Talk:Stellar evolution

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Contents 1 The Caption of the Crab Nebula 2 Random Question 3 White dwarfs 4 Removed 5 1st and 2nd Generation Stars 6 Removed 7 Universe's Age 8 Helium-Carbon Fusion 9 Heavier than iron 10 Life Cycle 11 giant molecular cloud 12 Brown dwarfs 13 Graphics

[edit] The Caption of the Crab Nebula

It states that the supernova occurred about 1000 years ago. However, the Crab Nebula is 6300 light years away. It seems like it should be written as the supernova occurring about 7000 years ago, unless this is just convention to use time of observation. 66.251.24.251 21:57, 16 March 2007 (UTC)

• It is indeed the convention to use the time of observation, since in astronomy distances aren't always well known. --Etacar11 23:45, 16 March 2007 (UTC)

[edit] Random Question

Random question from a reader, regarding the second paragraph of "Birth" talking about Giant Molecular Clouds, which reads: "The cloud is stable as its constituent molecules are too widely spaced apart for gravity to draw them closer." This doesn't really make sense, since gravity doesn't "cut off" at any distance. In contrast, the page actually *about* Giant Molecular Clouds, (at the bottom of the third paragraph in "General Information"), gives a different explanation: "The clouds have an internal magnetic field that provides support against their own gravity."

Of course that explanation doesn't cover (a) what would cause such a magnetic field and (b) how a magnetic field manages to keep a molecular cloud from collapsing since electrically neutral particles wouldn't be affected by magnetism -- but at any rate we have two pages linked to each other offering different explanations about what holds a molecular cloud apart. Anyone who actually understands astrophysics want to correct one or the other?

(Jan 8, 2005 -- It appears the "Birth" section has radically changed since this comment was posted and no longer contains the statement about particles being too far apart. Hence this comment is obsolete.)

[edit] White dwarfs

Updated first location where stability mechanism for white dwarfs is mentioned to link to degeneracy pressure. Although this entry does not yet exist a number of other astronomical entries link to it. It seems to have fallen down the cracks between the contributing astrophysicists and contributing quantum mechanics.

- Alan Peakall

[edit] Removed

Removed:

The ball, now a star, begins to shine.

The ball was already a star fusing hydrogen. Rednblu 11:02 20 Jul 2003 (UTC)

This text was replaced.

fusing iron does not liberate energy - because of the vast pressure and temperature, iron is actually forced to fuse. The supernova explosion is less than a fraction of a second away. Iron takes in energy when it fuses, it also takes in electrons. This energy and those electrons had been helping to support the star against it's own gravity. Now, with the support gone, the envelope of the star comes crashing down onto the core at a fine fraction of the speed of light. The implosion rebounds into a shock wave going through the star.

As the shock encounters material in the star's outer layers, the material is heated to billions of degrees, fusing to form the heavier elements. Indeed, all elements heavier than iron-56 are formed in supernovae explosions. In one of the most spectacular events in the Universe, the shock propels the material away from the star in a tremendous explosion called a supernova. The material spews off into interstellar space -- perhaps to collide with other cosmic debris and form new stars, perhaps to form planets and moons, perhaps to act as the seeds for an infinite variety of living things. Rednblu 11:57 20 Jul 2003 (UTC)

[edit] 1st and 2nd Generation Stars

looking for more info about 1st generation and 2nd generation stars, and what times these have existed. particularly, our sun is 2nd generation; how does its age compare with other 2nd generation stars? is it one of the older? or one of the younger of such stars?

I woudn't call our Sun second generation, but it's recycled material. The life time of stars strongly depends on their age. The more massive ones burn out rapidly, the lighter ones have very long life times (much longer than the age of the universe). There is an entire population of old stars in the globular clusters, which form a spherical halo around what one usually thinks of as the galaxy, namely the central bulge and the surrounding disc. These could be called first generation stars, I guess. Stars form permanently out of the galactic gas, which is (also permanently) being replenished by supernova explosions. The latter occur when massive (and thus short-lived) stars reach the end of their life time, or in binary stars with mass flow between the components. The Sun's age is about 4.5 billion years, which is half its expected life span and one third the age of the universe. Gas from supernova explosions contains more heavy elements (like iron) than the galactic gas contains on average (because supernovae is where those elements are created). Therefore, the gas out of which the stars form becomes more and more enriched with heavy elemnents as the galaxy ages. The stars that form today contain more heavy elements than our Sun, which, in turn, is more metal-rich than the stars in the globular clusters.

Yeah, but that's precisely the question *e's asking, even if *e doesn't know the correct terminology. *I* don't know the terminology either, but I'm also looking for the same information. After the big bang, you'd have lots of hydrogen, which could form into stars, go thru fusion, and make stuff up to iron via reactions. You don't get any heavier elements in the universe until those "1st generation" stars go supernova. When they do go supernova, you get trace amounts of heavier elements due to the supernova reaction. Those elements are then available in clouds which can form stellar systems which can now have planets which consist of more than just carbon, silicon, iron, and oxygen, hydrogen and a few other light elements.

So, how long would it take for the fastest stars first created to go supernova, their results turned into clouds, and first set of "2nd generation" suns (which we are? or are we a third?) to coalesce? Does that make Sol one of the first 2nd generation stars (via age), or middling old for a second generation star?

~ender 2006-12-04 23:02:PM MST

The term you're looking for is stellar population. That term redirects to one titled metallicity, which is unfortunate, but the discussion there is OK. BSVulturis 22:53, 12 January 2007 (UTC)

[edit] Removed

Removed statement about astronomers objecting to the name evolution. I've never heard any astronomer object, and if there are astronomers who do, they are small enough in number so that statement doesn't belong in the first paragraph. Roadrunner 18:02, 4 Mar 2004 (UTC)

[edit] Universe's Age

The article makes references to the age of the universe as though we are certain of it, but shouldn't it say that "it is believed that the universe is about 13 billion years old" and "The universe is not believed old enough for black dwarves to exist"?

We may not know the age of the universe for certain, but it's certain for all practical purposes (as certain as we are about anything else) that it's not old enough for black dwarves to exist. Black dwarves take hundreds to thousands of billions of years to come into existence.

As of 2003, we _have been_ certain of the age of the Universe to within 2%, thanks to WMAP data, and the scores of peer-reviewed professional journal publications such as this one detailing the data, its accuracy, confidence intervals, and normalization of its statistics to account for the models being considered. As far as the scientific community is concerned, the debate is over for the value of the age of the Universe. Of course, there are those who ignore scientific observations and data, and then there are the philosophers who can discuss the implications of this but science is science: the data has been taken, and it has been reported. It's up to you whether you accept the data and science, or reject it and believe something else in spite of scientific fact. But to do so would be a disservice to ourselves, and of course the Wiki articles to which we are contributing. Astrobayes 14:03, 20 June 2006 (UTC)

[edit] Helium-Carbon Fusion

Edited the statement about helium->carbon fusion lasting only "a few minutes" for a solar-mass star to the more generally accepted 1 billion years.

[edit] Heavier than iron

I'm confused with how an iron atom can be fused when being struck by a neutrino in supernova, I mean, wouldn't it break the nuclei apart? How does the additional protons and neutron fuse with the iron nuclei if just a neutrino made impact with it? (Sorry for asking easy questions I haven't studied in college yet)

- Protecter

I'm assuming its something related to neutron capture, just on a more fundamental scale. I mean, neutrons don't break the nucleus either, right? (Plus, they have more mass than a neutrino!) Perhaps the neutrino helps to form bonds or something. -- Natalinasmpf 14:42, 16 Apr 2005 (UTC)

I've changed neutrinos to neutrons in supernova section because neutrinos cannot form/alter nuclei at all and won't probably cause a shock wave compable to one caused by neutrons and photons. This should be reviewed though.

//lodin

• I believe neutrinos is correct here. --Etacar11 04:07, 11 May 2006 (UTC)

This is all quite confused in the article. The collapse of the iron core results in it being converted to chiefly neutrons, creating a core composed of neutronium. The neutron core collapse is halted by neutron degeneracy, which causes the momentum of the collapsing core to rebound, and which in turn causes the infalling stellar envelope (read: the rest of the star) to also rebound. The formation of the neutron core creates a massive amount of neutrinos, which are then emitted after it rebounds from its point of "maximum scrunch." This results in the neutrino pulse -- which is emitted before the electromagnetic event -- and actually contains most of the energy involved in the supernova itself. The rebounding envelope then creates what is traditionally thought of as the supernova, namely the star blowing itself apart. The pressures involved in the rebound, as well as induced reverse beta decay caused by the massive neutrino flux, are what synthesizer the heavier elements. What is left behind is the neutron core, which is called a neutron star. Xihr 06:46, 11 May 2006 (UTC)

• Yes, I think you are right, it might need some reworking, but when the person changed neutrino to neutron, that was incorrect. BTW, be careful about using the word "neutronium"--it is not really used in astronomy. Mainly you see it in science fiction. :) --Etacar11 22:24, 11 May 2006 (UTC)

It's also used by particle physicists, high-energy physicists, and astrophysicists dealing with collapsed stars. Don't believe everything you read in Wikipedia. ... But yes, your correction back to neutrinos was certainly correct. I was making a more general comment about the confusions that were going back and forth. Xihr 01:39, 12 May 2006 (UTC)

• Yeah, I get that. But I have to add: I'm not trying to start a fight, but I AM an astronomer and have NEVER heard one use the term "neutronium" seriously (and ADS finds no papers using it in the title) but hey YMMV (I can't speak for physicists [or all astronomers/astrophysicists] ;) ). --Etacar11 01:47, 12 May 2006 (UTC)

• The supernova article seems to get it right. The neutrinos do cause shock waves, so I was wrong in this part... However, how can neutrinos help creating heavier elements is still beyond my understanding, because you need baryons to do so -- and neutrinos are leptons, no matter what their energy is. //Lodin 2006-05-13 11:01 GMT

• Google "neutrino nucleosynthesis" and you'll find some info/references on it. --Etacar11 14:12, 13 May 2006 (UTC)

I already indicated how; induced reverse beta decay. nu + n --> p + e. Xihr 00:45, 14 May 2006 (UTC)

• Umm, stupid question maybe, but can someone provide actual papers and documentation on this. It seems like all this back and forth violates the WP:OR rules. No original research. If we don't know how it happens, we can't speculate. Either we know, and there's a citable source, or there's not, and don't include it. Maybe I'm misreading. But it seems like nobody ACTUALLY knows what goes on. Sounds like Pathological science to me. Pathological science, and Pseudoscience don't really belong in a good article (unless it's an article specifically about a pseudoscience claim or protoscience claim or religious claim [like Pastafarianism or Scientology]; assuming it's notable). So, if all we're doing is theorizing... Let's not. If it's controversial, it probably shouldn't be in there, or the controversy should be noted with notable resources on all applicable theories. Yeah? My 2c. Granted, I'm not a physicist. But when poeple start speculating, rather than using citable authoritative sources, Wikipedia says that's a 'no-no'... Mgmirkin 00:00, 19 October 2006 (UTC)

This discussion is not very speculative,at least not between physicists. I think this debatte has aroused because some people here tried to think about a supernovae without knowing all important processes. What comes out ist thinks like "oh i think this should be differt, because it can not be caused by the process i am familiar with". The only discussion i know about is whether neutronstar collisions provide a scenario likely to produce these heavy elements as well (or dominantly). Anyway if you want papers just go to google scholar and type "r process" almoust every paper you get (wich is not dealing with neutron stars) will tell you what i just told you. Here just some links:

http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v562n1/53839/53839.text.html http://www.journals.uchicago.edu/cgi-bin/resolve?id=doi:10.1086/174638&erFrom=2467619731031865010Guest http://www.journals.uchicago.edu/cgi-bin/resolve%3Fid%3Ddoi:10.1086/342230 Theo

Ok it goes like this: There is a shockfront of the infalling material wich bounces of the core. This shockwave now travelles back away from the core. Anyway simulations show that they would not have enough energy and stop at some time. Fortunally the core sends of a wast amound of neutrinos wich collide with the material giving it the extra drive to actually go out into space. Anyway they just give new impuls and might cause some inverse betas. Ok now that the SN finally explodes you have to understand what a heavy nucleii actually is. We can characterize a nucleii by two numbers: Number of Protons in it (=Z) and total number of protons and neutrons (=A). Only some compositions of these two numbers are stable, others will decay. Roughly one can say you always need little more neutrons than protons. So A = 2Z. All the incoming neutrinos can do is change neutrons in protons, but that leaves A unaffacted. If you want to create for instance lead (Z=82 A=206) you need to bombart Iron (A=56) with many many Neutrons. NEUTRINOS WILL NOT DO. Luckily there is a good flux of Neutrons appearing in a Supernova so Iron catches some neutrons and may create the heavier (bigger Z AND bigger A) elements. So i assure you Neutrons are correct. I know this because it is my topic of research. You will have noticed english is not my native language and i am kind of in hurry right now so feel free to correct anything you think is wrong with my language. hope i could help you a little. Greetings Theo

[edit] Life Cycle

I assume stars have a life cycle, yes? This article seems to cover mainly just the evolution of a star, classifying them on mass, reporting on its death, then touching lightly on that it can later after its death (or becomes a black hole) be fed into new stars sometime later. This is ambiguous though, but I assume there should be some sort of life cycle or we would have run out of stars at the Universe's current age? Also, about blackholes trigerring nebulae as well? -- Natalinasmpf 14:37, 16 Apr 2005 (UTC)

[edit] giant molecular cloud

For this phrase, is molecular cloud not better link as dark nebula? -- Harp 10:54, 19 May 2006 (UTC)

• I have a dumb question. Has it ever actually been shown that a giant molecular cloud collapses under its own gravity or whatnot to create a star (I mean have we ever actually seen gas collapse in a vacuum with no force other than gravity)? Or is that just a theory that's accepted as fact despite not ever having actually witnessed it? Just wondering... Never personally seen a cloud condense into a star. So, how do we know it happens? Or is it just in fashion until we figure out it doesn't really work that way, or we do figure out what ACTUALLY happens? If someone could point me to actual experiments showing how gases in a vacuum condense into solid matter (like Earth or the sun; okay, granted the sun's not "solid", but more so than ambient gas/plasma in a vacuum), I'd be interested to know. Just wondering. Mgmirkin 00:06, 19 October 2006 (UTC)

[edit] Brown dwarfs

Brown dwarfs heavier than 13 Jupiter masses (M_J) do fuse deuterium, and some astronomers prefer to call only these objects brown dwarfs, classifying anything larger than a planet but smaller than this a sub-stellar object.

Could someone clarify this please? Should the latter term read sub-brown dwarf? I was under the impression that brown dwarfs are sub-stellar objects... I'd make changes myself, but I don't actually know enough about brown dwarfs! --Xanthine 08:33, 6 September 2006 (UTC)

[edit] Graphics

The "Image:Sun Life.png" is really cool, can someone make a similar diagram for other stars with differing initial mass? --71.3.236.196 15:05, 9 September 2006 (UTC)

Like a comparison between red dwarfs, yellow dwarfs, etc? I would gladly, provided I have the spare time. :)

--Xanthine 16:45, 11 September 2006 (UTC)

Would this illustration be of use in this article?

Image:Stellar evolution L vs T.png

It's similar to, but not the same as, Image:Stellar evolution sun.svg. Thanks. — RJH (talk) 17:32, 20 December 2006 (UTC)

I think that figure's superb. BSVulturis 20:36, 17 January 2007 (UTC)