary Camera 2. That picture showed the concentric gas shells, the high-speed jets and the shock-induced knots for the first time.

The 1994 image used filters isolating hydrogen, oxygen and nitrogen, and it covers mostly the core. The 2002 image at the top of this page came from the wider field of the Advanced Camera for Surveys, under proposal 9026, with 1.2 hours of exposure through two oxygen filters and one for hydrogen-alpha and nitrogen. That wider field is what brought the full bull's-eye of outer rings into view. A separate composite paired a WFPC2 sulfur exposure with hydrogen and oxygen exposures taken in 2002 by the High Resolution Camera of the Advanced Camera for Surveys. In that version a thin red line crosses the frame, an artifact of the occulting finger that the High Resolution Camera uses to block the light of bright stars.

Ground-based images from the Nordic Optical Telescope on La Palma reach farther still. Taken in nitrogen and oxygen light, they show faint nebulosity extending well beyond the region Hubble photographed. Two published versions of those ground-based data make a useful lesson in color. Both use the same two narrowband exposures, yet one shows nitrogen in red and oxygen in shades of green and blue, while the other puts oxygen in blue and nitrogen in red. The structure is identical; only the palette differs. Are the Colors Real? looks at why such choices are made.

When the 2004 release went out, its description compared the nebula's glare to the disembodied eye of Sauron in the film version of The Lord of the Rings, a comparison suited to the bright, staring core. The Ring Nebula and NGC 5189 show other ways a dying star can shape its gas.

Questions readers ask

Why is it called the Cat's Eye Nebula?

The bright inner region of NGC 6543 is elongated and pointed at both ends, with overlapping loops that look like the slit pupil and iris of a cat's eye. The name describes that core. The fainter outer rings, revealed fully in 2002, make the whole object look more like a bull's-eye.

What made the rings around the Cat's Eye Nebula?

Each ring is the edge of a shell of gas and dust the star ejected roughly every 1,500 years. The cause of the regular rhythm is unknown. Candidates include a magnetic cycle in the star, a companion star, stellar pulsations, or waves forming later in a smooth outflow.

How far away is the Cat's Eye Nebula?

The distance published with the 2002 Hubble image is about 3,000 light-years, or 1,000 parsecs. The nebula is in the northern constellation Draco. The figure is approximate, as distances to planetary nebulae are hard to measure, and the image itself spans about 1.2 light-years.

Was the Cat's Eye the first planetary nebula discovered?

Yes. NGC 6543 holds the distinction of being the first planetary nebula ever found, yet it is also one of the most complicated ever seen. Hubble's 1994 image first showed its shells, jets and knots in detail.