The Trifid Nebula

Jack Kramer

The Trifid Nebula (M20) is a favorite object in the summer sky. The dark lanes in this diffuse nebula divide it into three principle parts and are visible under a reasonably dark sky in all but the smallest telescopes. (The word "trifid" means "divided into three parts".) The lanes actually create more than just three segments to the nebula; the others can be distinguished with a large enough telescope under dark skies. A "nebula" filter or light pollution reduction filter will provide an improved view where skyglow is a problem. There is both emission and reflection type nebulosity represented here, but since it's primarily an emission type nebula, Oxygen III and other narrow band filters work especially well.

This also happens to be one of those few nebulae in which visual observers sometimes report seeing a slight pinkish coloration. Along with some blue reflection nebulosity, astrophotos show strong red hydrogen emission nebulosity. The best way to see this color visually is to observe from a very dark site without any sort of filter. Since our eyes are less sensitive to the red end of the spectrum, you'll need as large a telescope as possible. A hint of pink coloration has been detected under ideal conditions in 13-inch scopes. However, such conditions appear to be very rare. The reflection part of the nebula is somewhat fainter, so it's unlikely that we would be able to detect any of the blue coloration visually. Have you ever seen any color?

This brings up the issue of whether it's possible to visually detect reddish coloration in any diffuse nebulae. (However, certain planetary nebulae are famous for showing color even in instruments of modest size.) Many experts say it's impossible for color to be seen in diffuse nebulas, especially with run-of-the-mill amateur instruments. I used to correspond once in awhile with the late Walter Scott Houston, the feisty deep sky columnist in Sky & Telescope. In one of his responses to me on this subject he wrote that seeing color in diffuse nebulas is not uncommon: "I and many others have seen the pink tinge in M42. I thought it was well known. Of course, weather plays a great role. No two nights are exactly the same." So carefully observe the Trifid in a dark sky; you never know what you'll find.

The Trifid is also a very active nebula. Hubble Space Telescope images show a region of ongoing star formation that is being torn apart by radiation from the bright star at the center of the nebula, which lies about 9,000 light-years away from Earth. Ultraviolet radiation from this star is also lighting the nebula, causing it to glow like a fluorescent light. As in Hubble images of M16 (nicknamed "The Pillars of Creation"), evaporating gaseous globules (EGGs) have also been found in the Trifid. One of these globules has a jet of material three-quarters of a light-year long, moving out at speeds up to about 800,000 miles per hour. This jet is a tip-off to the stellar object hidden from view within this EGG. It's estimated the jet turned on about 600 years ago, and is being powered by the star formation.

Within the next 10,000 years or so, the advancing ionization front from the nebula's main star will overrun the forming star in the EGG. Depending on the strength of the front, the newborn star's formation could stop dead in its tracks. This may have already happened to another star in the EGG. A tiny microjet pointing out from one of the "fingers" in the EGG may be the last gasp of a star that was cut off from its supply of nurturing gas and dust by the ionizing radiation more than 100,000 years ago.

Many stars like our Sun formed in regions like these, in the neighborhood of massive powerful stars. Luckily, our Sun survived it's birthing!