Messier 16
Written by Tammy Plotner
Object Name: Messier 16
Alternative Designations: M16, NGC 6611, Eagle Nebula (IC 4703)
Object Type: Open Star Cluster and Emission Nebula
Constellation: Serpens (Cauda)
Right Ascension: 18 : 18.8 (h:m)
Declination: -13 : 47 (deg:m)
Distance: 7.0 (kly)
Visual Brightness: 6.4 (mag)
Apparent Dimension: 7.0 (arc min)
Locating Messier 16: One of the easiest ways to find M16 is to identify the constellation of Aquila and begin tracing the stars down the eagle's back to Lambda. When you reach that point, continue to extend the line through to Alpha Scuti, then southwards towards Gamma Scuti. Aim your binoculars or image correct finderscope at Gamma and put it in the 7:00 position. In the finderscope, M16 will show as a faint haze and in binoculars? You can't miss. If Gamma is in the lower left hand corner of your vision – then M16 is in the upper right hand. For all optics, you won't be able to miss the open star cluster and the faint nebulosity of IC 4703 can be seen from dark sky locations. Large aperture telescopes will be able to see the nebula well, but sky conditions are everything when it comes to this one. The star cluster which is truly M16 will always be easy, but the nebula is a challenge.
What You Are Looking At: Hanging out in space some 7,000 light years away in the next inner spiral arm of the Milky Way galaxy is a huge cloud of interstellar gas and dust. It spans some 70 by 50 light years across and caught in its heart is cluster of stars. Born around 5.5 million years ago, this glittering swarm marks an area about 15 light years wide that has captured our imaginations like no other area in the sky… the "Pillars of Creation".
Here, star formation is going on. The dust clouds illuminated by emission light… excited by the high-energy radiation of its massive hot, young stars. Inside the pillars are evaporating gaseous globules called (EGGs) emerging from the womb and about to become stars. The interstellar gas is dense enough to collapse under its own weight, forming young stars that continue to grow as they accumulate more and more mass from their surroundings. As their place of birth contracts gravitationally the interior gas reaches its end and the intense radiation of bright young stars causes low density material to boil away.
"For a long time astronomers have speculated about what processes control the sizes of stars – about why stars are the sizes that they are," said Jeff Hester of Arizona State University, Tempe, AZ. "Now in M16 we seem to be watching at least one such process at work right in front of our eyes."
The Hubble has shown us what happens when all the gas boils away and only the EGGs are left. "It's a bit like a wind storm in the desert," said Hester. "As the wind blows away the lighter sand, heavier rocks buried in the sand are uncovered. But in M16, instead of rocks, the ultraviolet light is uncovering the denser egg-like globules of gas that surround stars that were forming inside the gigantic gas columns." And some of these EGGs are nothing more than what would appear to be tiny bumps and teardrops in space – but at least we are looking back in time to see what stars look like when they were first born. "This is the first time that we have actually seen the process of forming stars being uncovered by photoevaporation," Hester emphasized. "In some ways it seems more like archaeology than astronomy. The ultraviolet light from nearby stars does the digging for us, and we study what is unearthed."
History: The star cluster associated with M16 (NGC 6611) was first discovered by Philippe Loys de Chéseaux in 1745-6, but it was Charles Messier who was the very first to see nebulosity associated with it. From his notes: "In the same night of June 3 to 4, 1764, I have discovered a cluster of small stars, mixed with a faint light, near the tail of Serpens, at little distance from the parallel of the star Zeta of that constellation: this cluster may have 8 minutes of arc in extension: with a weak refractor, these stars appear in the form of a nebula; but when employing a good instrument one distinguishes these stars, and one remarks in addition a nebulosity which contains three of these stars. I have determined the position of the middle of this cluster; its right ascension was 271d 15' 3", and its declination 13d 51' 44" south."
Oddly enough, Sir William Herschel didn't seem to notice the nebula in his notes and Admiral Smyth just barely saw it as well: "A scattered but fine large stellar cluster, on the nombril of Sobieski's shield, in the Galaxy, discovered by Messier in 1764, and registered as a mass of small stars in the midst of a faint light. As the stars are disposed in numerous pairs among the evanescent points of more minute components, it forms a very pretty object in a telescope of tolerable capacity."
Remember, the nebula isn't an easy object and it depends greatly on sky conditions as to how well you can see it on any given night. As historical evidence suggest, only one of the two masters caught it… So take a lesson and return many times. One day you'll be rewarded!
B&W image thanks to Palomar Observatory, courtesy of Caltech, M14 core region by NASA and the Hubble Heritage Team (STScI/AURA) and color image thanks to T.A.Rector (NRAO/AUI/NSF and NOAO/AURA/NSF) and B.A.Wolpa (NOAO/AURA/NSF).
Filed under: Astronomy
