Three faces of a nebula

Posted: Wednesday, August 26, 2009 5:20 PM by Alan Boyle

ESO

The Trifid Nebula reveals three faces in this ESO view. Click on the image for a larger version.

The latest view of the Trifid Nebula serves as fresh evidence that good things definitely come in threes: This star-illuminated cloud of gas and dust gets its name from its three-lobed appearance (via the Latin word "trifidus"), and the European Southern Observatory's crowd-pleasing picture puts the "three faces" of the nebula on full display.

The Trifid Nebula, which lies thousands of light-years away in the constellation Sagittarius, was first observed by French astronomer Charles Messier in 1764 - who listed it as No. 20 in his famous catalog of interesting sky objects. It was English astronomer John Herschel who gave it the "Trifid" tag 60 years later.

In the centuries since then, the nebula has been imaged thousands of times, by the Hubble Space Telescope and other telescopes great and small. Today the ESO showed off its own view of the nebula, captured by the Wide-Field Imager attached to the MPG/ESO 2.2-meter telescope at the La Silla Observatory in Chile.

The three-lobed central region of the nebula is just one of the Trifid's three faces. As explained in today's image advisory from the ESO, that pinkish-red glow arises when young stars heat up the surrounding gas so much that it glows with the hot red signature given off by hydrogen. This is the Trifid's classic emission nebula.

The light-obscuring lanes of dust and cool gas that trisect the bright lobes represent another face, known as dark nebulae. Still more dark nebulae are scattered around the Trifid scene - and around the cosmos at large. The Horsehead Nebula, 1,500 light-years away in the constellation Orion, is the best-known example.

Blasts of stellar radiation can sculpt and squeeze the dense knots of nebular material into fresh batches of stars and planets. One potential example of this can be seen toward the lower part of the emission nebula, where a silhouetted finger (seen more clearly in this Hubble closeup) seems to point toward the nebula's bright central star. Actually, the star's radiation is carving away at the finger, leaving behind an evaporating gaseous globule, or EGG. Such EGGs are most famously on display in Hubble's iconic picture of the Eagle Nebula, known as the "Pillars of Creation."

The Trifid's third face is recognizable as a reflection nebula - that is, a cloud of gas that doesn't glow on its own but instead scatters the light filtering through from nearby stars. The effect can be seen just above and to the left of the emission nebula, where bluish clouds are lit up by sparkling stars.

"The largest of these stars shines most brightly in the hot, blue portion of the visible spectrum," ESO says in its advisory. "This, along with the fact that dust grains and molecules scatter blue light more efficiently than red light - a property that explains why we have blue skies and red sunsets - imbues this portion of the Trifid Nebula with an azure hue."

Eventually, the surrounding clouds will dissipate, leaving behind groupings of mature stars - just as a primordial nebula left behind our sun and its stellar neighbors billions of years ago. Billions of years from now, our sun - and some of the Trifid Nebula's hot young things - may well give rise to yet another type of nebula: the planetary nebulae created when dying sunlike stars blow off shells of colorful glowing gas.

You'll find all sorts of nebular faces in our Space Gallery, including our slideshow tribute to the "Pillars of Creation." But don't stop there: Check out the ESO's zoom-in video of the Trifid Nebula, look for planetary nebulae in msnbc.com's archive and learn about a brand-new breed of cosmic objects called "super-planetary nebulae."

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