The Cocoon Nebula. Photo: Marcel Drechsler.
The Cocoon Nebula widefield. Photo: Tommy Knutsen.
Today's APOD looks fine, but I think we need to take a more widefield approach to understand more about the nature of this nebula. Marcel Drechsler's image (top right) shows colorful nebulosity surrounding the rosy red emission nebula of IC 5146 proper. A bluish reflection nebulosity can be seen encircling the central red emission nebula, and two deep red "wings" of emission nebulosity protrude from the main body, giving the nebula the appearance of a bird. Or perhaps it is more like an airplane, because we can see a long thick "exhaust trail" of dusty gas flowing from the nebula toward the lower left.
Tommy Knutsen's image "puts the Cocoon Nebula in its place" some distance away from the North America Nebula and Deneb. We can see how small the Cocoon Nebula is, although appearances are somewhat deceiving: The way I understand it, the Cocoon Nebula is twice as far away from us as the North America Nebula. Note that the tiny Cocoon Nebula is located at the tip of a long dark dust lane.
But the Cocoon Nebula is
small. It is powered by a single hot star, and this star isn't even an O-type star. We are talking about B1V-type BD+46 3474 or TYC 3608-1446-1. The star is too faint in our skies to even have an HD or an SAO designation. According to Wikipedia, the size if the Cocoon Nebula is about 15 light-years, and the age of the ionizing star is only about 100,000 years.
I was struck by how much the Cocoon Nebula really resembles the Trifid Nebula:
Both the Cocoon Nebula and the Trifid Nebula have round red emission centers, ionized by a single star or a very small group of stars. Both nebulas and their ionizing stars are very young. Both nebulas display prominent dust lanes crisscrossing the red emission center. Both nebulas are surrounded by a bluish reflection nebulosity.
I googled "Trifid Nebula wiki" to see how big the Trifid Nebula is compared with the Cocoon Nebula, and to find out how old the ionizing central star of the Trifid Nebula is compared with BD+46 3474 of the Cocoon Nebula, which is apparently 100.000 years old.
Unfortunately, I could find no Wikipedia information on the age of the ionizing star of the Trifid, HD 164492A, which is however an O7.5III star, and thus much hotter than a B1V star. But the radius of the Trifid is said to be 21 light-years, compared with (the way I understand it) a 15 ly diameter
for the Cocoon. So clearly the Trifid Nebula is both larger and brighter than the Cocoon.
They do look similar, nevertheless.
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