The Trifid Nebula (Trifid literally meaning ‘divided into three lobes’) is a diffuse nebula located in the constellation Sagittarius (Frommert "Messier 20"). A diffuse nebula such as this is very basically a cloud of gas and dust (primarily Hydrogen gas) floating in space. If such a nebula is large and massive enough, it often becomes a breeding ground for new stars (Frommert "Nebulae"). The Trifid Nebula has been estimated at a wide variety of distances by different sources; however, the index on seds.org gives a distance of 5,200 light years. The Trifid Nebula was discovered originally by Charles Messier in 1764, who described it as “a cluster of stars.” It was later recorded byWilliam Herschel in 1786, who recorded it as four separate objects (likely due to its three-lobed appearance). William’s son, John Herschel, was the first to use the term “Trifid” to describe this particular nebula, and recorded it as a single object rather than four (Frommert "Messier 20").Diffuse nebulae can be classified as emission, reflection, or dark nebulae based on the light that is seen from the nebulae. The Trifid nebula, however, contains components of all three of these. In the above picture we can see a red emission nebula at the bottom. An emission nebula is a cloud of very hot Hydrogen energized by ultraviolet rays from a nearby star--in this case, likely the very bright star at the bottom of the picture. Since Hydrogen gas produces red emission lines when it is energized, emission nebulae appear red (Arnett). The Trifid nebula also contains what is called a reflection nebula (the blue region in the middle of the photograph), which is a cloud of dust that reflects light from a nearby star or stars. The light reflected is actually the same as that produced by the stars (and therefore would produce a continuous spectrum); however since scattering is more efficient for blue light, reflection nebula appear blue (Arnett; Frommert "Nebulae"). Finally, in the picture we can see dark nebulae (the apparent rifts in the red cloud at the bottom of the picture). Dark nebulae are not so different in nature from reflection nebulae—they are clouds of dust that block the light of whatever is behind them rather than reflect it, based solely on the geometry of the dust, the light source, and the earth (Arnett). Since the distance to this nebula is estimated to be 5,200 light years (Frommert "Messier 20"), we can determine a linear size of 11 light years from applying the maximum angular size of 446 arcseconds (determined from the above picture) to the small angle formula.
Arnett, Bill. "Types of Nebulae." The Web Nebulae. N.p., 10 Feb. 1997. Web. 19 Apr. 2011. <http://astro.nineplanets.org/twn/types.html>.
Frommert, Hartmut, and Christine Kronberg. "Messier 20." seds.org. N.p., 13 Aug. 2007. Web. 19 Apr. 2011. <http://seds.org/messier/m/m020.html>.
Frommert, Hartmut, and Christine Kronberg. "Nebulae." seds.org. N.p., 17 Nov. 2005. Web. 19 Apr. 2011. <http://seds.org/messier/nebula.html>.
|Right Ascension (J2000)||10:02:31.30|
|Filters used||B (Blue), C (Clear), R (Red), V (Green)|
|Exposure time per filter||B, V, R, and C (60s x 5)|
|Date observed||March 24, 2011 UT|