How big is Blinking Nebula? | Scale of the Universe

The Blinking Nebula, also known as NGC 6826, is a glowing shell of gas thrown off by a dying star in the constellation Cygnus. Its bright outer shell is about 5 to 6 trillion kilometers across, a little more than half a light-year, where a light-year is the distance light travels in a year. The nebula is named for the way it seems to vanish and reappear in a small telescope.

Size

NGC 6826 is a planetary nebula: the outer layers of a Sun-like star, blown off near the end of its life and glowing around the small, hot core left behind. Astronomers in the 18th century called these clouds planetary nebulae because in small telescopes they looked round, like distant planets such as Uranus.

Images from the Hubble Space Telescope show a bright inner rim about 19 by 10 arcseconds, inside an outer shell about 31 by 27 arcseconds. An arcsecond is 1/3,600 of a degree. A much fainter halo of gas reaches about 130 arcseconds from the star.

Turning these angles into lengths needs the nebula's distance. Measurements from the Gaia space telescope, analyzed in 2021, put it about 4,200 light-years away, give or take about 5%. At that distance the outer shell is about 5.9 by 5.1 trillion kilometers. Other estimates of the distance range from about 2,200 to 6,800 light-years, so sizes based on them differ too.

Size comparisons

The outer shell is about 37,000 times the average distance from Earth to the Sun.
About 600 copies of the orbit of Neptune around the Sun would fit side by side across it.
It is about three times the distance from Proxima Centauri to Alpha Centauri A, the next smaller object in the explorer.
NGC 2392, another planetary nebula that seems to blink, is about twice as wide.
If the nebula were shrunk to the width of a basketball, Earth's whole orbit around the Sun would be about 13 micrometers across, about one-seventh the width of a human hair. A micrometer is a thousandth of a millimeter.

Why it seems to blink

Looking straight at NGC 6826 through a small telescope, most observers see only its bright central star. Glancing slightly to one side brings the dimmer nebula into view, and looking back at the star makes it fade again. This effect earned it the name Blinking Planetary. The nebula itself does not change: the blinking is a trick of viewing.

William Herschel discovered the nebula in 1793. It is best seen from the Northern Hemisphere on summer nights.

Red knots on either side

Hubble's images show two red patches on opposite sides of the nebula. Astronomers call them FLIERs, short for fast low-ionization emission regions. They are young features, rich in nitrogen, in which electrically charged atoms stream outward faster than the speed of sound. Hubble photographed them in January 1996 to help astronomers learn how they form.