Are you seeing a ghost? It could be. When most people think of the Trifid nebula they think of the wild colored fantasy images they’ve seen taken with filtered, long exposure photography. But what happens when you combine science with imagery? Just ask Toni Heidemann. Toni may have made his living in Grenoble, France by studying cold neutron backscattering in a spectrometer, but when he retired in 2002 he did the world a favor by turning his interest in h-alpha celestial photography into works of art.
The Trifid Nebula is also known as Messier 20 and NGC 6514. But what is it? Behold a three-lobed, glowing cloud of gas and plasma where star formation is taking place. In the case of our ghostly apparition, this is a remarkable collection of open cluster, emission nebula and Barnard dark nebula (B85) combined. Buried in here are hot, young blue stars which formed from the gas itself and they are emitting unfathomable amounts of ultraviolet light and ionizing the nebulous sheath around them.
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Is M20 the ghost of the past – or the ghost of the future? The huge cloud of ionized molecular hydrogen may have already given birth to thousands of stars and may yet be the home of an eventual supernova. In a few more million years, the driving force of the stellar winds from the more massive stars will disperse the cloud, leaving only the cluster. But, for now, recent Hubble studies have shown NGC 6514 to be home to an EGG – an evaporating gaseous globule – a clump of gas so dense that not even the Trifid’s fueling star can destroy it.
Perhaps it is M20’s varying nature that makes its distance so hard to distinguish as a single object. Many times we disregard history’s teachers, such as Sir William Herschel, who instinctively chose to label the Trifid as four separate objects. Of course, why he did so may remain open to debate, but as a devotee of Herschel’s work, I’ve often found his assumptions have often remarkably been proved accurate. There is a star cluster in the center, surrounded by an emission nebula, enfolded in a reflection nebula and divided by a dark nebula. No wonder science can’t decide if its 2200 light years away or 7600! Some figures place it at 5200, others at 3140, and even recent Hubble studies can only say “about 9000 light years away”.
So why are images like Toni’s M20 really more exciting than the colorful Trifid renditions? By using h-alpha, he’s blocking most of the visible spectrum and centering on collecting specific photons. The h-alpha wavelength is a wonderful resource for studying the ionized hydrogen content of gas clouds like NGC 6514. Because it requires as much energy to excite the hydrogen atom’s electron as it does to ionize it, chances are slim that it will be removed from the equation. Once ionized, the electron and proton recombine to form a new hydrogen atom – perhaps emitting hydrogen alpha wavelengths and photons.
Want to know more? According to studies done by Yushef-Zadeh (et al), “Radio continuum VLA observations of this nebula show free-free emission from three stellar sources lying close to the O7 V star at the center of the nebula. We argue that neutral material associated with these stars is photoionized externally by the UV radiation from the hot central star. We also report the discovery of a barrel-shaped supernova remnant at the northwest rim of the nebula, and two shell-like features.” More features? “We also note a remarkable complex of filamentary and sheetlike structures that appear to arise from the edge of a protostellar condensation. These observations are consistent with a picture in which the bright massive star HD 164492A is responsible for the photoevaporation of protoplanetary disks of other less massive members of the cluster, as well as the closest protostellar condensation facing the central cluster.”
There is such a huge amount of information packed into what appears to be such a small area of space. According to Lefloch (et al), “The Trifid Nebula is a young H II region undergoing a burst of star formation.” Their far-infrared studies took a deeper look at the protostars surrounding the Trifid’s exciting star hiding behind the ionization front. “Inspection of their physical properties suggest that they are similar to the dust protostellar cores observed in Orion, although at an earlier evolutionary “pre-Orion” stage. The cores are embedded in a compressed layer of dense gas. Based on comparison with the models, we find that the cores could have formed from the fragmentation of the layer and that the birth of the protostars was triggered by the expansion of the Trifid Nebula.”
From studies that examine the internal dust which absorbs and scatters radiation from the H II region and central star to polarization studies which show the continuum is higher in emission lines for three regions in the southern part of the nebula, the M20 is still a wonderful, delightful and mysterious “Ghost Of Summer”… and meant to be enjoyed in exactly the color in which we see it.
Many thanks to Toni Heidemann and his outstanding h-alpha imaging work. Merci.