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Star formation
Sorry if this is an FAQ, but it5 has long made me wonder and I have never
seen an explanation. The Earth massive as it is, is too slight to have hydrogen in the atmosphere. I think that is because the speed of the molecules exceeds, just about, the escape velocity. Given that, how did stars form from hydrogen? |
#2
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Star formation
Wasn't it John Brockbank who wrote:
Sorry if this is an FAQ, but it5 has long made me wonder and I have never seen an explanation. The Earth massive as it is, is too slight to have hydrogen in the atmosphere. I think that is because the speed of the molecules exceeds, just about, the escape velocity. Given that, how did stars form from hydrogen? Stars formed from gas clouds that are considerably heavier than the Earth, and therefore have higher escape velocities. -- Mike Williams Gentleman of Leisure |
#3
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Star formation
John Brockbank wrote:
Sorry if this is an FAQ, but it5 has long made me wonder and I have never seen an explanation. The Earth massive as it is, is too slight to have hydrogen in the atmosphere. I think that is because the speed of the molecules exceeds, just about, the escape velocity. Given that, how did stars form from hydrogen? Surface gravity at the sun 27 G?(260 /s/s) And that is for a yellow dwarf. Even Jupiter/Saturn/neptune hold onto their hydrogen pretty well. |
#4
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Star formation
"John Brockbank" wrote in news:43fcaa89$1_3@mk-nntp-
2.news.uk.tiscali.com: Sorry if this is an FAQ, but it5 has long made me wonder and I have never seen an explanation. The Earth massive as it is, is too slight to have hydrogen in the atmosphere. I think that is because the speed of the molecules exceeds, just about, the escape velocity. Given that, how did stars form from hydrogen? Note that the planets that formed further out in the solar system (Jupiter, Saturn, Uranus and Neptune) still have plenty of hydrogen. This is mainly because of their greater mass but also the solar radiation is much lower so their atmospheres are cooler than the Earth's and thus a lower percentage of the hydrogen can reach escape velocity (which is considerably higher for those planets than for the earth). The details of star formation are still being worked out but the basic idea is that the increase in escape velocity caused by the accretion of matter exceeds the increase in the speed of the accreting molecules or atoms caused by increasing temperature, i.e no significant portion of the accreting matter heats up enough to gain a speed suffient to overcome the ever increasing escape velocity caused by accretion. Once the stars' core initiates nuclear fusion the radiation from the new star heats the atmosphere of any planets formed. In the case of the Earth, the heat from newly formed sun would have caused the Earth to gradually lose hydrogen from its' atmosphere. It is thought that hydrogen was probably being outgassed by volcanism on the Earth for some considerable time after its' initial formation. This is deduced from the probability that the Earth primarily formed from chondritic material. See: http://www.sciencedaily.com/releases...0911103921.htm and http://en.wikipedia.org/wiki/Star_formation Klazmon |
#5
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Star formation
"Sjouke Burry" wrote in message . .. John Brockbank wrote: Sorry if this is an FAQ, but it5 has long made me wonder and I have never seen an explanation. The Earth massive as it is, is too slight to have hydrogen in the atmosphere. I think that is because the speed of the molecules exceeds, just about, the escape velocity. Given that, how did stars form from hydrogen? Surface gravity at the sun 27 G?(260 /s/s) And that is for a yellow dwarf. Even Jupiter/Saturn/neptune hold onto their hydrogen pretty well. Of course once a star has formed it has a strong gravity field. However, presumably a vary large volume of hydrogen, having a huge mass, in fact will have a low gravitational force because it is spread out (inverse square law). Sorry, but I do not think this is a no-brainer. |
#6
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Star formation
Wasn't it John Brockbank who wrote:
"Sjouke Burry" wrote in message ... John Brockbank wrote: Sorry if this is an FAQ, but it5 has long made me wonder and I have never seen an explanation. The Earth massive as it is, is too slight to have hydrogen in the atmosphere. I think that is because the speed of the molecules exceeds, just about, the escape velocity. Given that, how did stars form from hydrogen? Surface gravity at the sun 27 G?(260 /s/s) And that is for a yellow dwarf. Even Jupiter/Saturn/neptune hold onto their hydrogen pretty well. Of course once a star has formed it has a strong gravity field. However, presumably a vary large volume of hydrogen, having a huge mass, in fact will have a low gravitational force because it is spread out (inverse square law). Sorry, but I do not think this is a no-brainer. Remember that it will also be very cold. There's no heat source until the star starts to form. So the hydrogen will be moving very much more slowly. Escape velocity is proportional to sqrt(mass/distance). We only have to consider molecules escaping from the outer fringes, and don't need to worry about the gravity in the interior of the condensing cloud. The typical speed of molecules in the gas is proportional to the square root of the absolute temperature. Presumably there's certain critical mass/distance/temperature ratio. If external forces cause a cloud to cross this boundary then it starts to contract under its own gravity. If a cloud stays outside the boundary, then it continues to be a cloud or may even disperse. -- Mike Williams Gentleman of Leisure |
#7
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Star formation
John Brockbank wrote:
"Sjouke Burry" wrote in message . .. John Brockbank wrote: Sorry if this is an FAQ, but it5 has long made me wonder and I have never seen an explanation. The Earth massive as it is, is too slight to have hydrogen in the atmosphere. I think that is because the speed of the molecules exceeds, just about, the escape velocity. Given that, how did stars form from hydrogen? Surface gravity at the sun 27 G?(260 /s/s) And that is for a yellow dwarf. Even Jupiter/Saturn/neptune hold onto their hydrogen pretty well. Of course once a star has formed it has a strong gravity field. However, presumably a vary large volume of hydrogen, having a huge mass, in fact will have a low gravitational force because it is spread out (inverse square law). For any given density of material there is some ultimate size beyond which a clump of matter will necessarily and inevitably contract into ever tighter clumps until stars are formed. There are some really nice simulations online at a cosmological scale and star cluster scales too. Sorry, but I do not think this is a no-brainer. It only requires that there is enough attractive force from the clump of material to ensure that parts of it slowly become ever denser. There was a very nice illustrated talk about this at Astrofest by Matthew Bates of Exeter, simulations in video for braodband only at: http://www.astro.ex.ac.uk/people/mbate/ Regards, Martin Brown |
#8
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Star formation
JRS: In article , dated
Thu, 23 Feb 2006 18:04:28 remote, seen in news:uk.sci.astronomy, John Brockbank posted : Of course once a star has formed it has a strong gravity field. However, presumably a vary large volume of hydrogen, having a huge mass, in fact will have a low gravitational force because it is spread out (inverse square law). Sorry, but I do not think this is a no-brainer. It's not the force that matters, but the integral of the force over the distance from the putatively-escaping particle to infinity. The field of a large volume drops off slowly. Most people find climbing a ladder easier than climbing Snowdon, although in the latter case the slope is generally much less. The larger Gas Giants of the Solar System hold hydrogen more because they are big than because they are cold. -- © John Stockton, Surrey, UK. Turnpike v4.00 MIME. © Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links; Astro stuff via astron-1.htm, gravity0.htm ; quotings.htm, pascal.htm, etc. No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News. |
#9
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Star formation
Martin Brown wrote in
: John Brockbank wrote: "Sjouke Burry" wrote in message . .. John Brockbank wrote: Sorry if this is an FAQ, but it5 has long made me wonder and I have never seen an explanation. The Earth massive as it is, is too slight to have hydrogen in the atmosphere. I think that is because the speed of the molecules exceeds, just about, the escape velocity. Given that, how did stars form from hydrogen? Surface gravity at the sun 27 G?(260 /s/s) And that is for a yellow dwarf. Even Jupiter/Saturn/neptune hold onto their hydrogen pretty well. Of course once a star has formed it has a strong gravity field. However, presumably a vary large volume of hydrogen, having a huge mass, in fact will have a low gravitational force because it is spread out (inverse square law). For any given density of material there is some ultimate size beyond which a clump of matter will necessarily and inevitably contract into ever tighter clumps until stars are formed. There are some really nice simulations online at a cosmological scale and star cluster scales too. Sorry, but I do not think this is a no-brainer. It only requires that there is enough attractive force from the clump of material to ensure that parts of it slowly become ever denser. Yes but I agree with the previous poster that it is not a no brainer. As the collapse procedes, the temperature rises and then the question is if the increase in temperature gives a significant portion of the molecules enough speed to exceed the escape velocity. The increase in temperature acts against further collapse but of course heat is also being radiated away. The radiation details depend on the composition of the collapsing nebula. It is a genuine complex problem, worthy of computer simulation which you point out below. Klazmon. There was a very nice illustrated talk about this at Astrofest by Matthew Bates of Exeter, simulations in video for braodband only at: http://www.astro.ex.ac.uk/people/mbate/ Regards, Martin Brown |
#10
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Star formation
"Mike Williams" wrote in message ... Wasn't it John Brockbank who wrote: "Sjouke Burry" wrote in message l... John Brockbank wrote: Sorry if this is an FAQ, but it5 has long made me wonder and I have never seen an explanation. The Earth massive as it is, is too slight to have hydrogen in the atmosphere. I think that is because the speed of the molecules exceeds, just about, the escape velocity. Given that, how did stars form from hydrogen? Surface gravity at the sun 27 G?(260 /s/s) And that is for a yellow dwarf. Even Jupiter/Saturn/neptune hold onto their hydrogen pretty well. Of course once a star has formed it has a strong gravity field. However, presumably a vary large volume of hydrogen, having a huge mass, in fact will have a low gravitational force because it is spread out (inverse square law). Sorry, but I do not think this is a no-brainer. Remember that it will also be very cold. There's no heat source until the star starts to form. So the hydrogen will be moving very much more slowly. Ah, yes, temperature is a big factor with gases. I wonder whether there is some sort of snowflake effect, like water droplets in a cloud. I will certainly look at the simulations, and do some more looking. Escape velocity is proportional to sqrt(mass/distance). We only have to consider molecules escaping from the outer fringes, and don't need to worry about the gravity in the interior of the condensing cloud. The typical speed of molecules in the gas is proportional to the square root of the absolute temperature. Presumably there's certain critical mass/distance/temperature ratio. If external forces cause a cloud to cross this boundary then it starts to contract under its own gravity. If a cloud stays outside the boundary, then it continues to be a cloud or may even disperse. -- Mike Williams Gentleman of Leisure |
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