# The largest star ever **URL:** \ **Category:** Factual Questions\ **Created:** [October 28, 2000, 1:05am UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687 "2000-10-28T01:05:38Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![howardsims](https://avatars.discourse-cdn.com/v4/letter/h/e8c25b/32.png) [@howardsims](https://boards.straightdope.com/u/howardsims)\ **Post date:** [October 28, 2000, 1:05am UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687/1 "2000-10-28T01:05:38Z")
After the big bang, when hydrogen formed and everything was still hot; wasn’t the universe basically a big star? And if so, why didn’t all the hydrogen burn up and turn into helium and eventually iron? ---
**Author:** ![Mahaloth](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mahaloth/32/1079_2.png) [@Mahaloth](https://boards.straightdope.com/u/Mahaloth)\ **Post date:** [October 28, 2000, 1:12am UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687/2 "2000-10-28T01:12:20Z")
People still believe in the Big Bang? My, my, the persistance people have in believing myths. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [October 28, 2000, 1:20am UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687/3 "2000-10-28T01:20:30Z")
Ha, ha. Yes, and that silly round earth thing, too. For it to have burned up all the hydrogen, it would have had to stay compact. There was so much energy that it all flew apart instead. Hence the term, “Big Bang.” I’m sure someone will pop by to explain it more rigorously. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [October 28, 2000, 1:20am UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687/4 "2000-10-28T01:20:48Z")
I thought Orson Wells was the largest star ever? ---
**Author:** ![DeutschFox](https://avatars.discourse-cdn.com/v4/letter/d/49beb7/32.png) [@DeutschFox](https://boards.straightdope.com/u/DeutschFox)\ **Post date:** [October 28, 2000, 1:29am UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687/5 "2000-10-28T01:29:23Z")
Subject: Largest star ever? > [@](#): > > \*Originally posted by Johnny L.A. \* > \*\*I thought Orson Wells was the largest star ever? > \*\* Kudos! ---
**Author:** ![Ankh\_Too](https://avatars.discourse-cdn.com/v4/letter/a/3ab097/32.png) [@Ankh\_Too](https://boards.straightdope.com/u/Ankh_Too)\ **Post date:** [October 28, 2000, 2:28am UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687/6 "2000-10-28T02:28:06Z")
> [@](#): > > \*Originally posted by howardsims \* > \*\*After the big bang, when hydrogen formed and everything was still hot; wasn’t the universe basically a big star? > And if so, why didn’t all the hydrogen burn up and turn into helium and eventually iron? \*\* Well, in a way, the very early universe was not unlike the interior of a star, albeit a star that was expanding at just under the speed of light. From 10[sup]-43[/sup] (Planck time) seconds after the Big Bang until until 10[sup]-6[/sup] seconds, the Universe was too hot (10[sup]32[/sup]K to 10[sup]12[/sup]K) for protons and neutrons to form, there were only quarks. By one second after the big bang, the electrons and positrons had collided and annihiliated, leaving electrons, protons, neutrons, photons and neutrinos. At 2 seconds after the Big Bang, the synthesis of light nuclei (hydrogen, helium, etc.) began. By 3 minutes after, the fusion of new nuclei has stopped. It’s my understanding, (perhaps mistaken) that fusion stops because the matter in the universe drops below the threshold for fusion (10[sup]8[/sup]K). But, it’s not for 100,000 years that the temperature drops low enough (3000 K)for the electrons to bind to the nuclei and form atoms. At that point, it’s a waiting game as to when the gravitational forces can bring together the dust and gas that will eventually become stars and galaxies. ~looks around wondering if there are any theoretical physicists who read the boards~ How’d I do? ---
**Author:** ![DeutschFox](https://avatars.discourse-cdn.com/v4/letter/d/49beb7/32.png) [@DeutschFox](https://boards.straightdope.com/u/DeutschFox)\ **Post date:** [October 28, 2000, 8:08am UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687/7 "2000-10-28T08:08:33Z")
> [@](#): > > \*Originally posted by Ankh\_Too \* > \*\* How’d I do? \*\* Great … very well done … but the temperature of fusion is not necessarily just based on the temperature … and probably includes pressure, concentrations, etc., in the equation … Our Sun is a fusion machine too … but the temperatures estimated at the surface and in the interior of the our Sun are, as I recall, significant lower. I don’t recall them even approaching 10^8K … I’m really confused on this??? Maybe someone could help us? ---
**Author:** ![Bobort](https://avatars.discourse-cdn.com/v4/letter/b/ac8455/32.png) [@Bobort](https://boards.straightdope.com/u/Bobort)\ **Post date:** [October 28, 2000, 8:42am UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687/8 "2000-10-28T08:42:01Z")
> [@](#): > > \*Originally posted by DeutschFox \* > \*\*Great … very well done … but the temperature of fusion is not necessarily just based on the temperature … and probably includes pressure, concentrations, etc., in the equation … Our Sun is a fusion machine too … but the temperatures estimated at the surface and in the interior of the our Sun are, as I recall, significant lower. I don’t recall them even approaching 10^8K … I’m really confused on this??? Maybe someone could help us? \*\* I’m no fusion expert, but I’ll have a go at this anyway. For a specific fusion reaction taking place in the plasma phase it seems to me like there would be a minimum temperature regardless of pressure, etc. The nuclei are going to have to have enough energy to overcome electrostatic repulsion to fuse, which seems to me to imply a minimum temperature (and I have no idea what this temperature would be). Greater pressure and concentration will certainly increase the _rate_ of the fusion reaction though. ---
**Author:** ![Ike\_Witt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ike_witt/32/283_2.png) [@Ike\_Witt](https://boards.straightdope.com/u/Ike_Witt)\ **Post date:** [October 28, 2000, 3:42pm UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687/9 "2000-10-28T15:42:25Z")
> [@](#): > > \*Originally posted by Mahaloth \* > \*\*People still believe in the Big Bang? My, my, the persistance people have in believing myths. \*\* What creation myth do you believe in? ---
**Author:** ![Ankh\_Too](https://avatars.discourse-cdn.com/v4/letter/a/3ab097/32.png) [@Ankh\_Too](https://boards.straightdope.com/u/Ankh_Too)\ **Post date:** [October 28, 2000, 7:15pm UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687/10 "2000-10-28T19:15:01Z")
The minimum enegry threshold for protons to over come the repulsion of the electric force and allow the strong nuclear force to initiate fusion is 3.5 x 10 [sup]6[/sup]K, while the core of the sun has a _calculated_ temerature of 1.5 x 10[sup]7[/sup]K. The pressure and density come into play, I _think_. The density of matter at the core of the sun is 160[sup]g[/sup]/cm[sup]2[/sup], for comparison, gold has a density of 19.3 [sup]g[/sup]/cm[sup]2[/sup]. The pressure at the core is 3.45 x 10[sup]11[/sup] atmospheres (345 **trillion** times the atmospheric pressure at sea level). The young universe, even at 2 or 3 seconds after the initial Big Bag, was still expanding outwards at nearly the speed of light, even though the inflationary phase of the growth was long over. The protons in the core of the sun are undergoing fusion at a (more or less) constant pressure. The same is not true of the young universe. The pressure and density of the matter was falling as the Universe expanded. By three minutes, it may have been that the matter concentration had dropped below the density required for the particles to be so close as to overcome the electromagnetic force and allow the strong nuclear force to fuse the protons into helium (and trace amounds of lithium). ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 28, 2000, 8:37pm UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687/11 "2000-10-28T20:37:35Z")
The pressure and composition determine the probability that two nuclei will meet, but it’s the average energy per particle (otherwise known as the temperature) that determines what will happen when they do meet. In fact, there was a good bit of fusion going on immediately after the Big Bang, and in fact most of the lithium and a good bit of the helium in existance today were formed at that time, but due to the expansion, it didn’t last long enough to use up all of the hydrogen. The temperature at the surface of the Sun, incidentally, is relatively low (5,770 Kelvins, cooler than a propane torch), but all of the fusiuon is going on in the core, which is much, much hotter. ---
**Author:** ![howardsims](https://avatars.discourse-cdn.com/v4/letter/h/e8c25b/32.png) [@howardsims](https://boards.straightdope.com/u/howardsims)\ **Post date:** [October 28, 2000, 11:39pm UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687/12 "2000-10-28T23:39:05Z")
For Ankh\_too, I think 3 minutes after the big bang for fusion to end is a little early. That would put the universe at a “radius” of only 3 light minutes (If you can call a closed system having a radius). Yet our own sun will one day expand and grow to even the distance of our earth which has a radius of 8 light minutes. And there was a heck of a lot more matter after the big bang than in our little puny sun. But you make a good point that matter was flying out and pressure dropping. I just wonder how long before it all became too distributed to still fuse. ---
**Author:** ![Ankh\_Too](https://avatars.discourse-cdn.com/v4/letter/a/3ab097/32.png) [@Ankh\_Too](https://boards.straightdope.com/u/Ankh_Too)\ **Post date:** [October 29, 2000, 1:25am UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687/13 "2000-10-29T01:25:08Z")
Yes, and no. The three to three and a half minute limit is pretty firm. The primary source for the ‘timeline’ on the early Universe was a website for the course [Astronomy 102: “Introduction to Stars, Galaxies, and the Universe”](http://www.astro.lsa.umich.edu/Course/Bernstein102/), specifically lecture [#33 _In the Beginning_](http://www.astro.lsa.umich.edu/Course/Bernstein102/Notes/notes10.html#_Toc416265815). The [Astronomy Tutorial](http://cassfos02.ucsd.edu/public/tutorial/BB.html) at University of California, San Diego; Center for Astrophysics & Space Sciences calls the period from 180 to 210 seconds (3 to 3 1/2 minutes) after _t_=0 as the **Era of Nuclear Reactions** , and saying it ended at approximately _t_ + 210 seconds. The [National Center for Supercomputing Applications’](http://www.ncsa.uiuc.edu/) project [Cosmos in a Computer](http://www.ncsa.uiuc.edu/Cyberia/Cosmos/CosmosCompHome.html) gives a value of aproximately “100 seconds” for the duration of nucleosythesis. > [@](#): > > _From the [Big Freeze Out](http://www.ncsa.uiuc.edu/Cyberia/Cosmos/BigFreezeOut.html) page at the NCSA:_ > \*\* > Almost all of the helium, deuterium (heavy hydrogen), and some of the lithium nuclei in our universe today were created during the “Era of Nucleosynthesis,” which began about 1 second after the Big Bang and ended just 100 seconds later (ordinary hydrogen nuclei, each consisting of just one proton, did not have to be “created;” they already existed!). \*\* My mistake came in theorizing _(wow, makes “Wild Ass Guess” sound so impressive, doesn’t it?)_ that nucleosynthesis ended because of a drop in pressure/temperature/density. The process ended because there were no more free neutrons with which protons could bind to create a nucleus. Given an initial ratio of 87% protons and 13% neutrons, it’s not hard to see where the neutrons would run out first. The rest of the free protons became hydrogen atoms when they, along with the newly created nuclei of duterium, helium and lithium, bound with electrons at _t_=10[sup]6[/sup]yr. So, my second post, while filled with interesting factoids, was basically useless as far as explaining the end of nucleosythesis. Ahh, well. I can live with 66.7%. ---
**Author:** ![howardsims](https://avatars.discourse-cdn.com/v4/letter/h/e8c25b/32.png) [@howardsims](https://boards.straightdope.com/u/howardsims)\ **Post date:** [October 29, 2000, 1:15pm UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687/14 "2000-10-29T13:15:40Z")
\>\>The process ended because there were no more free neutrons with which protons could bind to create a nucleus\<\< Hmm, then how can stars still have fusion if there are no more free neutrons? ---
**Author:** ![Akatsukami](https://avatars.discourse-cdn.com/v4/letter/a/ea666f/32.png) [@Akatsukami](https://boards.straightdope.com/u/Akatsukami)\ **Post date:** [October 29, 2000, 3:19pm UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687/15 "2000-10-29T15:19:08Z")
**howardsims** asks: > [@](#): > > Hmm, then how can stars still have fusion if there are no more free neutrons? A proton can be changed to a neutron (or vice versa) through a weak-force-mediated reaction (it involves a couple of leptons). OTOH, since this _is_ a weak-force-mediated reaction, it’s incredibly slow; the lifetime of the average proton in the solar core is estimated to be around 10[sup]9[/sup] years (when a proton is changed to a neutron, and forms a deuteron, however, that deutron is sucked up almost immediately). This is why current fusion research is almost completely directed at the D-T (deuterium-tritium) reaction, with a little, essentially theoretical, effort directed at the D-D and D-[sup]3[/sup]He reactions; none of these require p-\>n, and therefore are (probably) achievable with current or near-current technology (D-T _is_ achievable with current technology; it’s getting a controlled, sustainable , profitable (in both economic and energetic terms) reaction that poses problems). Controlled, etc., P-P fusion in less than stellar-sized masses is science fiction at this point. ---
**Author:** ![howardsims](https://avatars.discourse-cdn.com/v4/letter/h/e8c25b/32.png) [@howardsims](https://boards.straightdope.com/u/howardsims)\ **Post date:** [October 29, 2000, 10:55pm UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687/16 "2000-10-29T22:55:10Z")
Isn’t a hydrogen bomb a fusion reaction? I mean if it is, then we have already acheived fusion. ---
**Author:** ![Ankh\_Too](https://avatars.discourse-cdn.com/v4/letter/a/3ab097/32.png) [@Ankh\_Too](https://boards.straightdope.com/u/Ankh_Too)\ **Post date:** [October 30, 2000, 12:03am UTC](https://boards.straightdope.com/t/the-largest-star-ever/38687/17 "2000-10-30T00:03:24Z")
Controlled fusion reactions. The kind we might actually be able to harness.