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Stellar Structure: TCD 2006: 2.1 2 equations of stellar structure.

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Presentation on theme: "Stellar Structure: TCD 2006: 2.1 2 equations of stellar structure."— Presentation transcript:

1 Stellar Structure: TCD 2006: equations of stellar structure

2 Stellar Structure: TCD 2006: 2.2 a stellar interior

3 Stellar Structure: TCD 2006: 2.3 assumptions Isolated body – only forces are self-gravity internal pressure Spherical symmetry Neglect: rotation magnetic fields Consider spherical system of mass M and radius R Internal structure described by: rradius m(r)mass within r l(r)flux through r T(r)temperature at r P(r)pressure at r [  (r)density at r ] m,l,P,T r M,L,0,T eff X,Y,Z 0,0,P c,T c Centre: r=0 Surface: r=R

4 Stellar Structure: TCD 2006: 2.4 mass continuity Consider a spherical shell of radius r thickness  r (  r <

5 Stellar Structure: TCD 2006: 2.5 hydrostatic equilibrium Consider forces at any point. A sphere of radius r acts as a gravitational mass situated at the centre, giving rise to a force: g = Gm/r 2 If a pressure gradient (dP/dr) exists, there will be a nett inward force acting on an element of thickness  r and area  A: dP/dr  r  A   m /  dP/dr (element mass is  m =   r  A) The sum of inward forces is then  m ( g +1/  dP/dr ) = -  m d 2 r/dt 2 r  rr z P+  P P g In order to oppose gravity, pressure must increase towards the centre. For hydrostatice equilibrium, forces must balance: dP/dr = - Gm  / r 2 2.5

6 Stellar Structure: TCD 2006: 2.6 Virial theorem

7 Stellar Structure: TCD 2006: 2.7 Virial theorem (2)

8 Stellar Structure: TCD 2006: 2.8 Virial theorem: non-relativistic gas In a star, an equation of state relates the gas pressure to the translational kinetic energy of the gas particles. For non- relativistic particles: P = nkT = kT/V and E kin = 3/2 kT and hence P = 2/3 E kin /V2.7 Applying the Viral theorem: for a self-gravitating system of volume V and gravitational energy E grav, the gravitational and kinetic energies are related by 2E kin + E grav = 02.8 Then the total energy of the system, E tot = E kin + E grav = –E kin = 1/2 E grav 2.9 These equations are fundamental. If a system is in h-s equilibrium and tightly bound, the gas is HOT. If the system evolves slowly, close to h-s equilibrium, changes in E kin and E grav are simply related to changes in E tot.

9 Stellar Structure: TCD 2006: 2.9 Virial theorem: ultra-relativistic gas For ultra-relativistic particles: E kin = 3 kT and hence P = 1/3 E kin /V2.10 Applying the Viral theorem: E kin + E grav = Thus h-s equilibrium is only possible if E tot = 0. As the u-r limit is approached, ie the gas temperature increases, the binding energy decreases and the system is easily disrupted. Occurs in supermassive stars (photons provide pressure) or in massive white dwarfs (rel. electrons provide pressure).

10 Stellar Structure: TCD 2006: 2.10 conservation of energy Consider a spherical volume element dv=4  r 2 dr Conservation of energy demands that energy out must equal energy in + energy produced or lost within the element If  is the energy produced per unit mass, then l+  l = l +   m  dl/dm =  Since dm = 4  r 2  dr, dl/dr = 4  r 2   2.12 We will consider the nature of energy sources, , later. r  m rr l+  l l

11 Stellar Structure: TCD 2006: 2.11 radiative energy transport A temperature difference between the centre and surface of a star implies there must be a temperature gradient, and hence a flux of energy. If transported by radiation, then this flux obeys Flick’s law of diffusion: F = -D d(aT 4 )/dr where aT 4 is the radiation energy density and D is a diffusion coefficient. We state (for now) that D is related to the “opacity”  (actually: D = c/  ) The flux must be multiplied by 4  r 2 to obtain a luminosity l, whence L = - (4  r 2 c / 3  ) d(aT 4 )/dr  dT/dr = 3  /4acT 3 l/4  r a

12 Stellar Structure: TCD 2006: 2.12 radiative energy transport (2)

13 Stellar Structure: TCD 2006: 2.13 convective energy transport

14 Stellar Structure: TCD 2006: 2.14 convective energy transport (2)

15 Stellar Structure: TCD 2006: 2.15 convective energy transport (3)

16 Stellar Structure: TCD 2006: 2.16 energy transport

17 Stellar Structure: TCD 2006: 2.17 equations of stellar structure

18 Stellar Structure: TCD 2006: 2.18 ode’s: Lagrangian form

19 Stellar Structure: TCD 2006: 2.19 boundary conditions

20 Stellar Structure: TCD 2006: 2.20 constitutive relations

21 Stellar Structure: TCD 2006: equations of stellar structure -- review Assumptions: spherical symmetry, … Mass continuity Hydrostatic equilibrium Conservation of energy Radiative energy transport Convective energy transport The Virial theorem Eulerian and Lagrangian forms Boundary Conditions Constitutive equations


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