Nine-thousand light-years away in the direction of the constellation Sagittarius, lies the famous Trifid Nebula, so-called for the three dark “lanes” that trisect it.
The Trifid’s triple nature is not limited to the lanes though. It is also three different types of nebulosity in one…it is a reflection, emission and dark nebula in one neat package.
A nebula is a cloud of gas and sometimes dust, floating in interstellar space.
Reflection nebulae have a bluish colour. We see them because light from nearby stars is reflecting off them—the process preferentially reflects the blue wavelengths of the starlight.
Emission nebulae are pinkish. In this case they’re not reflecting light, but emitting their own pale glow. (In the case of the image above, the emission nebulosity looks orange due to the particular wavelength observation used to make the image.)
Silhouetted against brighter backgrounds, dark nebulae stand out like ghostly holes in space. In reality they are very dense clouds of gas and dust particles—they don’t give off or reflect any light to speak of.
The Trifid was discovered in 1764 by the French astronomer Charles Messier, who made it number 20 in his catalogue of “deep sky” objects…hence it’s other common name, M 20.
Messier was a comet hunter who had become frustrated by repeatedly coming across fuzzy blobs in the sky that didn’t turn out to be comets. He decided to make a catalogue of those blobs so that he and other astronomers could learn to ignore them in future.
Ironically, he is now better known for his list of 103 deep sky objects (more were added later by other astronomers) than he is for the 13 comets he discovered.
At the time, Messier thought M 20 was actually a small bunch of stars that couldn’t quite be seen individually. But he did notice the three dark lanes running through it, and gave it the name Trifid.
Story by Jonathan Nally, editor SpaceInfo.com.au
Image courtesy IAC / Daniel López.
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