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I Why there are different size stars?

  1. Apr 13, 2016 #1
    I just read something about the theory of stars formation that left me puzzled. I am easily puzzled so please bear with me.
    The theory goes somewhat like this:
    There is s nebula (gas) that is contracting and increasing mass and pressure at its center until they are so big that a nuclear reaction starts. Then the extra gas is blown away.
    OK, fine.
    Problem for me is that gigantic stars many, many times bigger than our sun exist. Why?
    Did they not go through the accumulation of mass that started our sun? So why they did not stop there and instead continued growing?
    Please, unpuzzle me.
    Thank you.
  2. jcsd
  3. Apr 13, 2016 #2

    Vanadium 50

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    Gas clouds are not all the same size.
  4. Apr 14, 2016 #3
    Yes, Vanadium 50, I know but the point is, once the nuclear explosion happens the rest of the cloud is blown away stopping the growth.
    Why then the gas is not blown away?
  5. Apr 14, 2016 #4
    Maybe it has something to do with the amount of cloud particles that are involved in the beginning process of formation. Say, if the nebula were to be very large, it would have a very large density. Gravity would also be much more, overwhelming the supporting gas pressure, in turn pulling towards the core a lot more particles compared to a smaller nebula. This would then form a larger star compared to a star formed by a smaller nebula. The then resulting nuclear explosion blows off all the gas that has not been necessary in the formation of the star.

    Hope this helps!
  6. Apr 14, 2016 #5
    Thanks GloomyGerman, that gave me something to ask.
    If the nebula is very large that does not implies that the density will be large.
    Now, if for some reason the WHOLE nebula starts contracting at once I see your explanation valid.
    On the other hand, if the condensation starts at the center and is attracting particles to it, then there will happen a mass large enough to start the explosion that blows away the rest of the gas.
    What can cause a whole humongous nebula to contract at once?
  7. Apr 14, 2016 #6
    British physicist Sir James Jeans determined that when interstellar gas cloud exceeded a certain mass, radius, and temperature it would become unstable. If the gas cloud reached a critical mass as a result of pressure (mass and radius) and temperature, it would begin to collapse. He devised a formula for determining when that instability in the gas cloud would occur. It is called Jeans Instability.

    Interstellar gas clouds also do not have a perfectly distributed mass, there are voids and clumps which causes the gas to collapse differently. As a result, more than one protostar could form from a single gas cloud, and the protostar(s) may vary significantly in size. The size of the protostar depends on the density of the gas cloud, and the angular momentum of the collapsing gas. As the gas collapses it begins to spin, the more it rotates the bigger the protostar will be. Furthermore, the more gas involved, the hotter the protostar will be. At first it will begin to fuse deuterium, then lithium, and finally when it reaches a core temperature of 2,500,000°K it will begin fusing hydrogen. It is at that temperature a star is born.

    Normally there is a maximum luminosity a star can achieve, when there is a balance of radiation acting outward counteracting the gravitational pull inward. This is known as the Eddington Luminosity, or Eddington Limit. However, it has been suggested that super-massive black holes could form by exceeding the Eddington Luminosity.

    Did supermassive black holes form by direct collapse? - American Institute of Physics Conference 990, 489 (2008); DOI: 10.1063-1.2905669 (arXiv free reprint)
    Last edited: Apr 14, 2016
  8. Apr 14, 2016 #7
    Now Glitch I almost got it! Good explanation! I think I got the info I needed. Thank you.
    Wondercosmos out.
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