How big is Rotten Egg Nebula? | Scale of the Universe

The Rotten Egg Nebula is a cloud of gas and dust being thrown off by a dying star, about 5,000 light-years from Earth in the constellation Puppis. A light-year is the distance light travels in one year, about 9.46 trillion kilometers. The nebula is long and narrow: it is about 1.4 light-years, or 13 trillion kilometers, from end to end. It is also called the Calabash Nebula, and astronomers know it as OH 231.8+4.2.

Size

The nebula has two lobes of gas streaming away from its star in opposite directions. Radio observations of this flow show it is about 57 arcseconds long and at most 6 arcseconds wide, where an arcsecond is 1/3,600 of a degree. At the nebula's distance, that is about 1.4 light-years long and 0.15 light-years wide. An infrared study gives a similar size, 60 by 10 arcseconds.

A 2025 study measured the distance with radio telescopes. It tracked tiny shifts in the positions of bright radio sources near the star as Earth orbited the Sun. The result, about 5,000 light-years, is uncertain by about 5%.

The lobes are tilted about 35 degrees away from the plane of the sky, so their true length is about 1.7 light-years. A faint halo of older gas around the middle of the nebula is about 1 light-year across.

Size comparisons

The Boomerang Nebula, also thrown off by an old red giant star, is about 2 light-years long. The Rotten Egg Nebula is about two-thirds of its length.
The nebula is about one-third of the distance from the Sun to Proxima Centauri, the nearest star, 4.2 light-years away.
If the nebula were shrunk to the height of a person 1.7 meters (5 feet 7 inches) tall, its central star would be about as wide as a human hair. The star is a swollen red giant about 4 astronomical units across, where 1 astronomical unit is the average distance from Earth to the Sun.
The gas shown in yellow in Hubble's picture moves at close to a million kilometers per hour (620,000 miles per hour). At that speed it would travel from Earth to the Moon in about 23 minutes.

A star between two stages

The nebula shows how a star like the Sun dies. Its star, QX Puppis, is a red giant that pulsates, and it has a smaller companion star. The nebula is changing from a red giant into a planetary nebula, the glowing shell of gas that such a star leaves behind as it sheds its outer layers.

This stage passes quickly, so astronomers rarely catch a star in it. The European Space Agency (ESA) expects the Rotten Egg Nebula to become a full planetary nebula within the next thousand years. The shape of its fast outflow was set about 800 years ago, in an event that lasted less than 150 years. The halo is older: it comes from a gentler wind that began at least 11,000 years ago.

Why it is named for rotten eggs

The nebula contains a lot of sulfur, an element that smells like rotten eggs when it is combined with other elements. Its gas holds sulfur-bearing molecules such as sulfur monoxide, sulfur dioxide and hydrogen sulfide. A 2018 study found that its envelope of gas has the richest chemistry known around any oxygen-rich star at this stage of life.

On the sky, the nebula lies in the same direction as the star cluster Messier 46. The 2025 study found that the two are at about the same distance, but they move through space at speeds that differ by 15 kilometers per second. The study concluded that the nebula does belong to the cluster. It suggested that the difference in speed might come from two stars that merged in the past.