All stars are not created equal, nor are their creators. By far the best-known stellar nursery, the Orion Nebula, has spawned thousands of young stars, large and small. It glows so brightly the naked eye can see it, even though it is 1,350 light-years away. On a clear, dark, moonless night the cloud of gas and dust that makes up the Nebula looks like a fuzzy star south of the highly visible three-star belt of Orion, a constellation prominent tonight in all populated regions of the world. Now a new imaging technique has revealed that this great nebula is just a small part of an enormous ring of dust stretching across hundreds of light-years. The discovery hints at the nebula’s origins: radiation and explosions of massive stars at the ring's center may have blasted gas and dust outward until some of the material collapsed to give birth to the famous star creator.
No one had previously noticed the ring because foreground and background dust obscures the newfound object. "We were completely surprised to find out there's this beautiful ringlike structure," says Eddie Schlafly, an astronomer at the Max Planck Institute for Astronomy in Germany. He and his colleagues found the ring using the 1.8-meter Pan-STARRS telescope in Hawaii to map interstellar dust. Dust reddens starlight—that's one reason the setting sun looks orange or red—so Schlafly's team observed the colors of stars over most of the sky in order to see where interstellar dust lurks. From the colors and distances of 23 million stars the team established how the dust is distributed in three dimensions in and around Orion.
These observations revealed that the Orion Nebula lies on the rim of a vast ring of dust that is 330 light-years in diameter—so large that much of it spills into Monoceros, a constellation east of Orion. If the ring were visible to the naked eye, it would look 27 times wider than the full moon. The Orion Nebula sits in one of its densest sections. The discovery appears in the February 1 The Astrophysical Journal.
John Bally, an astronomer at the University of Colorado Boulder who was unaffiliated with the discovery, calls the new dust-mapping technique that revealed the ring phenomenal. "It really allows us to measure the dust distribution in three dimensions for the first time," he says. "That I think is an astonishing result." The discovery hints at the Orion Nebula’s origins. One scenario: 10 million or 20 million years ago, long before the Orion Nebula existed, a group of massive stars arose. These stars were hot and luminous and the ultraviolet light they emitted stripped electrons from interstellar hydrogen gas in all directions. This radiation pushed interstellar gas and dust away in an expanding bubble, which was jolted even more when the stars exploded as supernovae. Parts of the bubble's surface grew dense enough to collapse, forming new stars—and an especially rich region of star birth set aglow the gas and dust we now call the Orion Nebula.
Bally and Christopher McKee, an astrophysicist at the University of California, Berkeley, say this scenario is plausible but requires confirmation. If the idea is right, the dust ring should be expanding, so scientists will have to measure the dust expansion speed to verify it. Those measurements would also indicate when the expansion began, thereby dating the sequence of events that may have led to the Orion Nebula’s formation.
Europe’s new Gaia spacecraft may lend further insight as it determines distances and motions of stars throughout the sky. Gaia may reveal stars that have moved away from the ring’s center, siblings of the deceased stars that created the ring, teaching us more about the formation process. The discovery of the Orion dust ring is an important piece of the puzzle, Bally says, although many aspects of star formation ecology in the region remain to be understood.