APOD: A Beautiful Trifid (2022 Aug 05)

[APOD Robot]

A Beautiful Trifid

Explanation: The beautiful Trifid Nebula is a cosmic study in contrasts. Also known as M20, it lies about 5,000 light-years away toward the nebula rich constellation Sagittarius. A star forming region in the plane of our galaxy, the Trifid does illustrate three different types of astronomical nebulae; red emission nebulae dominated by light from hydrogen atoms, blue reflection nebulae produced by dust reflecting starlight, and dark nebulae where dense dust clouds appear in silhouette. But the red emission region, roughly separated into three parts by obscuring dust lanes, is what lends the Trifid its popular name. Pillars and jets sculpted by newborn stars, above and right of the emission nebula's center, appear in famous Hubble Space Telescope close-up images of the region. The Trifid Nebula is about 40 light-years across. Too faint to be seen by the unaided eye, it almost covers the area of a full moon in planet Earth's sky. Open star cluster M21 just peeks into this telescopic field of view along the bottom right edge of the frame.

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[Ann]

M20-Trifid-Nebula-1024[1].jpg

A Beautiful Trifid. Image Credit & Copyright: Vikas Chander

Click to view full size image

That's a very fine portrait of the Trifid Nebula! The pink "main body" of the Trifid looks almost like a pink beach ball floating on somewhat choppy waters!

The colors of the APOD are great. Note the very pink hue of the central Trifid, which is due to cyan-colored hydrogen beta (at 486 nm, this color ███) being mixed with the dominant red hydrogen alpha (at 656 nm, this color ███). The blue "choppy waters" surrounding the "pink beach ball" is reflection nebulosity.

By the way, where does the blue reflection nebulosity come from? I'm going to hazard a guess. It comes from the same source that provides the ionization for main pink nebula, namely, from O7.5III star HD 164492A. For this to be true, the blue light from HD 164492A must "reach further" than the ultraviolet ionizing light from the same star.

Like the Trifid Nebula, the Cocoon Nebula is ionized by a single star, B1V-type BD+46 3474. Like the Trifid Nebula, the small Cocoon Nebula also has a surrounding blue reflection nebula, although this reflection nebula is quite faint and can only be brought out with careful processing.

Cocoon Nebula Marcel Drechsler.png

Cocoon Nebula. Image: Marcel Drechsler.

Interestingly, both the APOD and the Marcel Drechler image shows some interesting red stuff in the background. My impression of Marcel Drechler's image is that the main red "streak" are outflows or jets from the very young hot star, but there is generally a lot of red (probably quite faint) Hα mixed with the dust to the left of the Cocoon in Drechsler's image.

The red background stuff in the APOD is not so jet-like. We may also note that the color of this background ruddiness is not at all pink, but "all red". If this red stuff is faint, which it is, perhaps the proportion of hydrogen beta to hydrogen alpha is lower than it would be if the source of ionization was strong, as it is in the Trifid Nebula? Or are we talking about hydrogen beta being scattered away due to background dust reddening?

NGC 3603 and NGC 3576 Astrosurf.png

Background reddened NGC 3603 and foreground less reddened NGC 3576.
Photo: Astrosurf.

In Astrosurf's image, background NGC 3603 is at left, and foreground NGC 3576 is at right. The more distant nebula is more reddened, because almost all the hydrogen beta has been scattered away.

Ann

[orin stepanek]

M20-Trifid-Nebula-1024.jpg

Sorta reminds me of a Pink Pansy

th-2101133866.jpg

Like this;

th-1704427468.jpg

or this!

[johnnydeep]

Looks more like the Quadfid nebula to me! Also, in the "dark nebula" link - https://apod.nasa.gov/apod/ap090522.html - I see stars well within the darkest parts of the dark Pipe Nebula. Are those stars bright enough to be shining through it, or are they in front of it? (I tried posting that question at the discuss page for that APOD, but the "discuss" link takes me to elsewhere.)

Click to view full size image

[Chris Peterson]

Looks more like the Quadfid nebula to me! Also, in the "dark nebula" link - https://apod.nasa.gov/apod/ap090522.html - I see stars well within the darkest parts of the dark Pipe Nebula. Are those stars bright enough to be shining through it, or are they in front of it? (I tried posting that question at the discuss page for that APOD, but the "discuss" link takes me to elsewhere.)

Click to view full size image

When you see stars (especially unreddened ones) apparently in front of a dense dust nebula, they are almost certainly physically in front of it as well. It's not impossible they are just barely inside it, but there's a lot more space between us and the nebula than there is in the narrow shell where you might still see an interior star. Statistics favors it being outside the nebula.

[johnnydeep]

Looks more like the Quadfid nebula to me! Also, in the "dark nebula" link - https://apod.nasa.gov/apod/ap090522.html - I see stars well within the darkest parts of the dark Pipe Nebula. Are those stars bright enough to be shining through it, or are they in front of it? (I tried posting that question at the discuss page for that APOD, but the "discuss" link takes me to elsewhere.)

Click to view full size image

When you see stars (especially unreddened ones) apparently in front of a dense dust nebula, they are almost certainly physically in front of it as well. It's not impossible they are just barely inside it, but there's a lot more space between us and the nebula than there is in the narrow shell where you might still see an interior star. Statistics favors it being outside the nebula.

Ok, makes sense.