Starship Asterisk*

by **dougettinger** » Sun Apr 24, 2011 8:46 pm

[quote="Ann"]
I'm sure that I haven't read that massive stars suffer multiple core collapses. On the other hand, "core collapse" is only a term. You are definitely right that the core of a massive star must suddenly shrink and heat on several occasions, and after each "sudden shrinkage" and temperature increase, a new element can be created in the core through fusion.IAnn[/quote]

Hello Ann, I was not using any math in my presentation; my reference numbers are trying to show some resemblance of order in the expulsion of matter from an evolving star.

For me sudden shrinkage and core collapse are the same thing. A star uses all the hydrogen at a certain pressure and temperature due to fusion in the center of the star. There is certainly much more hydrogen at larger radii but it is not hot or pressurized enough to cause fusion. So when fusion ceases in the center of the star the core shrinks or collapses since radiation pressure has ceased and is not holding back the weight of the column of matter from above. The shrinkage or new compression creates higher temperature and pressure which starts a different fusion process that will convert helium to other higher metals. The new fusion process along with the implosion creates a dramatic expansion and rebound which in turn expels outer layers of gases into interstellar space. Due to convection the first sequence of expelled matter will be a mixture of mostly hydrogen and some helium. I am not argueing that the expelled materials will not be a mixture which in the first expulsion should be only hydrogen and helium. This process repeats itself each time at smaller periods. The next expulsion will have hydrogen, more helium than the first expulsion, and some carbon and nitrogen in its composition. The third explusion will have hydrogen, less helium, more carbon and nitrogen than the previous expulsion, and some neon which is now being fused in the core of the star. This process can continue through O and some Mg, Si and some S, and finally Fe and some Ni depending on how large the star.

So you are correct that the expulsion of materials from each explosion will be a mixture of elements, but the predominant element for each successive explosion should show a certain order that agrees with the processes of fusion as we know it.

Ann, please refer to my next topic, the Cat's Eye Nebula which shows superbly successive explosions. However, these expulsions are probably due to a smaller star going through frequent periods of instability.

by **Ann** » Sun Apr 24, 2011 6:53 am

Doug, I'm sorry that I haven't answered you before. One reason is that I don't reason mathematically the way you do, so therefore it becomes hard or impossible for me to put my argument in mathematical terms. What I think I know about astronomy comes from reading about it many times and sorting it out in my head, if you get what I mean.

I'm sure that I haven't read that massive stars suffer multiple core collapses. On the other hand, "core collapse" is only a term. You are definitely right that the core of a massive star must suddenly shrink and heat on several occasions, and after each "sudden shrinkage" and temperature increase, a new element can be created in the core through fusion.

I believe you are right that each time the core shrinks the outer layers will swell and expel some gas. I'm not sure, however, that the elements are "lined up" in really neat shells inside the star, so that each expulsion of gas will expel only or mostly one element. There is quite a bit of mixing of elements inside a star thanks to convection. In a massive star, I think the convective zone is deep inside the star, where the elements are produced. On their way to the surface of the star, these elements could become well mixed. See this page: http://answers.yahoo.com/question/index ... 432AAwabjg

To put it briefly, I haven't heard that the different gaseous shells expelled sequentially by massive stars are clearly made up of different elements. The only thing I know for sure is that type Ia supernovae contain no hydrogen. The reason for this is believed to be that the progenitor of the type Ia supernova is a white dwarf, the core of a former relatively (but not incredibly) massive star, which has indeed converted all of the hydrogen in its core and shed all of the hydrogen in its atmosphere before it exploded.

Ann

by **dougettinger** » Fri Apr 22, 2011 3:18 pm

Ann wrote:
Why do different elements become ionized at different distances from a star? Does each element as you see it farther from a star need less UV and less heat to be ionized ?
The way I understand it, nitrogen requires less UV than hydrogen to become ionized, and therefore ionized nitrogen will typically be found farther from the central star of a planetary nebula than ionized hydrogen. Check out this link:

http://apod.nasa.gov/apod/ap090122.html

When it comes to oxygen, it can only become ionized in very rarefied conditions, so that you won't see oxygen emission inside dust clouds, for example. Ann

My understanding is that for very massive stars, each subsequent core collapse should produce the following elements starting with the first core collapse: H, He, C with some N, Ne, O with some Mg, Si with some S, and finally Fe with some Ni. Of course, for less massive stars this chain of fused elements does not go as far as iron. For our Sun, supposely, it should only collapse once or twice to produce H, He, and perhaps C with some N.

The elements as they are expelled from the various core collapses, will start cooling because they are moving farther away from the heat of the remaining star, and because the longer time they move away the ionization energy is lost due to radiant heat transfer. For the first nth ionization energies the above elements require the following descending energies. As these elements move farther away from the star and time, t, for their respective expolsion they will continually lose ionization energy. Those desending energies are 2372 kJ/mol for He, 2080 for Ne, 1402 for N, about 1312 for H and O, about 1040 for C and S, and about 770 for Fe and Si.

Knowing these facts, assuming that they are logical and correct, one can analyze the planetary nebula NGC 2818, 4 ly in diameter, that you have referenced. The source star has already imploded at least 5 times for H, He, C, Ne, and O since ionization of O can be readily observed. During the explosion for carbon, some nitrogen was produced which one can still observe because the ionization energies have not dropped below 1402 kJ/mol. Some hydrogen is always emitted with each explosion and can be observed because the its ionization energy is below 1402 kJ/mol. Helium was the second element expelled and cannot be detected because it has cooled below its ionization energy of 2372. Neon, the third element to be expelled, cannot be detected because it also has cooled below it very high ionization energy of 2080.

The shock front for the explosion that created oxygen has already caught up to the shock front that produced Ne and C since the nitrogen that was produced in the carbon explosion is observed to be farther from the source star than oxygen. This indicates that later shock fronts accelerate faster than shock fronts from previous explosions.

Ann, I hope you can undertand this petit dissertation. I am not sure that it is completely correct. You have been very helpful in explaining the detection of the different elements from the colors that are recorded. I hope to solicit more of your commentary and also any criticisms from expert star evolutionists.

I used Wikepedia's stellar evolution, ionization energy, and the various elements as some of my references.

by **Ann** » Thu Apr 21, 2011 6:17 am

Why do different elements become ionized at different distances from a star? Does each element as you see it farther from a star need less UV and less heat to be ionized ?

The way I understand it, nitrogen requires less UV than hydrogen to become ionized, and therefore ionized nitrogen will typically be found farther from the central star of a planetary nebula than ionized hydrogen. Check out this link:

http://apod.nasa.gov/apod/ap090122.html

When it comes to oxygen, it can only become ionized in very rarefied conditions, so that you won't see oxygen emission inside dust clouds, for example.

Fascinatingly, although hydrogen requires decidedly more UV to become ionized than nitrogen, ionized hydrogen and ionized nitrogen emit light of almost exactly the same color. I haven't checked it out, but I think that ionized hydrogen emit light of 656 nm, whereas inonized nitrogen emit light of 658 nm.

Ann

by **dougettinger** » Thu Apr 21, 2011 4:39 am

Ann wrote: Interestingly, planetary nebulae often show what appears to be a sequential arrangement of elements in the gaseous shells around the star. You find ionized oxygen and sometimes ionized helium closest to the star. The you find ionized hydrogen, if there is any hydrogen there at all, and then you find ionized nitrogen or, I think, possibly ionized sulphur. But the reason for this elegant sequential order of elements is not that they were expelled in that order, but that the various elements require different amounts of ultraviolet emission to get ionized at all. Oxygen requires the most, and oxygen also can't get ionized except in when the oxygen gas is extremely rarified. That is why you find oxygen emission closest to the white dwarf, where the ultraviolet emission is strongest and and the stellar wind from the white dwarf itself makes the surrounding gases most rarified. Some distance farther out there is more gas, and oxygen can't get ionized here. Hydrogen, however, can, and this part of the nebula is perfect for ionizing hydrogen. Still further out there is less ultraviolet emission, and hydrogen won't get noticably ionized any more. But here nitrogen can get ionized. Ann

Why do different elements become ionized at different distances from a star? Does each element as you see it farther from a star need less UV and less heat to be ionized ? Why don't elements expelled from an explosive star remain ionized for long periods of time since they possess the energy of the explosion and remain heated as they are being propelled through space ? How many AU or light years does it take expelled elements from a supernova explosion to cease being a plasma and cool down to being the average temperature of an typical interstellar molecular cloud ?

I appreciate you explaining the sequence due to ionization potential. Perhaps some of each subsequent element as it is being produced by fusion in the star is being expelled in the order that is being made, but also each new element is also being mixed with the next new main element being expelled. My reasons are based on the majority of each expulsion being made from the equatorial latitudes of the star. Then the higher latitudes of elements remaining do migrate to the lower latitudes to be expelled in the next implosion/explosion. This reasoning is based on the numerous cylindrical shapes of novae that are observed.

4/20/2011

by **Chris Peterson** » Mon Apr 04, 2011 11:43 pm

Ann wrote:
jabm67 wrote:AFAIK, all planetary nebulae have substantial hydrogen emission. Images are taken in other spectral lines to map out the thermal and density structure of the nebulae. The hydrogen lines are so strong that extracting that kind of information from them is much more difficult than using other lines; there are other details of the atomic physics which make lines other than the Balmer lines the things you want to use if you're after the physical structure of the nebulae.
You may well be right, but why is it, then, that some planetary nebulae never seem to show any red emission in any broadband images? I think that is true about Abell 43.

Abell 43 has a PG 1159 type star at its center- an unusual type of hydrogen deficient star. It makes sense that planetary nebulas surrounding such stars are also hydrogen deficient. But most planetary nebulas do have strong hydrogen emission lines.

by **Ann** » Mon Apr 04, 2011 11:18 pm

jabm67 wrote:AFAIK, all planetary nebulae have substantial hydrogen emission. Images are taken in other spectral lines to map out the thermal and density structure of the nebulae. The hydrogen lines are so strong that extracting that kind of information from them is much more difficult than using other lines; there are other details of the atomic physics which make lines other than the Balmer lines the things you want to use if you're after the physical structure of the nebulae.

You may well be right, but why is it, then, that some planetary nebulae never seem to show any red emission in any broadband images? I think that is true about Abell 43.

Ann

by **jabm67** » Mon Apr 04, 2011 3:39 pm

AFAIK, all planetary nebulae have substantial hydrogen emission. Images are taken in other spectral lines to map out the thermal and density structure of the nebulae. The hydrogen lines are so strong that extracting that kind of information from them is much more difficult than using other lines; there are other details of the atomic physics which make lines other than the Balmer lines the things you want to use if you're after the physical structure of the nebulae.

There's mounting evidence that planetary nebulae generally have cylindrical, not spherical, symmetry, and that they generally show signs of episodic mass ejection from the progenitor star. Bruce Balick and collaborators have a glorious collection of deep planetary nebula images; some of them are at http://www.astro.washington.edu/users/balick/WFPC2/ . In this cylindrical-symmetry concept, M57, the Ring, is one planetary seen more or less straight down the symmetry axis.

by **Ann** » Sat Apr 02, 2011 5:41 am

Hence, due to the theory of fusion processes, hydrogen is first expelled, then hydrogen and helium, then some H and He along with large portions of carbon, then some H and He along with large portions of Neon, etc. until the final fused material, iron, is reached.

I think you are mostly right about this, too, but I don't think that elements are expelled perfectly sequentially. Certainly hydrogen is expelled first, which is why supernovae are placed in two main categories: Type I, which lack hydrogen in their spectrums, and type II, which do contain hydrogen. Interestingly, relatively faint supernova SN 1987A, whose progenitor star had shed a lot of of its outer atmosphere and thus a lot of its hydrogen, nevertheless had hydrogen in its spectrum. So SN1987A was a type II supernova with hydrogen.

I found this Chandra X-ray Telescope image at http://www.nmm.ac.uk/explore/astronomy- ... va-remnant:

The caption describes the image like this:

Bluish knots of emission are scattered across the Chandra image. These contain material which is highly enriched in newly created oxygen, neon and magnesium. These heavy elements were produced deep within the original star and ejected by the supernova explosion. Elsewhere there are whitish coloured and yellow regions. This material is of a more standard composition than that seen elsewhere and is much less enriched in heavy elements. It represents either the pre-existing matter which surrounded the star or the outer layers of the star itself, lost before it exploded as a supernova.

You can see that there doesn't appear to be sequential shells of previously ejected material with higher and higher amounts of ever-heavier elements. Admittedly the supernova explosion itself would very efficiently mix up and destroy previously existing shells.

Interestingly, planetary nebulae often show what appears to be a sequential arrangement of elements in the gaseous shells around the star. You find ionized oxygen and sometimes ionized helium closest to the star. The you find ionized hydrogen, if there is any hydrogen there at all, and then you find ionized nitrogen or, I think, possibly ionized sulphur. But the reason for this elegant sequential order of elements is not that they were expelled in that order, but that the various elements require different amounts of ultraviolet emission to get ionized at all. Oxygen requires the most, and oxygen also can't get ionized except in when the oxygen gas is extremely rarified. That is why you find oxygen emission closest to the white dwarf, where the ultraviolet emission is strongest and and the stellar wind from the white dwarf itself makes the surrounding gases most rarified. Some distance farther out there is more gas, and oxygen can't get ionized here. Hydrogen, however, can, and this part of the nebula is perfect for ionizing hydrogen. Still further out there is less ultraviolet emission, and hydrogen won't get noticably ionized any more. But here nitrogen can get ionized.

: Click to view full size image

Beautifully "sequential" Ring Nebula. The filter used here were, Red: F658N ([N II]), Green: F501N ([O III]), Blue: F469N (He II). You have to wonder why there was no filter detecting ionized hydrogen. Perhaps the Ring Nebula contains only small amounts of hydrogen?

Ann

by **dougettinger** » Fri Apr 01, 2011 2:09 pm

I will make three conclusions from the observed data that you previously presented. But, of course, these conclusions are premature without collecting more matching data. Many thanks for entering the discussion about O and B type stars.

1. Supernova candidates expel matter in an episodic manner prior to its total collapse that results in a quark star, neutron star, or black hole. The periods of these episodes many vary from thousands to millions of years.

2. The pre-dominant observed metal or metals varies with each episode.

3. These irregularly spaced episodes may intersect each other due to their possible differences in energy release and the resulting acceleration of expelled particles. (This conclusion needs a little more work)

Do you know of any data that may counter these conclusions ?

04/01/2011

by **Ann** » Fri Apr 01, 2011 5:11 am

I wish I took a greater interest in some of the details of how things happen in the cosmos than I do, Doug. I'm not too interested in the itty bitty things, the things that can't be seen and don't have any interesting colors. That means I don't feel any pressing need to learn how atoms and molecules behave in different situations, or to learn why O and B stars have the death spasms that they do.

But you are definitely right that massive stars expel gas before they die. For planetary nebulae, that has the interesting consequence that many of them are red from hydrogen emission, but a few of them seem to lack hydrogen emission altogether. In the thread at the Asterisk Café where you and I are discussing now, I showed you a picture of planetary nebula Abell 43. Its color is all greenish from oxygen emission. There is no trace of red hydrogen light at all. This planetary nebula must definitely have shed its hydrogen before it became a planetary nebula. The red planetary nebulae, however - and a few of them appear to be all red - are either energized by a central star which is too cool to ionize oxygen, or else they are indeed surrounded by a planetary nebula which is mostly lacking oxygen.

Hydrogen-rich red Sharpless 2-71.

: Click to view full size image

Oxygen-rich green Abell 43.

At least one planetary nebula which is quite red appears nevertheless to be hydrogen-poor, and the red color comes mostly from nitrogen. Look at these narrow-band images of Jones Emberson 1, and see how weak the hydrogen is in all three different narrow-band images:

http://www.narrowbandimaging.com/pk164_page.htm

The expulson of matter from stars is often episodic rather than continuous. An example of episodic expulsion of matter is found in Asymtotic Giant Branch star Mira. Mira is speeding through space, but it is also "hiccuping" and periodically shedding a lot of gas. This gas, which travels along at Mira's speed, is colliding with the interstellar medium ("the gas out there"), and the collision creates ultraviolet light. So Mira has a long ultraviolet tail. But this tail is clumpy:

: Click to view full size image

Mira's clumpy tail.

Famous supernova 1987 A had definitely shed a lot of matter before it blew up. That is why ththe progenitor star was blue instead of red. But it probably also explains why the supernova was rather faint. The progenitor star simply didn't have such a thick atmosphere to ionize and energize and start fusion processes in and light up in various interesting ways.

Ann

by **dougettinger** » Wed Mar 30, 2011 9:54 pm

I have very strong suspicions based on some data that the very massive blue stars that die by core collapse do so in a certain sequence of steps. My steps are not really too dis-similar to the accepted orthodoxy - that the core's supply of H is exhausted and the core begins to collapse, causing a rise in temperature and pressure which becomes great enough to ignite and start a helium-to-carbon fusion cycle, creating enough outward pressure to halt the collapse. The core expands and cools slightly, with a hydrogen fusion outer layer and a helium fusion center. The difference for me is that the core rebounds with so much energy that a huge proportion of the outer hydrogen layer is expelled. And each time this process repeats itself a subsequent portion of the next outer layer is expelled. I also suspect that for each expulsion after a core collapse that most of the material is expelled from the equatorial regions due the centripetal forces of this spinning object.

Hence, due to the theory of fusion processes, hydrogen is first expelled, then hydrogen and helium, then some H and He along with large portions of carbon, then some H and He along with large portions of Neon, etc. until the final fused material, iron, is reached. At this point the final supernova explosion takes place mixing iron with some of the other already fused materials in the ejecta. The energy released also produces minute amounts of higher metals than iron due to nucleosynthesis.

The data that reinforces my idea are the Wolf-Rayet stars that are the normal stage for the death of very massive stars. These stars have strong emission lines that reflect either helium and nitrogen ( a WN sequence), or helium, carbon, and oxygen ( a WC sequence ), or helium, carbon, oxygen, and silicon ( WO sequence ). No order of these sequence events has yet been verified for any one star, although the fusion process should dictate that order. The Luminous blue variables, LBV's, constantly eject matter and are believed to evolve into Wolf-Rayet stars. Outbursts by LBVs are believed to produce Supernova impostors which are examples of large ejections of matter prior to the final expulsion that either causes a neutron star or black hole. Two specific stars that illustrate a sequence of outbursts are Eta Carinae and SN 2006jc.

I am looking forward to receiving criticism or any commentary on this topic.

03/30/2011

Starship Asterisk*

The death spasms of O and B type stars

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