APOD: IC 4592: The Blue Horsehead Nebula... (2023 Sep 26)

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APOD: IC 4592: The Blue Horsehead Nebula... (2023 Sep 26)

Post by APOD Robot » Tue Sep 26, 2023 4:06 am

Image IC 4592: The Blue Horsehead Reflection Nebula

Explanation: Do you see the horse's head? What you are seeing is not the famous Horsehead nebula toward Orion, but rather a fainter nebula that only takes on a familiar form with deeper imaging. The main part of the here-imaged molecular cloud complex is reflection nebula IC 4592. Reflection nebulas are made up of very fine dust that normally appears dark but can look quite blue when reflecting the visible light of energetic nearby stars. In this case, the source of much of the reflected light is a star at the eye of the horse. That star is part of Nu Scorpii, one of the brighter star systems toward the constellation of the Scorpion (Scorpius). A second reflection nebula dubbed IC 4601 is visible surrounding two stars above and to the right of the image center.

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Ann
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Re: APOD: IC 4592: The Blue Horsehead Nebula... (2023 Sep 26)

Post by Ann » Tue Sep 26, 2023 6:39 am

Today's APOD is all about reflection nebulas. That is to say that the APOD is about a few stars, and about dust that has or hasn't scattered light from these stars (or has the light been refracted, Chris?).


I'll leave it to you to explain the difference between reflection, scattering and refraction, Chris, and I'm going to use the words reflection and scattering. But let's look at the dust first. Dust both scatters and "kills" light. Shortwave (bluer) light is scattered away first, and longer and longer wavelength (redder) light is scattered away later. In the end, light is "killed", because it can't penetrate the thick dust.

Interstellar dust reddening 2Mass.png

In this picture from 2MASS (I think), you can see how a dust cloud reddens the light from the stars behind it. The light from stars behind the dust goes from yellow (thin dust) to orange (thicker dust) to red (even thicker dust) to black, where the light from the stars is extinguished altogether.

Take a look at the fascinating dust lane of galaxy NGC 5866:


Look closely, and you'll see that only the middle part of the dust lane is "pitch black". There are little brown curlicues rising from the dust lane. These things are most likely dusty remnants of past supernovas exploding in the dust lane. Right in the middle of the dust lane, the fluff rising from the dust lane becomes yellow-orange. This is because light from the bright core of NGC 5866 is being reflected in the dust.

Yes, but please note that parts of the dust lane are bluish. Look carefully (enlarge the picture) and you can see individual stars in the dust lane. What we are seeing is a faint bluish reflection nebula almost along the entire length of the dust lane. Dust in front of some of these stars - and the dust must not be too thick - scatters blue light from these stars.

Also note that where the dust lane ends, diffuse faintly bluish light seems to continue in the same direction as the dust lane on both sides of it, as if the dust lane had been "extended by light". (Or if someone had lit up a light sword inside the dust lane.)

What we are seeing is stars that are slightly hotter than the stars around them. I'd say that the slightly hotter stars are typically spectral class F, and they are remnants of star formation in the dust lane that took place when the dust lane was much larger. Now the dust lane has shrunk, because its gas has been used up in star formation, and the hot stars that were once born there have died. But the F-type stars remain, now nakedly deprived of their cover of dust, like narrow but diffuse beams of ever so slightly bluish light.

Can't keep a larger picture of NGC 5866 away from you. Note the dark dust lane and the thin bluish beams of light emerging from the ends of it (and the tidal mess of streamers and strands of faint yellow stars creating a large cloud around the galaxy):


So let's finally return to today's APOD!

APOD 26 September 2023 annotated .png

The large blue reflection nebula, IC 4592, is lit up by blue light scattered by dust from the hot bright blue B2V-type star Nu Scorpii. But do note three other reflection nebulas as well. Nebulas vdB 102 and vdB 103 are lit up by smallish stars of spectral class B9V, which are just a tad hotter and brighter than Vega.

Perhaps even more interesting is reflection nebula vdB 101, which is yellow-white in color. Shouldn't reflection nebulas be blue? Yes, they typically are, but not if the star whose light they reflect is very non-blue. The star lighting up vdB 101 is HD 146834, a modest yellow-orange K0III star, just a tad cooler and brighter than Pollux.

Interestingly, if you check up HD 146834 in Simbad's Astronomical Database, you find a picture of this star surrounded by faintly bluish strands of reflection nebulosity. Oh well!

Yellow star in reflection nebula vdB 101 DSS.png

At the very least, Simbad admits that the reflection nebulosity surrounding red supergiant Antares is orange in color!

Ann
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Re: APOD: IC 4592: The Blue Horsehead Nebula... (2023 Sep 26)

Post by De58te » Tue Sep 26, 2023 1:16 pm

Wow 2 horse head nebulae!

Just an off the cuff observation but to me that looks rather like a grayhound dog's head running after a rabbit.
Just like Sirius the dog star, could there be a dog nebula as well? Google search tells me there is a "Running Dog Nebula."

Also called the Heart Nebula.

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Re: APOD: IC 4592: The Blue Horsehead Nebula... (2023 Sep 26)

Post by Christian G. » Tue Sep 26, 2023 2:27 pm

Lovely dreamy nebula! Kudos to the husband and wife team who imaged this (a happy looking couple... I have a hunch stargazing can sometimes be an issue in couples, "Not tonight honey, I see clear skies!")

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Re: APOD: IC 4592: The Blue Horsehead Nebula... (2023 Sep 26)

Post by orin stepanek » Tue Sep 26, 2023 4:15 pm

BlueHorse_Grelin_1080.jpg
It is amazing how much it does look like a horse!
funnyface.jpg

Horse feathers! :evil:
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Re: APOD: IC 4592: The Blue Horsehead Nebula... (2023 Sep 26)

Post by johnnydeep » Tue Sep 26, 2023 6:51 pm

I'm always amazed that so many stars are really multiple star systems. From the Wikipedia entry for Nu Scorpii:
Nu Scorpii (ν Scorpii, abbreviated Nu Sco, ν Sco) is a multiple star system in the constellation of Scorpius. It is most likely a septuple star system,[5] consisting of two close groups (designated Nu Scorpii AB and CD) that are separated by 41 arcseconds.[5] Based on parallax measurements,[7] it is approximately 470 light-years from the Sun.
Also, the "two stars" of the smaller reflection nebula IC 4601 are really three (and perhaps some of those are even multiples themselves?). From the astrobin link:

ic 4601.jpg
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Re: APOD: IC 4592: The Blue Horsehead Nebula... (2023 Sep 26)

Post by johnnydeep » Tue Sep 26, 2023 6:57 pm

Oh, and I think this looks more like the head of a Miniature Bull Terrier than a horse:

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Re: APOD: IC 4592: The Blue Horsehead Nebula... (2023 Sep 26)

Post by starsurfer » Sat Sep 30, 2023 10:32 pm

I miss seeing images of this that are north up. :cry: