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Hu, man[edit]

Hu, man, my understanding is that in the first stage of evolution stars only fuse hydrogen into helium. It is only when hydrogen is depleted, the reaction slows and the star colapses, that the core of the star is pushed inwards with enough pressure to start the next reaction up the table, fusing helium into Lithium. If the star is massive enough, the cycle continues until iron is produced, at which point the cycle ends. Of course, along the way, novas or supernovas can occur, and stars can collapse into neutron stars or black holes, all depending on the original mass.And all the elements heavier than iron are produced in the incredible pressure and heat of novas.PoorEd 15:49, 25 February 2008 (EST)

Hi Ed, thanks. I suppose we both ought to dig out reading material and get it right ;) As I understand it, the first half dozen elements can be formed in stars - but there is a roadblock around oxygen or somewhere like that - there isn't a stable "next step" nucleus that can be formed under stellar conditions. It takes nova conditions to make anything heavier (or human lab conditions!). I'll see if I can find where I think I learnded that. humanUser talk:Human 16:13, 25 February 2008 (EST)
You're right, more research is called for. Maybe you're right, the roadblock is oxygen rather than iron. But my understanding was that each stage of a star's evolution consists of only one reaction. Thus my suggestion that most stars are only transforming hydrogen to helium. I always liked Joni Mitchell's reminder in "Woodstock" that we are all made of star dust - that many of the atoms in us were made during novas and supernovas. PoorEd 16:22, 25 February 2008 (EST)
Tentatively, you appear to be broadly correct - but it's all a matter of scale, it seems. The heavier a star, the heavier the elements it can produce. (from wp:nucleosynthesis#Stellar_nucleosynthesis "The first direct proof that nucleosynthesis occurs in stars was the detection of technetium in the atmosphere of a red giant in the early 1950"). I once thought that iron was the roadblock myself, 'til I read something somewhere - but it may have been about stars in the same mass range as ours. And, yes, various elements are produced under specific conditions in the lifetime of a star. Anyway, as far as the article, since it is a lightweight piece that is mostly for fun, we can just be vague ("lighter" and "heavier" elements?) rather than describe all the nuclear physics? I'll try to find a good reference for the whole process somewhere, as I am sure you are also doing. humanUser talk:Human 16:53, 25 February 2008 (EST)

So - what goes on in a star, and possibly the confusion over oxygen above. There is the CNO cycle which is short for Carbon Nitrogen Oxygen. In this system, carbon acts as a catalyst for fusion of hydrogen into helium:

  1. 12C + 1H -> 13N + gamma ray + 1.95 MeV
  2. N13 -> 13C + e+ + νe +2.22 MeV
  3. 13C + 1H -> 14N + gamma ray +7.54 MeV
  4. 14N + 1H -> 15O + gamma ray +7.35 MeV
  5. 15O -> 15N + positron + electron neutrino +2.75 MeV
  6. 15N + 1H -> 12C + 4He +4.96 MeV

You will note that you are back at carbon 12 to repeat the cycle.

The other type of fusion found in stars is the proton - proton chain. It is basicaly smash two protons together and convert one of them to a neutron in the process, then smash the deuterium together to form helium. This is common in sun sized and smaller stars. 98% of the fusion in the sun is pp, and 2% CNO. Heavy stars have mostly CNO fusion. The CNO cycle releases more energy and thus needs a larger star (more gravity) to balance it out.

Once the hydrogen supply in the core is used up, the star stars burning other things. First is helium burning to produce beryllium and quickly fuse it (before it decays in about 10-16 seconds) to form carbon 12. One bit to realize though that 4+ solar mass star will start carbon burning at 'birth'. Carbon burning produces neon, sodium, oxygen and magnesium. These reactions only release 4.6 MeV to 2.2 MeV (much less than the CNO cycle above). Once the carbon builds up the star cannot produce more energy and so the star contracts and heats up again to where it can burn neon.

While the star is fusing carbon into heavier elements (for example) at the core, there is a shell of helium around it. In this shell, it will be fusing helium into carbon.

Eventually, none of the reactions is exothermic. This is at iron and the star collapses. This collapse smashes together the atoms and forms heavier elements.

Further reading:

--Shagie 16:57, 25 February 2008 (EST)

I may have been partially confusing myself with big bang nucleosynthesis: "Big Bang nucleosynthesis produced no elements heavier than beryllium, thanks to a bottleneck due to the absence of a stable nucleus with 8 nucleons. In stars, the bottleneck is passed by triple collisions of helium-4 nuclei, producing carbon (the triple-alpha process). However, this process is very slow, taking tens of thousands of years to convert a significant amount of helium to carbon in stars, and therefore it made a negligible contribution in the minutes following the Big Bang." humanUser talk:Human 17:00, 25 February 2008 (EST)

Stars are suns[edit]

Admittedly this is a very minor point, but I've always assumed the term "sun" was reserved for stars around which a planet or planets are revolving. Rational Ed5 or 6 edits 12:18, 27 March 2008 (EDT)

Is there even a "scientific" distinction? I suspect an astronomer might use a phrase like "a star like our sun", or "our sun, which is a blah mass star on the main sequence". It's sort of like the fuss over the word "planet", as far as our solar system (which just lost one!). Once upon a time, these words made sense - the sun was the bright thing in the daytime sky, stars were tiny twinkling points of light in the nighttime sky. Planets were nighttime points of light that "wandered" relative to the rest of the "fixed" stars. These words come from "common usage", not scientific analysis, and so are rife with error and not very useful as "scientific" categories. Even though now we have a "definition" of a "planet", it is more for schoolage science than adult endeavors.
At one level you may be correct - from the perspective of any planet orbiting a star, that star is its "sun" in the same sense that ours is our "sun". But they are all just stars in the end, some with orbital detritus, some without. humanUser talk:Human 14:04, 27 March 2008 (EDT)
The term sun is not a scientific term, and I have only heard scientists use it in reference to the closest star to earth. Rational Ed5 or 6 edits 19:39, 27 March 2008 (EDT)

The First Science[edit]

It took me awhile to realize this, but Astronomy's impact on the rest of science has been nothing short of gigantic. Think about it, from the complex calculations needed to create calendars to Galileo's astronomical observation's which changed the course of western history. Astronomy can literally be called "the first modern science." Should I include that in the article? Alsto003 (talk) 10:42, 28 August 2014 (UTC) Alex

Astronomy woo[edit]

Should there be such a listing here - Planet X and Nibiru, faked moon landings, Immanuel Velikovsky and suchlike?

And has any woo on this beastie been developed yet? Anna Livia (talk) 12:15, 10 November 2017 (UTC)

Link to relevant categories instead if more appropriate - and the (514107) 2015 BZ509-woo has already started. Anna Livia (talk) 18:09, 22 May 2018 (UTC)

Will add [1] and [2] for incorporation when appropriate. Anna Livia (talk) 00:02, 17 January 2019 (UTC)