Messier Monday: The Omega Nebula, M17

Perhaps the most spectacular example of rebirth in our galaxy, this cavernous nebula is giving rise to the next generation of stars!

Image credit: ESO, via http://www.eso.org/public/images/eso0925a/.


“For in the final analysis, our most basic common link is that we all inhabit this small planet. We all breathe the same air. We all cherish our children’s futures. And we are all mortal.” -John F. Kennedy

Imagine you could go back in time, 4.6 billion years ago, to when the giant molecular gas cloud that would give rise to the Sun, Earth, and around a thousand other stars first began to collapse. Initially overdense regions attracted more and more matter, accruing larger amounts of mass in a runaway gravitational process, until finally the temperatures and densities at the center were enough to ignite nuclear fusion. After that, a race begins: between the newly formed star(s) emitting radiation energetic enough to destroy the young nebula, and gravitational attraction in other overdense regions to form new stars.

Image credit: Mike Keith’s Periodic Table of Messier Objects, via http://cosmicneighbors.net/PeriodicMessier.htm.

Although this story only represents 7 of the 110 objects in the Messier catalogue, these seven objects are all contained within our galaxy and are not only home to some of the youngest stars we know of, but home to some of the most likely candidates to be the very next supernova in the Milky Way, and visible to the naked eye!

Today’s object — Messier 17, the Omega Nebula — is not only one of the most spectacular examples of this, it’s also one of the most accessible to amateur and professional skywatchers. Here’s how to find it.

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

The constellation of Sagittarius is incredibly rich in terms of stars, star clusters and new, star-forming nebulae. This shouldn’t come as a surprise: that’s the direction that the center of our galaxy is located! There’s simply more opportunity here for such objects to be found, and the Universe rewards those who look in likely places.

Sagittarius is well-known for its collection of stars that look like a teapot, most clearly visible above the southern horizon in summer but still visible in the southwest in the early part of the night here in October. The “top” of the teapot’s dome is marked by the star Kaus Borealis, which will make an easy-to-find guide towards Messier 17.

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

If you rise “up” from the top of the teapot’s dome, you’ll run into the prominent star μ Sagittarii, and then if you continue upwards and curve to the left you’ll run into the dimmer γ Scuti, which is still prominent due to the fact that it’s significantly brighter than all the other stars in its vicinity. If you follow that curve from μ Sagittarii to γ Scuti, you’ll find a number of faint (but still naked-eye) stars between them, with the one closest to γ Scuti — HIP 89851, about 2.5° west/southwest of this latter star — the one you’ll want to focus in on.

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

Because adjacent to this star lies exactly the faint, fuzzy, nebular object we’re setting our sights on: Messier 17, the Omega Nebula. It was catalogued by Messier back in 1764, where he described it as:

A train of light without stars, of 5 or 6 minutes in extent, in the shape of a spindle, & a little like that in Andromeda’s belt but of a very faint light…

It’s hard to blame him for such a description; through a modern small telescope (with a good camera), it looks something like this.

Image credit: Enzo De Bernardini, via http://www.astrosurf.com/astronosur/galeria/M17_20100809.htm.

But this “smudge” on the sky is much, much more than what it appears to be to the naked eye. What appears to be a small, irregular cloud emitting light and darkened by a few light-absorbing dust lanes is instead a giant cosmic star factory viewed from a great distance: some 5,500 light years away!

If we’re willing to look in a variety of different wavelengths of light and expose some essential elements, we can literally open up and peer into an entire new Universe.

Image credit: Andrea Tamanti, via http://www.tamanti.it/Nebulae/M17_sfull.htm.

Even with the breathtaking backdrop of the galactic plane behind it, and the thousands of stars visible to a quality telescope, it’s impossible to overlook this wonderland of new star birth. Just 15 light-years in diameter, the nebula proper (the bright part) contains anywhere from a few hundred stars all the way up into possibly the low thousands, at the center of a diffuse molecular cloud extending for at least an additional 25 light-years across and containing an estimated 30,000 solar masses worth of matter.

The bright red glow you see is indicative of ionized hydrogen, recombining to emit a characteristic spectrum of light, the brightest visible component of which is at 656.3 nanometers: the red you see above.

Image credit: European Southern Observatory, via http://www.eso.org/public/images/eso0416a/.

If we look at the right combinations of wavelengths, we can take a peek at the stars inside the nebula, finding around 100 bright, blue, young stars, with a number of evolved blue hypergiants and a total of nine O-class stars, all candidates for becoming Type II supernovae in the not-too-distant future! That could be tomorrow or in a few hundred-thousand years, but on astronomical timescales, these stars are destined for a spectacular funeral visible across the galaxy.

Image credit: NASA, ESA and J. Hester (ASU), via the Hubble Space Telescope at http://hubblesite.org/newscenter/archive/releases/2003/13/image/a/.

The brightest, hottest stars (like the one at the top left, above) are the ones that emit the most intense ionizing radiation, burning off the gas that’s working to collapse, form new stars and grow already existing stars, some of which are beginning to peek through the “evaporating gaseous globules” in the ribbon running from the bottom left to the top right of the image above. Although estimating the total number of stars is very difficult, based on the size, frequency and brightness of the whole nebula and the brightest stars within it, it probably has significantly more stars within it than the entire Orion Nebula, which would place the total number somewhere between 3,000 and 5,000, although it’s difficult to know for certain, as the lowest-mass stars are obscured beyond observation with current technology.

Image credit: VLT’s Survey Telescope, ESO/INAF-VST/OmegaCAM. Acknowledgement: OmegaCen/Astro-WISE/Kapteyn Institute.

There’s a slew of interesting phenomena going on in here, including:

  • the ionized hydrogen plasma emitting red light due to electrons recombining with the ionized hydrogen nuclei,
  • a large amount of blue light coming from various parts of the nebula, as the blue light from the brightest stars reflects off of the neutral gas,
  • newborn stars — most of which are under 1,000,000 years old — mostly obscured by the neutral gas of the nebula,
  • ionizing radiation working to burn off the gas and put a stop to the new star formation,
  • gravitation working like mad to attract as much of the gas as possible and bind it up in new stars, and
  • multiwavelength spectra revealing the ubiquitous presence of heavy elements beyond hydrogen, including carbon, oxygen, sulphur, silicon and more!
Image credit: Ignacio de la Cueva Torregrosa, via http://apod.nasa.gov/apod/ap070810.html.

The image above highlights a few of these — oxygen in blue, sulphur in red, and hydrogen in green — but that’s something that we don’t even need to look beyond the visible to discover. It’s long-exposure images like this that can bring out the faint details at the outskirts, teaching us that the total extent of the nebula ranges from 40-to-50 light years in diameter, or three times the size of the bright core.

But we’re not restricted to the visible; we can look in hugely different wavelengths all over the electromagnetic spectrum. And when we do, we can learn even more.

Images credit: ROSAT, Digitized Sky Survey, Anglo-Australian Observatory, 2MASS, IRAS, and VLA, all courtesy of IPAC/Caltech/CoolCosmos, via http://coolcosmos.ipac.caltech.edu/cosmic_classroom/multiwavelength_astronomy/multiwavelength_museum/m17.html.

The longest wavelength IRAS image (in the far-IR) reveals interstellar dust, which accounts for the dark dust lanes obscuring our view of portions of this nebula. That dust is invisible in the near-IR (the 2MASS image), and hence is lit up brightly. The X-ray shows either a faint source at the center (possible a black hole already!) or perhaps a background, coincidental quasar. The gas revealed in the mid-IR is the right temperature to form new stars, and likely represents the most active regions of star formation, while the radio wavelengths showcase the most heavily ionized regions.

This is as much a glimpse into the formation of new stars today as it is a window into the conditions under which our Solar System first formed a third of the age of the Universe ago! Have a look at the most spectacular view of this nebula I’ve ever seen, which showcases just how powerful the stellar winds from the most massive stars are, and the cavernous structures created in the nebula’s gas as a result!

The brightest stars here — the blue and red giants — will all be gone in just a few million years, resulting in between one-and-three dozen supernovae that will illuminate not only our skies, but the skies of watchers across our galaxy. In a few tens of millions years more, the entire nebula will be gone as well, having evaporated away completely. At that time, all that will be left is a bright open cluster of thousands of stars, which will slowly dissociate due to gravitational interactions, eventually leading to thousands of isolated star systems not so different from our own: with planets, asteroids, comets, heavy elements, organic molecules, and chances for life.

And with that final glimpse into not only our cosmic past but the future of our local Universe, we come to the end of one of the most spectacular Messier Mondays in recent memory. With the Omega Nebula complete, we have only six Messier Mondays left before our series is done! Take a look back at all our previous entries here:

And join us next week for one of the last remaining wonders in this catalogue!


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