How big is Homunculus Nebula? | Scale of the Universe

The Homunculus Nebula is a cloud of gas and dust thrown off by the giant double star Eta Carinae during an enormous outburst in the 1840s. It has two round lobes, one on each side of the star, and is about 6.5 trillion kilometers long from the tip of one lobe to the other. That is about 0.7 light-years, where a light-year is the distance light travels in a year, about 9.46 trillion kilometers.

Size

Each lobe is a thin, hollow bubble. In 2006, the astronomer Nathan Smith mapped their true three-dimensional shape by measuring the speeds of glowing hydrogen molecules across the nebula. He found that each lobe reaches about 21,700 astronomical units from the star along the nebula's axis and is up to about 20,000 astronomical units wide. One astronomical unit is the average distance from Earth to the Sun, about 150 million kilometers.

Seen from Earth, the nebula is about 18 to 20 arcseconds long, where an arcsecond is 1/3,600 of a degree. Turning that angle into a length needs the distance. The same study put it at about 7,700 light-years, give or take about 2%. The distance comes from the nebula itself: its expansion can be measured both across the sky and toward or away from Earth, and for a symmetric shape the two must agree.

The lobes' outer skin is only about 300 to 600 astronomical units thick, 2 to 3% of their radius. Most of the nebula's mass is in that skin, and the nebula as a whole holds more than 10 times the mass of the Sun.

Size comparisons

From end to end, the nebula is about 43,000 times the average distance from Earth to the Sun.
It is about two-thirds of a light-year long.
It is about 3.6 times the distance from Proxima Centauri to Alpha Centauri A.
If the Sun were shrunk to the size of a basketball, the Homunculus would be about 1,100 kilometers (700 miles) long.
The tips of the lobes are moving outward at about 650 kilometers per second. At that speed, something could cross the distance from Earth to the Moon in about 10 minutes.

The Great Eruption

Eta Carinae was a faint, unremarkable star in the early 1800s, but it grew steadily brighter. By April 1843 it was the second-brightest star in the sky, outshone only by Sirius, almost a thousand times closer to Earth. It then faded, and by the 20th century it could no longer be seen without a telescope. Astronomers call this outburst the Great Eruption.

Measurements of how fast the Homunculus is growing show that its gas was thrown out around 1847, in the middle of the eruption. The star survived and can again be seen with the unaided eye on a dark night. Astronomers expect the larger of its two stars to explode as a supernova, the explosion that ends the life of a very massive star, at some point in the future.

How it got its name

Between 1900 and 1930, astronomers noticed small objects right next to Eta Carinae, the first sign that it was more than a single point of light. Around 1950, the cloud was recognized as a nebula. The astronomer Gaviola named it the Homunculus, because on his photographs it looked like a small, plump man. Images from the Hubble Space Telescope later showed its two lobes in fine detail.