Transcript Initial Evolution-The Main Sequence

Stellar Evolution
It’s all about Gravity
The evolution of a star governed by a balance between the effects of
nuclear reactions, pressure, quantum mechanics, and GRAVITY.
Stellar Evolution
It’s all about Gravity
Two scenarios: stars with as mass less than 8 times the mass of the
sun and stars with a mass greater than 8 times the mass of the sun.
Stellar Evolution
It’s all about Gravity
In General: stars live their lives producing energy through the nuclear
fusion of elements. While on the main sequence, hydrogen is fused to
produce helium.
Once the hydrogen fuel depletes, a series of events occur which
involves the fusion of heavier elements, changes in luminosity,
contractions and expansions, and ultimately the final resting place of
the star that began its life on the main sequence.
On the main sequence – stars are fusing hydrogen, and
are in hydrostatic equilibrium.
The amount of time spent on the main sequence is
determined by the mass of the star.
Stellar Evolution
More Nuclear Physics
Why do stars eventually burn out and die?
* High pressures and temperatures in the core break atoms into a soup
of protons (hydrogen nuclei) and electrons.
* Electrons do not feel a strong force, and therefore cannot fuse.
* Protons do feel the strong force and can fuse to heavier elements.
Nuclei that are involved in the fusion process are
called parent nuclei
Nuclei that are the result of a fusion process are
called daughter nuclei
Stellar Evolution
More Nuclear Physics – The Nuclear Binding Curve
Can elements fuse indefinitely? No.
* When lighter elements fuse, they form an element which is “more
stable” than the parent elements, and in the process release energy.
* The MOST STABLE of all elements is iron.
* Beyond iron, elements no longer undergo fusion. It is more
energetically favorable for heavier elements to split and release energy,
a process called fission.
Stellar Evolution
More Nuclear Physics – The Nuclear Binding Curve
Stellar Evolution
More Nuclear Physics
Notice, however, that when particular parent nuclei fuse, the daughter
nucleus contains more protons that the parent.
* Because of electrical repulsion, heavier nuclei are harder to fuse.
* Once the hydrogen is fused, hydrostatic equilibrium can no longer be
maintained
* The core will collapse (gravity cannot be turned off) until temperatures
and pressures become high enough to fuse heavier elements.
* The dance between collapse and the fusion of heavier nuclei is the
hallmark of the evolution of a star once it leaves the main sequence.
Stellar Evolution
More Nuclear Physics
How it gets to the end point is
where the action is….