Starship Asterisk*

Horsehead and Flame

Explanation: The Horsehead Nebula, famous celestial dark marking also known as Barnard 33, is notched against a background glow of emission nebulae in this sharp cosmic skyscape. About five light-years "tall" the Horsehead lies some 1,500 light-years away in the constellation of Orion. Within the region's fertile molecular cloud complex, the expanse of obscuring dust has a recognizable shape only by chance from <a href"https://earthsky.org/astronomy-essentia ... -sun/">our perspective</a> in the <a href"https://www.nasa.gov/mission_pages/sune ... tml">Milky Way</a> though. Orion's easternmost belt star, bright Alnitak, is to the left of center. Energetic ultraviolet light from Alnitak powers the glow of dusty NGC 2024, the Flame Nebula, just below it. Completing a study in cosmic contrasts, bluish reflection nebula NGC 2023 is below the Horsehead itself. This well-framed telescopic field spans about 3 full moons on the sky.

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One of my favorites! Reminds me of a tree afire!

For people spending a lot of time looking at astronomical images, the skyscape portrayed in today's APOD is very familiar, and for me, it becomes somewhat hard to say something new and inspiring about it.

That's why I'm posting the picture by Terry Hancock, where you can see Sigma Orionis. This star plays a key role in shaping of a lot of the nebulosity that we are seeing near the Horsehead Nebula and the Belt of Orion.

Let's take a look at two other images that offer a key to what role Sigma Orionis is playing in the magnificent constellation of the Hunter.


Note in both the Mvln/Wikipedia and Zdeněk Bardon/ESO images how red the Sigma Orionis nebula is. The Sigma Orionis nebula is the nebula that creates the backdrop for the Horsehead Nebula. Nothing else in the familiar Orion constellation is that red.

The red color in the Sigma Orionis nebula means that we are looking at a rather pure Hα nebula, whose red light comes almost entirely from ionized hydrogen. The Orion nebula, by contrast, emits a lot of OIII light as well, which dilutes its red color. And Barnard's Loop and the Lambda Orionis nebula, while red, are fainter than the red backdrop of the Horsehead Nebula.

Note in the Mvln/Wikipedia image some pink and blue nebulous fluff near Alnilam (the middle Belt star) which could possibly be lit up and ionized by Sigma Orionis. (Alnilam itself is believed to be a background star relative to the other Belt stars and Sigma Orionis.)

Note in the Zdeněk Bardon/ESO image that Sigma Orionis seems to sit square in the middle of Barnard's Loop, if Barnard's Loop had been a full circle and not an arc. I'm not saying that Sigma Orionis is the star that is ionizing Barnard's Loop (certainly not on its own), but it does look, to me, as if Sigma Orionis had played a large part in driving away the gas and dust until it piled up to form Barnard's Loop. (It's unlikely that Sigma Orionis has done that this is still a main sequence star, fusing hydrogen to helium in its core, and I don't know how long an 18 solar masses O9.5V star can shine so brightly and blow such a strong wind as to dispel the gas around it so powerfully and blow it into such a wide arc before it leaving its main sequence life behind it.)

Anyway, Sigma Orionis seems to be centered on the arc of Barnard's Loop. Another image, which I can't find now, showed numerous little pillars along the edge of the bright red Sig Ori nebula, all pointing at the Sigma Orionis.

My point is that Sigma Orionis is a very important star in the Orion constellation. It plays a huge part in shaping the nebulosity there. And the Horsehead nebula is just a pillar, similar to the Pillars of Creation in the Eagle Nebula.

Oh, and... our resident astronomer, Chris Peterson, insists that the Horsehead Nebula should really be called the "Horse-rump" nebula. I'll leave it to Chris to post a picture to prove his point!

Ann

I like this 3D representation. JWST will be the next horse whisp-erer.

Fred the Cat wrote: ↑Thu Dec 29, 2022 5:15 pm I like this 3D representation. JWST will be the next horse whisp-erer.

but what if Horsehead has in fact the 3d form of a fragment of a wristband?

Okay, so why is the Flame Nebula weird? It's because it is a weird color, and because the star ionizing it is off to one side.


So Alnitak, the ionizing star, is off to one side, and the Flame Nebula is a blob of thick dusty nebulosity completely detached from Alnitak. This is most unusual.

Normally, hot stars and their nebulas look like this:


So, let's talk about these "normal" nebulas. The Cocoon Nebula is being ionized by a single star, B1V-type BD+46 3474, which has just been born from the nebula that it is now ionizing.

The Flaming Star Nebula is being ionized by O9.5V-type runaway star AE Aurigae, which is just passing through this particular dust cloud and lighting it up and ionizing it on its way through.

The California Nebula is being "snowplowed" into an elongated wall of gas and dust, and it is being ionized by the O7.5Iab runaway star Xi Per that is doing the snowplowing.

The amazing image by Miguel Claro shows Betelgeuse surrounded by a thin bell-shaped string of dust. We can also see the Rosette Nebula at upper right. The Rosette Nebula is older than the Cocoon Nebula, and the cluster of stars have begun clearing away their birth nebula and ionize the dispersing bubble surrounding them. Near bottom is the Lambda Orionis Nebula, which has been dispersed even further than the Rosette Nebula. Near the left edge of the picture is a part of Barnard's Loop. Near Barnard's Loop is a little yellow reflection nebula, vdB 62.

But the Flame Nebula is a compact blob of gas and dust which is being ionized from outside by O9.5Ib star Alnitak and from the inside by the stars being born inside it, whose light is reddened to an orange hue by the surrounding dust:


So the Flame Nebula is being made to glow red from hydrogen alpha as Alnitak is ionizing it, but it is also glowing from inside from dust-reddened and therefore orange-tinted starlight.


I found a picture that actually reminds me of Alnitak and the Flame Nebula, and I can't keep it from you:


NGC 3077 and HD 86677 are nothing like Alnitak and the Flame Nebula. A galaxy is of course unimaginably larger and more powerful than a nebula (even if NGC 3077 is smallish), and HD 86677, which is located some 200 light-years from the Earth, is downright puny and unimpressive compared with Alnitak.

Still, this star-galaxy pair looks much the same as Alnitak and the Flame Nebula. NGC 3077 has got a dusty center, just like the Flame Nebula, and hot young stars are being born there, just like stars are being born in the Flame Nebula, and the stars are being reddened by dust, just like the stars inside the Flame Nebula. And NGC 3077 is located to one side of foreground star HD 86677 in our skies. We might almost imagine that HD 86677 is actually ionizing NGC 3077, but it goes without saying that nothing like that is going on.

Ann

I have never been able to get an answer to "what is the gas density the we see in Nebula?" Is it like earth's air of is it much thinner? Or thicker? Any enlightenment would be appreciated. Tried to help a kid in science, and this question has bedeviled me for decades.

Sense1 wrote: ↑Fri Dec 30, 2022 2:23 am I have never been able to get an answer to "what is the gas density the we see in Nebula?" Is it like earth's air of is it much thinner? Or thicker? Any enlightenment would be appreciated. Tried to help a kid in science, and this question has bedeviled me for decades.

It's a very hard vacuum... harder than we can easily achieve in a lab. Millions of times less dense than our atmosphere.

Sense1 wrote: ↑Fri Dec 30, 2022 2:23 am I have never been able to get an answer to "what is the gas density the we see in Nebula?" Is it like earth's air of is it much thinner? Or thicker? Any enlightenment would be appreciated. Tried to help a kid in science, and this question has bedeviled me for decades.

I wish Chris was here, our resident scientist. (Where is everybody?)

I'll have a go at answering you.

Planetary nebula - McGill School Of Computer Science wrote:

A typical planetary nebula is roughly one light year across, and consists of extremely rarefied gas, with a density generally around 1000 particles per cm³ - which is about a million billion billion (10²⁴) times less dense than the earth's atmosphere. Young planetary nebulae have the highest densities, sometimes as high as 10⁶ particles per cm³. As nebulae age, their expansion causes their density to decrease.

For comparison, according to this page, the density of the Earth's atmosphere at sea level is about 2.7x10¹⁹ molecules per cm³(cubic centimeter) or 4.4x10²⁰ molecules per inch³(cubic inch).

You may also want to look at this page, where two people offer their own answers to how dense nebulas are.

Some nebulas must collapse, otherwise there wouldn't be any more star formation. According to this site, a molecular core (a high density region of a nebula containing molecules, e.g. molecular hydrogen) has reached a critical density and will collapse at 10⁸ cm^-3 - and the unit they talk about here is particles (or molecules), I guess.

Once a molecular core starts collapsing it must eventually get denser than the Earth's atmosphere. After all, we are talking about the conversion of gas and dust into actual stars.

NASA wrote:

The temperature at the very center of the Sun is about 15,000,000° C (27,000,000° F) and the density is about 150 g/cm³ (approximately 10 times the density of gold, 19.3 g/cm³ or lead, 11.3 g/cm³). Both the temperature and the density decrease as one moves outward from the center of the Sun. The nuclear burning is almost completely shut off beyond the outer edge of the core (about 25% of the distance to the surface or 175,000 km from the center). At that point the temperature is only half its central value and the density drops to about 20 g/cm³.

That's the best I can do for you!

Ann

Are the glowing waves from the most upper left reflections or bow chocks?

beryllium732 wrote: ↑Fri Dec 30, 2022 6:14 pm Are the glowing waves from the most upper left reflections or bow chocks?

Are you talking about these things?


They are definitely not bow shocks. I don't think they are real, or rather: I think we are seeing some sort of reflections from inside the camera. And the light that is being reflected is the light from bright blue star Alnilam, the middle star of the Belt stars.

Take a look at the picture below where you can see Alnilam as well as the two reflection nebulas that you can see at left in the small picture I just posted. Now take a look at this picture:


You can find bright Alnilam at upper center in the image above. The two small reflection nebulas that you could see at left in the first picture I posted can be seen to the left (or lower left) of Alnilam. As you can see, there are no waves or rays between Alnilam and the two small reflection nebulas.

Here is another example of how light from a bright star creates strange reflections in a picture whose purpose is to show us a nebula:


The Hubble picture shows a strangely stretched clump of nebulosity very close to fourth magnitude star Merope in the Pleiades. The nebulosity itself is seen at lower left. At upper right we can see a large number of bright reddish rays. They are reflections from inside the Hubble itself caused by the light from star Merope, which is too bright for Hubble. (As a Color Commentator I would like to point out that Merope is a blue star, even though the reflected rays in the Hubble picture are red.)

The picture by Adam Block shows bright star Merope (above center) and a large "wave" of nebulosity below it. But immediately below Merope is a tiny white blob, almost touching the bright star, and this is the little nebula seen in the Hubble image. As you can see, there is no multitude of red rays in Adam Block's image, just four diffraction spikes (which are also reflections from inside a camera).

The rays in the Hubble image don't exist in reality, and instead they are reflections inside Hubble's camera. In the same way, the strange "waves" and rays seen at upper left in this APOD are almost certainly reflections inside the camera from bright star Alnilam.

Ann

Ann wrote: ↑Sat Dec 31, 2022 5:59 am

beryllium732 wrote: ↑Fri Dec 30, 2022 6:14 pm Are the glowing waves from the most upper left reflections or bow chocks?

Are you talking about these things?


Part of APOD 29 December 2022 with arrows.png


They are definitely not bow shocks. I don't think they are real, or rather: I think we are seeing some sort of reflections from inside the camera. And the light that is being reflected is the light from bright blue star Alnilam, the middle star of the Belt stars.

Take a look at the picture below where you can see Alnilam as well as the two reflection nebulas that you can see at left in the small picture I just posted. Now take a look at this picture:

Orion's Belt with Alnilam at upper center and two small blue reflection nebulas to the lower left of it. Image credit: Mvln at Wikipedia.

---

You can find bright Alnilam at upper center in the image above. The two small reflection nebulas that you could see at left in the first picture I posted can be seen to the left (or lower left) of Alnilam. As you can see, there are no waves or rays between Alnilam and the two small reflection nebulas.

Here is another example of how light from a bright star creates strange reflections in a picture whose purpose is to show us a nebula:

Merope Nebula Adam Block.png

Small white clump IC 349 just below bright star Merope.
Image credit: Adam Block.

IC 349 near Merope in the Pleiades. Credit: NASA and the Hubble Heritage Team.

---

The Hubble picture shows a strangely stretched clump of nebulosity very close to fourth magnitude star Merope in the Pleiades. The nebulosity itself is seen at lower left. At upper right we can see a large number of bright reddish rays. They are reflections from inside the Hubble itself caused by the light from star Merope, which is too bright for Hubble. (As a Color Commentator I would like to point out that Merope is a blue star, even though the reflected rays in the Hubble picture are red.)

The picture by Adam Block shows bright star Merope (above center) and a large "wave" of nebulosity below it. But immediately below Merope is a tiny white blob, almost touching the bright star, and this is the little nebula seen in the Hubble image. As you can see, there is no multitude of red rays in Adam Block's image, just four diffraction spikes (which are also reflections from inside a camera).

The rays in the Hubble image don't exist in reality, and instead they are reflections inside Hubble's camera. In the same way, the strange "waves" and rays seen at upper left in this APOD are almost certainly reflections inside the camera from bright star Alnilam.

Ann

Thank you for the throughout answer! Yes it must be the diffraction spikes or artifacts from the star Alnilam inside Hubble. Sometimes i wished you would get a second bigger picture encompassing a bigger wider area if there's one avaliable so you get sense what some things are.