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The size (and mass of stars)



 
 
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  #1  
Old August 11th 03, 05:13 AM
J. Scott Miller
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Default 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  
Old August 11th 03, 08:30 AM
Tom Kerr
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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  
Old August 11th 03, 09:33 PM
eyelessgame
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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  
Old August 11th 03, 10:39 PM
G=EMC^2 Glazier
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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  
Old August 11th 03, 11:55 PM
Bill Duncan
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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  
Old August 12th 03, 07:08 PM
G=EMC^2 Glazier
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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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