Messier Monday: The Eagle Nebula, M16

Ethan Siegel
Oct 20, 2014 · 11 min read

With the iconic pillars and fairy inside, this star-forming region in our galactic plane just might be the most spectacular of them all.

Image credit: ESO, via http://www.eso.org/public/images/eso0926a/.

“The most amazing lesson in aerodynamics I ever had was the day I climbed a thermal in a glider at the same time as an eagle. I witnessed, close up, effortlessness and lightness combined with strength, precision and determination.” -Norman Foster

The night is dark, but for those of us who’ve taken the time to get to know not only the stars and planets, but also the deep-sky objects that lie scattered across the skies, it’s full of wonder. To the first surveyers of the night skies with telescopes, faint, extended objects popped out in their fields-of-view, with the most common objects being open star clusters, globular clusters and distant galaxies lying well beyond our own. But also lying thousands of light-years away in the skies are star-forming nebulae, giving birth to the youngest, newest stars in the entire galaxy.

Image credit: Ole Nielsen, of http://www.ngc7000.org/ccd/messier.html.

One of the most famous of these is the Eagle Nebula, the 16th object in Messier’s original catalogue of deep-sky objects. Like many star-forming regions, there already is a cluster of hot, young stars in there, and in fact that’s how this object was first discovered: by Jean-Philippe Loys de Chéseaux in the 1740s. It wasn’t for another two decades that the nebula was found, by Messier himself in 1764.

But the full wonder of this object is accessible to even amateurs with good skies, like the kind you’ll see shortly after sunset tonight. Here’s how to get there.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

Fresh off its extraordinarily close encounter with a comet, Mars appears low over the southwest horizon just as the skies darken tonight. Just to the east of it, the collection of stars making the teapot in Sagittarius shines prominently. If you move either from Mars up towards the zenith or from the “top” of the teapot (the star Kaus Borealis) up away from its base — about 15° either way — you’ll find yourself in a region of space that’s relatively dark, but surrounded by bright stars.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

In particular, there are three stars at the edges of three different constellations that seem to engulf this dark area of the night sky: μ Sagittarii (at bottom-center, above), α Scuti (at top-center, above), and ν Ophiuchi (towards the right and slightly above-center). These are the three brightest stars in this region of space, and they’ll help guide you towards Messier 16, the Eagle Nebula.

If you draw an imaginary line connecting α Scuti to μ Sagittarii, you’ll find the fainter (but still clearly naked-eye) star γ Scuti just to the east of that line. And just 2.5° to the west of that line, where the line of stars that culminates in γ Scuti seems to point, lies Messier 16, the Eagle Nebula.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

Looking through the best optics at his time, Messier was able to identify both a cluster and a nebula, although perhaps he didn’t seem all that confident in the latter, writing:

A cluster of small stars, enmeshed in a faint glow, near the tail of Serpens, at little distance to the parallel of Zeta of this constellation; with an inferior telescope this cluster appears like a nebula.

In fact, through a small, amateur telescope today, both the cluster and nebula are clearly visible.

Image credit: Rogelio Bernal Andreo, via http://www.deepskycolors.com/archivo/2008/06/07/messier-16-The-Eagle-Nebula.html.

The Eagle Nebula isn’t the brightest nebula in the sky, nor the closest, nor the youngest, nor is it the largest. In fact, it doesn’t even hold any of those distinctions among nebulae in the Messier Catalogue, of which there are only seven!

But the reason this object is so spectacular isn’t because of how extreme it is in any particular way, but because it simultaneously illustrates all of the different stages that occur in a star-forming region.

Image credit: Bryan Bradley, via http://www.bryanbradley.com/nebula.htm.

The cluster at the core of this nebula is on the small side, consisting of approximately 460 stars at last count, but dominated by a few bright, blue O-class stars, the most massive of which is approximately 80 times the mass of the Sun. Based on the bright stars that aren’t there, we can say that this cluster is at least a million years old, with parts of it likely older: perhaps between two and five-and-a-half million years for the oldest ones!

But young stars aren’t the only thing of note here; most of the region that appears the most “nebulous” is made out of hydrogen atoms that are constantly being ionized by ultraviolet radiation from these hot stars. And when the ionized electrons recombine with their atomic nuclei — protons, mostly — they emit a characteristic wavelength of light: 656.3 nanometers.

Image credit: Ole Nielsen, retrieved from his website at http://www.ngc7000.org/ccd/messier.html.

That’s the brightest emission line in the visible spectrum for hydrogen atoms, but if we looked in the ultraviolet we’d find an even stronger line! If this were one of the most pristine regions of gas in the young Universe, forming stars for perhaps the first time, hydrogen would be the vast majority of what’s there, with virtually no signs of atoms heavier than helium.

But other atoms in various ionized states also have characteristic emission lines at specific frequencies, and when we look for them, we find them in great abundance.

Image credit: T.A.Rector (NRAO/AUI/NSF and NOAO/AURA/NSF) and B.A.Wolpa (NOAO/AURA/NSF), via http://www.noao.edu/image_gallery/html/im0725.html.

These elements include carbon, oxygen and sulphur, among many others. Both the young stars presently visible as well as the ones that still have yet to form in this nebula contain solar-system-like abundances of these heavy elements, meaning that — much like our neighborhood — we have every reason to believe that not only planets, but rocky planets with the raw ingredients for life abound with great frequency around practically every one of these new stars.

Image credit: ESA/Hubble & NASA, via http://www.spacetelescope.org/images/potw1033a/.

But one of the things I mentioned is one of the most spectacular things in the Eagle Nebula: the stars that have yet to fully form. There are a few regions in this nebula that consist of gas-and-dust of much greater density than normal. And it’s these regions where the ultimate cosmic race-against-time happens in earnest: the race between gravity to form and grow new stars, while radiation from all the stars both young and old works to heat up and accelerate that matter away, preventing further star growth.

Image credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).

Huge dust spires, like the “fairy” shown above, are indeed home to newly forming stars. Much lower in temperature than the hot surroundings of the interstellar medium making up the majority of the nebula, a cool region like this one has all the ingredients necessary to contract down and have the densest regions collapse, giving rise to new stellar life.

But the most spectacular region in this nebula — and perhaps the most famous photo of them all — belongs to the pillars of creation that lie within the Eagle Nebula.

Image credit: Salvatore Grasso, with the pillars at center, via http://sgastrophotography.com/Sgastrophotography/Messier_16.html.

These three dust columns look like stalagmites to some, and different astrophotographers have focused on different properties, elements or features. But by far the most iconic image of all was taken in 1995 by Hubble, and is arguably the greatest Hubble image of all-time.

Image credit: Jeff Hester and Paul Scowen (Arizona State University), and NASA/ESA, via http://www.spacetelescope.org/images/opo9544a/.

What you’re looking at is three competing processes going on at once:

  • The neutral gas you see here working hard to collapse into gravitational clumps, with the largest clumps growing into the largest stars.
  • The young proto-stars inside emitting intense radiation, destroying these pillars from the inside. In the peak of the largest spire, you can clearly see the starlight trying to peek through the gas.
  • And finally, the ultraviolet radiation from outside the pillars, from the other hot stars in and around the nebula, works to evaporate the gas in the pillars externally.

You might think that the inference that there are significant numbers of proto-stars inside these pillars is too great an assumption, but direct images in the infrared:

Image credit: European Southern Observatory (ESO), via http://en.wikipedia.org/wiki/File:M16_-_Eagle_nebula.jpg.

as well as from the Chandra X-ray Telescope — overlayed with the Hubble image, below — show that they’re really there!

Image credit: X-ray: NASA/CXC/U.Colorado/Linsky et al.; Optical: NASA/ESA/STScI/ASU/J.Hester & P.Scowen.

And that’s the fabulous story of the Eagle Nebula, in all its glory. From gas that’s still collapsing to active star formation to growing proto-stars to a young, fully-formed star cluster, this is one Messier object that has it all, and at some point in the next few hundred thousand years, will have a number of supernovae go off on top of it all!

And that will take us to the close of another Messier Monday. We’ve only got five objects left now, so don’t forget to take a look back at the 105 we’ve covered so far:

With a bright, young Moon out next week (and Halloween approaching), the skies will have an awful lot to offer. Don’t miss next Monday’s treat, and until then, enjoy your skies!


Starts With A Bang!

The Universe is out there, waiting for you to discover it.

    Ethan Siegel

    Written by

    The Universe is: Expanding, cooling, and dark. It starts with a bang! #Cosmology Science writer, astrophysicist, science communicator & NASA columnist.

    Starts With A Bang!

    The Universe is out there, waiting for you to discover it.