How big is Eskimo Nebula? | Scale of the Universe

NGC 2392, long nicknamed the Eskimo Nebula, is a glowing shell of gas thrown off by a dying star in the constellation Gemini. Its faint outer shell is about 12 trillion kilometers across, about 1.2 light-years, where a light-year is the distance light travels in a year. The nebula lies about 5,600 light-years from Earth.

Size

NGC 2392 is a planetary nebula: the outer layers of a Sun-like star, cast off near the end of its life and made to glow by ultraviolet light from the star. It has two main parts. A bright inner shell about 18 by 15 arcseconds surrounds the star, and a fainter, nearly round outer shell reaches about 23 arcseconds from it. An arcsecond is 1/3,600 of a degree.

Turning these angles into lengths needs the nebula's distance. Measurements from the Gaia space telescope, analyzed in 2021, put it about 5,600 light-years away, give or take about 8%. At that distance the outer shell is about 12 trillion kilometers across, and the inner shell about 4.7 by 3.9 trillion kilometers. Before Gaia, estimates of the distance ranged from about 3,600 to 5,900 light-years, so most sizes based on them are smaller.

Size comparisons

The outer shell is about 78,000 times the average distance from Earth to the Sun.
It is about 1.2 times the length of a light-year, one of the next smaller objects in the explorer.
It is about twice as wide as the Blinking Nebula, another planetary nebula.
It is about 6.5 times the distance from Proxima Centauri to Alpha Centauri A.
If the nebula were shrunk to the width of a basketball, the orbit of Neptune around the Sun would be about 0.2 millimeters across, about twice the width of a human hair.

Comets and jets

Hubble's image from January 2000 shows a ring of comet-shaped knots in the outer shell, their tails pointing away from the star like the spokes of a wheel. The heads of the knots all seem to lie at about the same distance from the star, a clue to how they formed. One idea is that they form where fast-moving gas collides with slower gas.

The outer shell is expanding at about 16 kilometers per second, the inner shell at up to about 120 kilometers per second, and a pair of narrow jets at about 180 kilometers per second. X-rays seen by NASA's Chandra X-ray Observatory show gas heated to about a million degrees near the center. The star began shedding its outer layers about 10,000 years ago.

Discovery and name

William Herschel discovered NGC 2392 in 1787. Like the Blinking Nebula, it seems to fade when an observer looks straight at its central star through a telescope, and to reappear when the observer looks slightly aside.

The nickname came from its appearance: astronomers describe the outer shell as a hood, and the ring of comet-shaped knots in it as the hood's fur. In 2020, NASA stopped using the nickname Eskimo Nebula. It said the word is widely seen as a colonial term with a racist history, imposed on the Indigenous peoples of the Arctic. NASA said it would use only the official name, NGC 2392, from then on.