How big is Helix Nebula? | Scale of the Universe

The Helix Nebula is a planetary nebula, the glowing gas around a dying star, about 650 light-years from Earth in the constellation Aquarius. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. The nebula's faint outermost ring is about 5.3 light-years across, or about 50 trillion kilometers. The bright ring seen in most photographs is less than half as wide.

Size

A nebula has no sharp edge, so its size depends on which part is measured. A 2004 study combined images from the Hubble Space Telescope with wide views from a ground-based telescope. It found that the bright main ring is really two parts: an inner disk and, around it, an outer ring tilted almost at right angles to the disk. A much fainter outermost ring surrounds both, flattened on the side where it runs into the thin gas between the stars.

Those parts are measured by how wide they look in the sky, and their true size then depends on the distance. The Gaia spacecraft measured the position of the nebula's central star precisely enough to put it about 199 parsecs away, where a parsec is about 3.26 light-years. That is about 650 light-years. At that distance the inner disk is about 1.6 light-years across, the outer ring about 2.3 light-years and the outermost ring about 5.3 light-years.

The 2004 study used an older distance of 213 parsecs, which made the outermost ring about 5.7 light-years across. Infrared images from the Wide-field Infrared Survey Explorer show an even fainter halo around the whole nebula, about 7.5 light-years across at Gaia's distance.

Size comparisons

The outermost ring is about one and a quarter times the distance from the Sun to Proxima Centauri, about 4.2 light-years, which is the next smaller object in the explorer.
It is about 340,000 times the average distance from Earth to the Sun.
If the Sun were shrunk to the size of a basketball, the Helix Nebula would be about 8,700 kilometers (5,400 miles) across, about two-thirds the width of Earth.
Seen from Earth, the bright ring is so close and so large that it appears nearly half as wide as the full Moon.

How the nebula formed

A planetary nebula forms when a star of medium mass nears the end of its life and sheds its outer layers of gas into space. The gas glows because it is lit by energy from the dying star at the center. Despite the name, planetary nebulae have nothing to do with planets. Many look like round disks in small telescopes, as planets do, which is how they got the name.

The 2004 study measured how fast the parts of the Helix are expanding and worked back to when they formed. It estimated that the inner disk is about 6,600 years old and the outer ring about 12,000 years old. The German astronomer Karl Ludwig Harding discovered the nebula in 1823.

Cometary knots

About 3,500 comet-shaped knots of gas are scattered through the nebula, most thickly along the inside of the bright ring. Astronomers call them cometary knots, because each has a glowing head and a long, faint tail, like a comet. Each tail stretches about 160 billion kilometers, about a thousand times the distance from Earth to the Sun.

Hubble images suggest the knots form where hot gas streaming from the dying star crashes into cooler gas the star threw off about 10,000 years earlier. The knots are expected to fade away into the space between the stars.