The Cat's Eye Nebula is a planetary nebula, a cloud of glowing gas thrown off by a dying star, about 4,300 light-years from Earth in the constellation Draco. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. Including the faint halo of gas around it, the nebula is about 6.9 light-years across, or about 65 trillion kilometers. The bright inner nebula seen in most pictures is less than a light-year across.

Size

A nebula has no sharp edge, and the Cat's Eye has several layers. Its bright core reaches about 20 arcseconds from the central star, where an arcsecond is 1/3,600 of a degree. Around the core, Hubble images show at least eleven faint rings, and deep images from a ground-based telescope suggest they reach out to about 50 arcseconds. Beyond them lies a much larger and fainter halo of wispy gas, with a radius of about 165 arcseconds.

Those are angles on the sky, so the true sizes depend on the distance. The Gaia spacecraft measured the position of the central star precisely enough to put it about 1,300 parsecs away, where a parsec is about 3.26 light-years. At that distance the halo is about 6.9 light-years across, the ring system about 2 light-years and the bright core about 0.8 light-years.

Older estimates placed the nebula closer. A 1999 study that measured how fast the core is growing in Hubble images found a distance of about 1,000 parsecs, give or take 270. At that distance the halo would be about 5.2 light-years across.

Size comparisons

The halo is about 1.3 times as wide as the faint outermost ring of the Helix Nebula, another planetary nebula and the next smaller object in the explorer.
It is about 1.6 times the distance from the Sun to Proxima Centauri, which is about 4.2 light-years.
The halo is about eight times as wide as the bright core.
If the Sun were shrunk to the size of a basketball, the halo would be about 11,000 kilometers (7,000 miles) across, almost as wide as Earth.

Rings from a dying star

A planetary nebula forms when a star like the Sun nears the end of its life and pushes its outer layers of gas into space. Each ring around the Cat's Eye is the edge of a spherical bubble of gas, seen projected against the sky. The pattern suggests that the star threw off material in pulses about every 1,500 years. Each pulse made a shell holding about as much mass as all the planets of the Solar System put together.

About 1,000 years ago the pattern changed, and the bright core began to form inside the shells. It has been growing ever since, as Hubble images taken between 1994 and 2002 show. The halo is older still. Its gas is hotter than the core's, about 14,400 degrees Celsius against about 7,600, and it holds more mass than the core does.

Discovery

William Herschel, the astronomer who also discovered the planet Uranus, found the Cat's Eye in February 1786. It was among the first planetary nebulae ever discovered. The English amateur astronomer William Huggins was the first to study its spectrum, the colors that make up its light. His work showed that planetary nebulae are made of gas.

Hubble images show shells of gas, jets of fast-moving gas and knots shaped by shock waves. In 2004, ESA and NASA described the Cat's Eye as one of the most complex planetary nebulae ever seen.

Sources

Dying star creates fantasy-like sculpture of gas and dust. ESA/Hubble, 2004.
Caldwell 6. NASA Hubble Space Telescope, Hubble's Caldwell Catalog.
The giant haloes of NGC 6543 and 6826. Middlemass, Clegg and Walsh, Monthly Notices of the Royal Astronomical Society, 1989.
Rings in the haloes of planetary nebulae. Corradi et al., Astronomy & Astrophysics, 2004.
HST Measurements of the Expansion of NGC 6543: Parallax Distance and Nebular Evolution. Reed et al., The Astronomical Journal, 1999.
Gaia EDR3 distance to the central star of the Cat's Eye Nebula, source 1633325248915154176. Bailer-Jones et al., via CDS VizieR, 2021.
Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3. Bailer-Jones et al., The Astronomical Journal, 2021.
Unraveling the Helix Nebula: Its Structure and Knots. O'Dell et al., The Astronomical Journal, 2004.
Gaia EDR3 distance to the central star of the Helix Nebula, source 6628874205642084224. Bailer-Jones et al., via CDS VizieR, 2021.
The Nearest Neighbor Star. NASA Goddard Space Flight Center, Imagine the Universe, 2020.
Measuring the Universe. International Astronomical Union.
Sun Fact Sheet. NASA, 2024.
Earth Fact Sheet. NASA, 2024.
Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.

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