“Fried Egg” Nebula Shows Rare Phase in Star Evolution

Like children finding shapes in the clouds, astronomers seem to have quite a bit of fun nicknaming the fabulous nebulae they find scattered across the cosmos.

For the most part the names are fairly straightforward, based either on location—a la the Orion Nebula in the constellation Orion—or on the most obvious shape that comes to mind, including Horsehead, Cat’s Eye, Crab, and even Running Chicken.

Astronomers must have been pretty hungry the day they first saw this new picture of the star IRAS 17163-3907 taken with ESO’s Very Large Telescope: They dubbed it the Fried Egg Nebula.

Picture courtesy ESO/E. Lagadec

How can a star also be a nebula, you ask? After all, nebulae are supposed to be cosmic clouds of dust and gas.

In this case, the new picture—the clearest yet taken of this object—revealed that the massive star lies at the center of two huge, nearly spherical shells of debris.

The star and shells together make up the nebula, and their combined shape inspired the breakfast-food moniker.

Perhaps the most exciting element is that the new picture allowed the team to identify the star as a yellow hypergiant, a rare type of star that’s on an evolutionary path to death by supernova.

Although it’s faint in visible light, considering it’s about 13,000 light-years away, the star had been discovered in 1983 glowing brightly in infrared in the constellation Scorpius.

The faint Fried Egg would lie inside the red circle. Star chart courtesy ESO, IAU and Sky & Telescope

The new data show that the star is about 20 times as massive as the sun and shines roughly 500,000 times brighter.

In addition, the dusty double shells indicate that the star is ejecting bursts of material: In fact, it appears to have lost about four times the mass of the sun in the space of just a few hundred years.

This sort of activity is a sure sign that the star is in its death throes.

When stars ten times as massive as the sun or more run out of hydrogen, they switch to burning helium and swell to become red supergiants. The bright star Antares, also in Scorpius, is a famous example of a red supergiant.

Once the helium runs out, a supergiant star progressively burns heavier elements until it builds up a mostly iron core, at which point the core collapses, triggering a Type II supernova.

(See “Supernova’s Beginning Blast Shown in 3-D—A First.”)

In rare instances, though, the red supergiant can evolve even further before it explodes, becoming an active yellow hypergiant, then a luminous blue variable, then a Wolf-Rayet star, which is characterized by fierce stellar winds ejecting material in all directions at 300 to 2,400 kilometers a second.

Yellow hypergiants are not only rare, they last just a few million years, so finding one is no small feat.

Based on this star’s activity, the research team thinks it won’t be long until IRAS 17163-3907 explodes—or maybe it already has and we simply haven’t seen it yet.

The cosmic Frying Pan? The star field in the direction of the galactic center that includes the Fried Egg. IRAS 17163-3907 is indistinguishable from the thousands of other stars in this frame. Picture courtesy ESO and Digitized Sky Survey 2

The Fried Egg Nebula is described in a new paper posted online at arXiv.org and submitted for publication in the journal Astronomy & Astrophysics.

Victoria Jaggard is the space editor for National Geographic News. Email blog tips to vjaggard@ngs.org, and follow Victoria on Twitter @vmjaggard99.

Changing Planet