The Trifid Nebula is so named because it is divided into three regions. The red color comes from a cloud of gas illuminated by a star in the center of the nebula, where the star excites the gas and causes emission radiation of the gas, which is called an H II region. The blue color is simply a reflection of star light from a gas cloud that is not close enough to nearby stars to be heated like the red colored cloud, and thus does not produce emission radiation. This object also includes dark nebula regions as well. The contrast between the emission nebula cloud and the reflection nebula cloud is a beautiful feature of the Trifid Nebula. This is one of my favorie objects because it has all three different types of nebulae. In this case, the H II region of the nebula is a cloud of gas that has been ionized by a monster star known as HD 164492A. The main source of illumination for the blue reflection nebula is a star known as HD 164514. The dark nebula lanes running through the red area are cataloged as Barnard 85.
Click on the above image for a larger version of the image.
The Trifid Nebula is the result of two molecular clouds that collided with each other. As an H II region, it is rare -- it is only the second cloud-cloud collision H II region discovered. Most H II regions are the result of a molecular cloud that gravitationally collapses as the result of a nearby supernova explosion or some sort of galatic density wave that triggers the collapse. The Trifid Nebula is in the same region of the sky and at about the same distance as the Lagoon Nebula, between 2,500 light-years and 5,000 light-years away from us. The Trifid Nebula was probably first discovered by French astronomer Le Gentil in 1747. It is believed that it was first named the "Trifid Nebula" by John Herschel, the son of the famous astronomer William Herschel, in the mid-1800's. Charles Messier included it in his first list published in 1769.
The monster star HD 164492A is part of a multiple star system at the center of the red H II region of the Trifid Nebula, with seven of the stars being visible in amateur size telescopes. HD 164492A and HD 164492C are the brightest in the group. All of the stars in this group as estimated to be only about 300,000 years old.
This is an LRGB composite CCD image taken with the 32-inch Schulman Telescope on the summit of Mount Lemmon, Arizona, at the University of Arizona SkyCenter. I operated the telescope remotely.
M20 (NGC 6514)
Constellation: Sagittarius
RA: 18h 02m 18s Dec: -23d 02' 00" (J2000)
July 2, 2016
Image by Sid Leach
Mount Lemmon, Arizona
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