Chris Peterson wrote: ↑
Sat Apr 17, 2021 4:36 am
This certainly demonstrates why telescopes operating in the IR are so useful. So much more information here than we can get in visible bands alone.
Indeed. IR can provide us with "a deeper look" that optical images can ever achieve.
Speaking of the devil, though. I recently posted a smaller version of today's APOD
in a discussion thread, and I'm sure you can guess which one: http://asterisk.apod.com/viewtopic.php?p=312401#p312401
I have been looking at a lot of optical Flame Nebula and Horsehead Nebula region images, and I can this: The color of the Flame Nebula varies in these images. In a few of them, the Flame nebula is almost lemon yellow, while in others, it is as red as the background of the Horsehead Nebula. In most of them, however, the Flame Nebula is
yellower than the background of the Horsehead Nebula.
I think there are interesting similarities between the Flame Nebula and the starforming region inside the well-known "Cone Nebula and Christmas Tree Cluster", or NGC 2264 . The similarity is that there is an embedded region of star formation in NGC 2264 too, which can only be seen in infrared light. This region of star formation in NGC 2264 has been named after the distinctive shape of the newborn cluster inside, the Snowflake Cluster (after the stars radiating out from the center like a snowflake).
Infrared image of NGC 2264 with young
infrared "Snowflake Cluster" at center. Photo: Spitzer.
Optical image of NGC 2264. The position of the Snowflake Cluster
is visible as a yellowish smudge at left. Photo: Bach Zoltan.
Let's begin by looking at the Flame Nebula and the Snowflake Cluster in infrared light.
As you can see, the Flame Nebula looks a lot more "energetic" than the region immediately surrounding the Snowflake Cluster. There seems to be a mighty outflow from the center of the Flame Nebula, which gives the Flame Nebula a bit of a butterfly (or bug) shape. Please note that the outflow in the Flame Nebula seems to be originating from the center of the nebula where the youngest star formation has been taking place. In other words, it looks to me as if most of the infrared energy of the nebula comes from within (the embedded cluster), not from without (hot star Alnitak).
When it comes to the Snowflake Cluster region, there are fewer signs of the background nebulosity being strongly disturbed by the presence of the cluster. Yes, there is a lot of infrared energy in NGC 2264, but not so much around the Snowflake Cluster itself. Yes, we can see what looks like two ridges below the Snowflake Cluster that may have been violently "forced apart" by stellar winds and jets from the cluster. But there are no signs of the mighty outflow of energy that we can see in the Flame Nebula.
Now let's look at the optical images of the Flame Nebula and NGC 2264. The Flame Nebula is yellow, or at least more yellow than the red background of the Horsehead Nebula. If you look at the optical image of NGC 2264 by Bach Zoltan, you can spot some yellowish flecks whose location correspond to the Snowflake cluster (and a few other infrared-bright spots).
So, in short: The embedded star formation in NGC 2264 creates yellowish flecks in an optical image. That's because the mostly blue-white light from newly formed young stars gets reddened to a deep yellow hue by the time it penetrates its dusty cocoon.
I think the same thing is happening in the Flame Nebula. The very young stars inside the Flame Nebula produce a lot of blue-white light, but only the yellow, orange and red light can penetrate the dust.
At the same time, both the Flame Nebula and the Snowflake Cluster region in NGC 2264 are (more or less strongly) affected by bright ultraviolet light from an O-type star: Alnitak when it comes to the Flame Nebula, and 15 Mon when it comes to the Snowflake Cluster. There is definitely hydrogen alpha light in these nebulas.
The combination of hydrogen alpha light due to irradiation by a nearby hot star, and visible and ultraviolet but highly dust-reddened light from within, creates the yellowish color of both the Flame Nebula and the Snow Flake cluster region in NGC 2264.
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