APOD: NGC 7822 in Cepheus (2016 Nov 11)

[APOD Robot]

NGC 7822 in Cepheus

Explanation: Hot, young stars and cosmic pillars of gas and dust seem to crowd into NGC 7822. At the edge of a giant molecular cloud toward the northern constellation Cepheus, the glowing star forming region lies about 3,000 light-years away. Within the nebula, bright edges and dark shapes stand out in this colorful skyscape. The image includes data from narrowband filters, mapping emission from atomic oxygen, hydrogen, and sulfur into blue, green, and red hues. The emission line and color combination has become well-known as the Hubble palette. The atomic emission is powered by energetic radiation from the central hot stars. Their powerful winds and radiation sculpt and erode the denser pillar shapes and clear out a characteristic cavity light-years across the center of the natal cloud. Stars could still be forming inside the pillars by gravitational collapse but as the pillars are eroded away, any forming stars will ultimately be cutoff from their reservoir of star stuff. This field of view spans over 40 light-years at the estimated distance of NGC 7822.

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[jambo]

Clicking "discuss" brings this error message:
You are probably trying to discuss an APOD which does not yet exist. You want to discuss the APOD from November 12th, 2016.
If you are sure this is correct, try again later. Otherwise, head over to The Asterisk to discuss an APOD which does exist.

I was able to get to the proper discussion page by clicking on "The Asterisk."

[geckzilla]

Clicking "discuss" brings this error message:
You are probably trying to discuss an APOD which does not yet exist. You want to discuss the APOD from November 12th, 2016.
If you are sure this is correct, try again later. Otherwise, head over to The Asterisk to discuss an APOD which does exist.

I was able to get to the proper discussion page by clicking on "The Asterisk."

Yeah. Thanks. The wrong link was used.

[MarkBour]

. . . but as the pillars are eroded away, any forming stars will ultimately be cutoff . . .

I see this kind of note regularly in APODs that show star-forming regions. It stands to reason, though, that there is another side to this. The dust that stars push away does not really evaporate, of course, it just moves. Eventually, that dust could become part of another dense region. I assume that the dynamics of stars pushing away dust must -- on some occasions -- contribute to the formation of a star. Now it would take me a lot more thought to form a guess as to whether that is truly a very rare occurrence (stellar winds contributing to a star's formation) by comparison with the one that is easily observable and oft-mentioned (stellar winds thwarting a star's formation). Are the stellar winds actually a significant force in the shaping of galactic dust lanes? What about other dust? Does it eventually form a tenuous shell of "grit" around galaxies, which in general is not as concentrated as visible dust lanes?

[neufer]

. . . but as the pillars are eroded away, any forming stars will ultimately be cutoff . . .

I see this kind of note regularly in APODs that show star-forming regions. It stands to reason, though, that there is another side to this. The dust that stars push away does not really evaporate, of course, it just moves. Eventually, that dust could become part of another dense region. I assume that the dynamics of stars pushing away dust must -- on some occasions -- contribute to the formation of a star. Now it would take me a lot more thought to form a guess as to whether that is truly a very rare occurrence (stellar winds contributing to a star's formation) by comparison with the one that is easily observable and oft-mentioned (stellar winds thwarting a star's formation). Are the stellar winds actually a significant force in the shaping of galactic dust lanes? What about other dust? Does it eventually form a tenuous shell of "grit" around galaxies, which in general is not as concentrated as visible dust lanes?

• 1) These cosmic pillars are mostly made of gas not of dust.
  
  2)These pillars are indeed being pushed away rather than "evaporating" as we normally think of it...but they are being pushed away by radiation pressure rather than by stellar winds.

[Chris Peterson]

2)These pillars are indeed being pushed away rather than "evaporating" as we normally think of it...but they are being pushed away by radiation pressure rather than by stellar winds.

To some degree the dust really is evaporating (although "sublimating" might be a more accurate description). In this environment dust grains don't generally come together, but their surfaces are eroded away into smaller particles or even free atoms by the high energy particles that strike them.

[MarkBour]

• 1) These cosmic pillars are mostly made of gas not of dust.
  2)These pillars are indeed being pushed away rather than "evaporating" as we normally think of it...but they are being pushed away by radiation pressure rather than by stellar winds.

2)These pillars are indeed being pushed away rather than "evaporating" as we normally think of it...but they are being pushed away by radiation pressure rather than by stellar winds.

To some degree the dust really is evaporating (although "sublimating" might be a more accurate description). In this environment dust grains don't generally come together, but their surfaces are eroded away into smaller particles or even free atoms by the high energy particles that strike them.

Ulp! Looks like I have to go do some more remedial reading this weekend. I see I have confused some quite distinct things. Also, I had no idea that the dust grains actually break down from the interactions.

(I do wish I could change my label from "science officer", which makes it appear that I have some astronomy expertise, to something more appropriate. Like maybe "pollster".)

[Boomer12k]

And to my old eyes... looks like a TURKEY... the heads is on the left, and the neck goes back to the right, and then the body and feathers fan out.... AWESOME... saw it right off...
OK, or maybe a Pheasant...

Going to miss the Super Full Moon, as Sunday night, rain... hope to get a glimpse, but no real hope.
:---[===] *

[Chris Peterson]

2)These pillars are indeed being pushed away rather than "evaporating" as we normally think of it...but they are being pushed away by radiation pressure rather than by stellar winds.

To some degree the dust really is evaporating (although "sublimating" might be a more accurate description). In this environment dust grains don't generally come together, but their surfaces are eroded away into smaller particles or even free atoms by the high energy particles that strike them.

Ulp! Looks like I have to go do some more remedial reading this weekend. I see I have confused some quite distinct things. Also, I had no idea that the dust grains actually break down from the interactions.

There's still a lot more dispersion than there is disruption down to to individual atoms. I was just pointing out that both processes are present.

BTW, I don't think this "evaporation" contributes directly to new star formation. The dust really is dispersed, and ends up in very large, thin clouds which are eventually recompressed (in zones) by the shockwaves from supernovas. Those newly compressed regions- without hot stars to blow them apart- can now start coming together, along with the much more significant gas they are embedded in, to become the seeds of new stars.

[Chris Peterson]

Going to miss the Super Full Moon, as Sunday night, rain... hope to get a glimpse, but no real hope.

I wouldn't get too upset. These things are pretty overblown. Unless you're a very experienced lunar observer, it's doubtful you'd really notice the difference between an apogee full Moon and a perigee one. The difference from the average size isn't all that significant.

[MarkBour]

BTW, I don't think this "evaporation" contributes directly to new star formation. The dust really is dispersed, and ends up in very large, thin clouds which are eventually recompressed (in zones) by the shockwaves from supernovas. Those newly compressed regions- without hot stars to blow them apart- can now start coming together, along with the much more significant gas they are embedded in, to become the seeds of new stars.

Interesting that you're talking abut dust here and used the term "seeds". My novice thinking is that stars would form best from a cloud of pure hydrogen. And that dust and any other gases would be contaminants that would "weaken" the resulting star with impurities. I don't know what proportion of atoms/molecules other than H (or H2) were there when our Sun was formed, probably someone has figured that out, though (?) And of course impurities had to be there in our solar system for us to have ever come to be, so I wouldn't really wish for pure H in our case. But again, regarding the term "seeds". Did you mean to suggest that larger non-gas particles in protostar regions can help begin the process of coalescing? (I'm told that dust particles in earthly clouds provide beginning points for condensation.) Or did you use the term "seeds" without that connotation?

[neufer]

2)These pillars are indeed being pushed away rather than "evaporating" as we normally think of it...but they are being pushed away by radiation pressure rather than by stellar winds.

To some degree the dust really is evaporating (although "sublimating" might be a more accurate description). In this environment dust grains don't generally come together, but their surfaces are eroded away into smaller particles or even free atoms by the high energy particles that strike them.

• The refractory dust grain 'cores' aren't evaporating...just (perhaps) their icy coats:

<<The large grains in interstellar space are probably complex, with refractory cores that condensed within stellar outflows topped by layers acquired subsequently during incursions into cold dense interstellar clouds. That cyclic process of growth and destruction outside of the clouds has been modeled to demonstrate that the cores live much longer than the average lifetime of dust mass. Those cores mostly start with silicate particles condensing in the atmospheres of cool oxygen rich red-giant stars and carbon grains condensing in the atmospheres of cool carbon stars. The red-giant stars have evolved off the main sequence and have entered the giant phase of their evolution and are the major source of refractory dust grain cores in galaxies. Those refractory cores are also called Stardust, which is a scientific term for the small fraction of cosmic dust that condensed thermally within stellar gases as they were ejected from the stars. Several percent of refractory grain cores have condensed within expanding interiors of supernovae, a type of cosmic decompression chamber. And meteoriticists that study this refractory stardust extracted from meteorites often call it presolar grains, although the refractory stardust that they study is actually only a small fraction of all presolar dust. Stardust condenses within the stars via considerably different condensation chemistry than that of the bulk of cosmic dust, which accretes cold onto preexisting dust in dark molecular clouds of the galaxy. Those molecular clouds are very cold, typically less than 50K, so that ices of many kinds may accrete onto grains, perhaps to be destroyed later.>>

[BillBixby]

2)These pillars are indeed being pushed away rather than "evaporating" as we normally think of it...but they are being pushed away by radiation pressure rather than by stellar winds.

To some degree the dust really is evaporating (although "sublimating" might be a more accurate description). In this environment dust grains don't generally come together, but their surfaces are eroded away into smaller particles or even free atoms by the high energy particles that strike them.

ignorance is looking for an answer from those who have more knowledge; as the dust and gas is driven away the concentration lessons. Instead of blue stars (hot and going out in a blaze of glory), with less concentration stars more like our Sol would be developed? Supporting the development of planets more like our Earth? An open ended question looking not for argument (though welcome,) but for elaboration.

[Chris Peterson]

BTW, I don't think this "evaporation" contributes directly to new star formation. The dust really is dispersed, and ends up in very large, thin clouds which are eventually recompressed (in zones) by the shockwaves from supernovas. Those newly compressed regions- without hot stars to blow them apart- can now start coming together, along with the much more significant gas they are embedded in, to become the seeds of new stars.

Interesting that you're talking abut dust here and used the term "seeds". My novice thinking is that stars would form best from a cloud of pure hydrogen.

The amount of heavier elements is really not a factor in the formation of the stars themselves. But dust appears to be important in catalyzing the gravitational collapse of the gas that ultimately becomes new stars. That process is somehow seeded by the presence of the dust, even though it represents only a tiny fraction of the total mass of the ingredient mix.

[Chris Peterson]

To some degree the dust really is evaporating (although "sublimating" might be a more accurate description). In this environment dust grains don't generally come together, but their surfaces are eroded away into smaller particles or even free atoms by the high energy particles that strike them.

• The refractory dust grain 'cores' aren't evaporating...just (perhaps) their icy coats:

No, it's more than that. Spallation removes atoms and molecules of the core material from the surface. It's just a very slow process and there's not enough time for it to significantly ablate the grains, so most of the mass survives in that form.

[Ann]

I think the Orion Nebula region can help us understand a bit more about the role that stars and dust play for star formation.

Click to view full size image

The Horsehead Nebula.
Photo: Marco Burali, Tiziano Capecchi, Marco Mancini

The best-known of all dust pillars in the sky is probably the Horsehead Nebula in Orion. We don't usually think of it as the same kind of pillar as the Pillars of Creation in the Eagle Nebula, but that is what it is.

The visible light image at left is actually a portrait of the evaporation (or, if you prefer it that way, the sublimation) of gas and dust in this high-energy environment. The Horsehead region is dominated by the harsh ultraviolet radiation from nearby hot stars. Note how red filaments of ionized hydrogen are rising like red smoke from the "wall" of dust running diagonally behind the Horsehead. What we are seeing is the wall eroding away, but the Horsehead, which is a part of this extended dust feature, still stands relatively tall, because it is the densest part of the upper layer of this wall of dust.

Note how the top of the Horsehead Nebula seems to emit white light. That is because all sorts of chemical reactions are taking place here, as the Horsehead meets the onslaught of ultraviolet light from O-type star Sigma Orionis, which is outside of the frame at top left in this picture.

Click to view full size image

Closeup of the tallest "Pillar of Creation". Photo:
NASA, ESA, and the Hubble Heritage Team

But are pillars like the Horsehead and the Pillars of Creation efficient star formation factories?

I would say no, particularly when it comes to the formation of massive stars. That is precisely because these pillars are evaporating away. The formation of massive stars requires massive, cold cores, and they are not to be had in a pillar being battered this way and that by fierce stellar winds and ultraviolet light.

Click to view full size image

Visible and infrared composite of the Horsehead region.
ESO/J. Emerson/VISTA. Acknowledgment: Cambridge Astronomical Survey Unit.
Composite Assembly and Processing: Robert Gendler.

But take a look at the visible and infrared composite image of the Horsehead Nebula region at left. The Horsehead itself is at upper right, and you can just make out a small star being born at the top of its head.

At lower left of the Horsehead is a bright nebula with a star inside. The nebula is reflection nebula NGC 2023, and the star is HD 37903, a newborn early B-type star, far more massive than the puny thing being born at the top of the Horsehead Nebula. NGC 2023 is deeper down in the dust and far more protected from the immediate onslaught of stellar wind and radiation than the Horsehead. So could the birth of massive HD 37903 actually have been facilitated by the hot stars in the region? I think that is a definite possibility. The cold core that became HD 37903 might well have been compressed and extra massive because of the "push" that was exerted on it by relatively nearby massive stars.

Click to view full size image

Horsehead and Flame Nebula region.
Photo: Terry Hancock.

Finally, take a look at the image at right of the Horsehead and Flame Nebula region. You can see the Horsehead, and just to the lower left of it, the rather small blue NGC 2023 reflection nebula. But farther to the left is a much more impressive, large, yellow nebula crisscrossed by dark dust lanes: the Flame Nebula. Clearly a lot more star formation is going on here than anywhere else in the Horsehead Nebula region.

What causes this bonanza of star formation? I'm going to have a guess. Almost immediately above the Flame Nebula is a blue star. The picture doesn't give this star justice, since it is suppressing the brightness of the star in order to bring out details in the nebula. The star is actually Alnitak, one of the brilliant stars of Orion's Belt. According to Jim Kaler, Alnitak is some 20 times more massive than the Sun and 100,000 times more luminous, most of its energy radiated as ultraviolet light.

Alnitak is an incredibly powerful star, and I think it has a huge impact on the Flame Nebula. But it doesn't seem to be evaporating the Flame Nebula. The nebula seems to be mostly in front of the brilliant star, probably protected by an intervening wall of dust, but it is almost certainly somewhat compressed by the fierce stellar winds from the star.

So massive hot stars can both impede and facilitate star formation, by either evaporating or compressing dust.

Ann