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The size (and mass of stars)
Dave Merrall wrote:
As far as I understand things: Stars accrete their mass and once they become dense enough, reactions take place within their core. Once this happens there is a balancing act between the pressure they excert outwards, and the gravitational effect of their mass. Acretion then stops and the starts go on to burn for eons. So: why are stars all of different sizes and mass? Pockets of condensing matter can range in accumulated mass once instabilities in the nebula from which they are formed allow them to contract. At the upper end, there may be a limit on the order of 100 times the mass of the Sun because of radiation pressure building up within the condensing cloud, forcing the material away from condensing. At the low mass range, insufficient mass would have the effect of not sustaining thermonuclear fusion. But, since there is no restriction on the size of the clouds of condensing gas and dust that will lead to the formation of stars in general (other than the limits mentioned above), then there can be a range of collapsing stars with different masses all condensing at different rates based on self gravity. |
#2
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In article , Llanzlan Klazmon The 15th wrote:
"Dave Merrall" wrote in : As far as I understand things: Stars accrete their mass and once they become dense enough, reactions take place within their core. Once this happens there is a balancing act between the pressure they excert outwards, and the gravitational effect of their mass. Acretion then stops and the starts go on to burn for eons. So: why are stars all of different sizes and mass? I would have guessed that, going by the priniple of how they come to be, there would be little or no variation in size since they would all begin nuclear reactions at some critical point directly related to how much mass they contain. [...] This is a good question, and it's actually a very active area in astrophysics research right now. Although we have a good idea of how stars form in general, the details are still missing, and quite how stars of various masses form is an open one. What does seem evident is that 1) the more turbulent or disturbed (e.g., by shocks) a region of star formation is, the more likely it is massive stars will form, and 2) star forming regions with higher accretion rates, which is driven by the "pressure" within the molecular cloud, are likely to also form higher mass stars. Ignoring point one, it's probably a high accretion rate that helps form more massive stars than the sun for instance. In other words, the stuff falling onto the young star exceeds the amount that is blown off by the nuclear reactions in the core, so the young star gains mass. It depends on the local density of the forming nebula. It is reasonably well modelled for stars up to ten solar masses but gets tricky after that as the radiation pressure should quickly build up to blow away any remaining nebula. Maybe in dense clusters, multiple proto stars merge to form giant stars? I agree that the local density is likely to determine the outcome of stellar masses, but it's not as simple as that. The Orion nebula, for instance, is a region of massive star formation and is also more dense than the Taurus region, which is almost exclusively a low-mass star forming region. However, quite how or why turbulence plays a role is unclear, but it clearly does. FYI, I think some recent research has indicated that the merging of proto-stars is not significant in forming massive stars, and that it's the accretion rate that's the main factor. I'll try and dig out the reference if you're interested. |
#3
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"Dave Merrall" wrote in message ...
As far as I understand things: Stars accrete their mass and once they become dense enough, reactions take place within their core. Once this happens there is a balancing act between the pressure they excert outwards, and the gravitational effect of their mass. Acretion then stops and the starts go on to burn for eons. So: why are stars all of different sizes and mass? I would have guessed that, going by the priniple of how they come to be, there would be little or no variation in size since they would all begin nuclear reactions at some critical point directly related to how much mass they contain. I am obviously missing something here. Can anyone put me right? I think the basic answer is: stars don't stop having gravity when they start nuclear fusion. The outward pressure of the fusion stops more matter from falling *into the core*, and remain in the middle and outer convective layers in the body of the star. But more matter can still join the body of the star beyond the minimum required to start fusion in the core. Also, there's a time delay involved. I suspect there's a point at which nearby matter is blown away by the solar wind, sometime after the star forms, making it more difficult for additional nebular gas to join the star -- but there's a period of time during which the star is, well, "gearing up" for lack of a better term, where more mass can continue to fall into the star even though there's sufficient matter to (a) start fusion and (b) stop internal collapse. It may depend a lot on the shape, lumpiness, density, relative motion, etc. of the cloud from which the star formed. Of course, not being a professional astronomer, I could be entirely wrong. eyelessgame |
#4
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Hi Dave Mass density of stars is proportional to mass density of
nebular. In the early universe( a few billion years after the BB the universe was much smaller,and that means more condensed. The nebular created supernova stars(lots of mass density. When these stars exploded in a short period of time they sent shock waves through space that helped nebular to create stars faster and seed the universe with heavy atoms so I could type this to you. The last 5 billion years were medium star structures(our sun) and they are long lasting because they burn slow. Gravity gave them a density that gave them time to go from bacteria life form to have a brain and a finger to post this to you Dave I'm glad that we share this spacetime together I think of it as a very personal link that goes at light speed,and that is fast enough for me Bert. |
#5
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In article ,
G=EMC^2 Glazier wrote: Hi Dave Mass density of stars is proportional to mass density of nebular. In the early universe( a few billion years after the BB the universe was much smaller,and that means more condensed. The nebular created supernova stars(lots of mass density. When these stars exploded in a short period of time they sent shock waves through space that helped nebular to create stars faster and seed the universe with heavy atoms so I could type this to you. The last 5 billion years were medium star structures(our sun) and they are long lasting because they burn slow. Gravity gave them a density that gave them time to go from bacteria life form to have a brain and a finger to post this to you Dave I'm glad that we share this spacetime together I think of it as a very personal link that goes at light speed,and that is fast enough for me Bert. And after our naps boys and girls we'll all have some milk and cookies.Bill. |
#6
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Hi Bill Can I have fruit cake instead of cookies? I'm sure you would
not mind(right?) Bert |
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