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    Stellar Nucleosynthesis: Formation of Heavy Elements During Stellar Evolution

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    Marina Aban
    Stellar nucleosynthesis involves nuclear fusion reactions inside stars that build heavier elements from lighter ones. Low-mass stars end as white dwarfs, while high-mass stars end as neutron stars or black holes after supernovae. Nuclear fusion sequentially converts hydrogen to helium, then helium to carbon and heavier elements up to iron. For massive stars, further fusion of heavier elements like carbon, neon and silicon occurs. However, iron cannot release energy through fusion, causing gravitational collapse and supernovae that produce still heavier elements through neutron capture processes.

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    Stellar Nucleosynthesis: Formation of Heavy Elements During Stellar Evolution

    Uploaded by

    Marina Aban
    230 views11 pages
    Stellar nucleosynthesis involves nuclear fusion reactions inside stars that build heavier elements from lighter ones. Low-mass stars end as white dwarfs, while high-mass stars end as neutron stars or black holes after supernovae. Nuclear fusion sequentially converts hydrogen to helium, then helium to carbon and heavier elements up to iron. For massive stars, further fusion of heavier elements like carbon, neon and silicon occurs. However, iron cannot release energy through fusion, causing gravitational collapse and supernovae that produce still heavier elements through neutron capture processes.

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    SN

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    Stellar nucleosynthesis involves nuclear fusion reactions inside stars that build heavier elements from lighter ones. Low-mass stars end as white dwarfs, while high-mass stars end as neutron stars or black holes after supernovae. Nuclear fusion sequentially converts hydrogen to helium, then helium to carbon and heavier elements up to iron. For massive stars, further fusion of heavier elements like carbon, neon and silicon occurs. However, iron cannot release energy through fusion, causing gravitational collapse and supernovae that produce still heavier elements through neutron capture processes.

    Copyright:

    © All Rights Reserved
    Download as PPTX, PDF, TXT or read online from Scribd
    Download as pptx, pdf, or txt
    230 views11 pages

    Stellar Nucleosynthesis: Formation of Heavy Elements During Stellar Evolution

    Uploaded by

    Marina Aban
    Stellar nucleosynthesis involves nuclear fusion reactions inside stars that build heavier elements from lighter ones. Low-mass stars end as white dwarfs, while high-mass stars end as neutron stars or black holes after supernovae. Nuclear fusion sequentially converts hydrogen to helium, then helium to carbon and heavier elements up to iron. For massive stars, further fusion of heavier elements like carbon, neon and silicon occurs. However, iron cannot release energy through fusion, causing gravitational collapse and supernovae that produce still heavier elements through neutron capture processes.

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    Stellar Nucleosynthesis: Formation of heavy

    Elements during Stellar Evolution


    What eventually happens to stars depend on their masses.
    - A low - mass star ends up as white dwarf
    - A high- mass star ends up either as a neutron star { after a supernova
    explosion} or a black hole

    - A star is formed when a cloud of gas and dust collapse to the point
    where the material in the center of the clump is so dense and hot that
    nuclear fusion of hydrogen nuclei and helium nuclei occurs.

    - The outflow of energy released by these reactions provides the


    pressure necessary to halt the collapse.The pressure and gravity are in
    hydrostatic equilibrium.
    Stellar Nucleosynthesis
    -is the collective term for the nuclear reactions taking place in stars to
    build the nuclei of the heavier elements.

    The important nuclear fusion processes in stellar nucleosynthesis are:


    1.Hydrogen burning {proton-proton chain,carbon cycle}
    2.Helium burning { triple –alpha process}
    3.Burning of heavier elements {lithium-burning,carbon-burning,neon-
    burning,oxygen –burning,silicon-burning}
    Hydrogen-burning
    This first stage begins with the proton-proton chain where hydrogen is converted
    to helium.This is the dominant process in stars that have a similar mass as the
    sun.
    The second stage involves the carbon cycle,the cycle of reactions results to the
    formation of helium nucleus.This is dominant process in stars more massive
    {1.3x} than the sun.
    Helium burning
    The second stage begins after most of the hydrogen is burned.Helium burning is
    done either through the alpha process or the triple-alpha process.
    In the triple-alpha process three helium-4 nuclei or alpha particles, fuse to form
    a carbon nucleus.
    As soon as the carbon is present,the alpha process begins where the formation
    of neon,oxygen and silicon takes place.
    Burning of heavier elements
    If the star is massive {>8x solar masses},another set of the nuclear fussion
    reactions begins.
    These includes the burning of carbon,neon,oxygen and silicon,which lead
    to the formation of heavier elements until finally produces iron.

    There are several nucleur synthesis routes and various nuclei are formed
    as by-products.Stellar nucleosynthesis is responsible for the production of
    heavy elements up to iron.
    Stars evolve so that they have “onion skin-like”shells of thermonuclear
    combustion with differing nuclear chemistry.
    After the stars consumes its “fuel” for a certain stage,it consumes its next
    fuel to continue its burning process and to sustain its life.
    As a final note,astronomical periodic table is the origin of the elements.

    Remember:
    SN is the collective term for the nuclear reactions which take place in
    stars to build the nuclei of the heavier elements.
    The nuclear fusion processes in SN are:
    Hydrogen burning
    Helium burning
    Burning of heavier elements if the star is a high-mass star
    Describing how elements heavier than iron are formed
    Recall:Stellar Evolution
    The fate of stars depends upon their masses- low-mass stars end up as
    a white dwarf while high mass stars end up either as a neutron star
    {after a supernova explosion} or a black hole.
    Recall: SN
    New heavier elements are created from the nuclear fusion of lighter
    elements.
    In the case of the lifetime of stars, the lighter elements proceed to
    fusion until those elements- the star’s “ fuel” – have been exhausted
    The formation of an iron core signals the end of a massive star’s life.
    • Iron cannot burn and release energy since it is the most tightly bound
    of all nuclei.As a result,attempting to fuse additional protons to it ,
    weakens the bond and absorb energy rather than realeasing
    it,Nuclear fusion stops with iron,and a star with an iron core is out of
    fuel.
    • Iron is at the peak of the binding energy curve which indicates that
    further fusion process {the process of creating elements}would
    decrease the binding energy splitting the atom.
    • CORE COLLAPSE OF MASSIVE STARS
    • With the stars core pressure suddenly dropping,nothing remains to
    support the stars and so its interior begins to collapse.
    • The force of gravity,meanwhile, is emmense that it crushes the core.
    • The star’s outer layers also contribute to the core’s immense
    crushing.as an effect, the infalling gas is heated to billions of degrees.
    • With the pressure surging,it lifts the outer layers away from the star
    through a titanic explosion- a supernova.
    • A supernova is the explosion of a star,it is the largest explosion that
    takes place in space.It happens where there is a change in the core,or
    center,of a star
    • Elements throughout the iron are created throughout the stellar
    evolution.
    • This synthesis of new elements are achieved through neutron
    capture either through the s-process or r-process.

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