A Stellar Collision Birthed the ‘Dragon’s Egg’ Nebula, a Puzzling Structure in the Milky Way

The colorful cloud of gas and dust has a violent origin—and this explains the unusual traits of two massive stars within it, astronomers say

A close-up of the Dragon's Egg Nebula, with two "mismatched" stars inside.
A close-up of the Dragon's Egg Nebula, with two mismatched stars inside. ESO/VPHAS+ team. Acknowledgement: CASU

Some 3,800 light-years away from Earth, in the Southern Hemisphere constellation Norma, lies a massive cloud of gas and dust. Nicknamed the “Dragon’s Egg” nebula for its proximity to the Fighting Dragons of Ara nebula complex, this mysterious cosmic structure has baffled astronomers. Now, researchers have discovered new evidence to support the idea that the breathtaking purple, pink and orange orb was born from a violent past.

A stellar tango flows beautifully inside the Dragon’s Egg. Two massive stars reside, each bound by the other’s gravitational pull, but they are rather unalike. One is about 30 times more massive than the sun, magnetic and 2.7 million years old; the other is 26.6 times the sun’s mass, non-magnetic and 4.1 million years old.

These conditions—two massive stars existing within a nebula and their very different properties—are outside the norm. And at 7,500 years old, the nebula itself is much younger than both stars inside. Combined, these cosmic rarities stood out to scientists.

“When doing background reading, I was struck by how special this system seemed,” Abigail Frost, an astronomer at the European Southern Observatory (ESO), says in a statement. “It really made us feel like something cool had to have happened in this system. When looking at the data, the coolness only increased.”

Three panels show an artist's interpretation of how two stars merged to create the larger, younger, magnetic star currently residing inside the Dragon's Egg Nebula binary system.
Three panels show an artist's interpretation of how two stars merged to create the larger, younger, magnetic star currently residing inside the Dragon's Egg Nebula binary system. The fourth panel depicts a real image of the nebula. ESO/L. Calçada, VPHAS+ team. Acknowledgement: CASU

To make sense of this system, Frost and other ESO astronomers analyzed nearly a decade’s worth of data collected by the Very Large Telescope in Chile’s Atacama Desert, as well as a swath of archival data from a second, nearby observatory called FEROS.

In a study published last week in the journal Science, the team suggests the system within the Dragon’s Egg used to have three stars around four million to six million years ago—until two of them crashed together and merged into one. That collision would have created the larger, magnetic, younger star observed today—and it would explain the recent formation of the nebula, as well as the difference they observed between the stars’ ages. These observations, backed by theoretical models the team created, suggested a stellar merger had occurred.

“Our study is the observational smoking gun confirming this scenario,” senior author Hugues Sana, an astronomer at KU Leuven in Belgium, tells Reuters Will Dunham.

Artist's animation: the violent history of stellar pair HD 148937

“The more distant star formed a new orbit with the newly merged, now-magnetic star, creating the binary that we see today at the center of the nebula,” Sana adds to Space.com’s Keith Cooper.

The research suggests the crash between stars tossed material out into the surrounding space, making the nebula. Today, astronomers observe gases such as nitrogen, carbon and oxygen in the Dragon’s Egg, although these gases are usually found within stars. Such a collision would explain how these elements were set free.

It would also explain how one of the nebula’s massive stars became magnetic. Most stars the size of our sun have a magnetic field. But only about 7 percent of massive stars, which are at least eight solar masses, have this characteristic. If the massive, magnetic star in the Dragon’s Egg nebula was indeed formed by a stellar merger, it offers compelling evidence that such collisions can lead this unusual—and fleeting—condition to appear.

“Magnetism in massive stars isn’t expected to last very long compared to the lifetime of the star, so it seems we have observed this rare event very soon after it happened,” says Frost in the statement.

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