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=The Stability of Uniform-Density Spheres=
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<font size="-1">[[H_BookTiledMenu#Stability_Analysis|<b>Sterne's<br />Analytic Sol'n<br />of Eigenvalue<br />Problem</b>]]<br />(1937)</font>
|}
As far as we have been able to determine, {{ Sterne37full }} was the first to use linearized perturbation techniques and, specifically, the [[SSC/Perturbations#2ndOrderODE|''Adiabatic Wave Equation'']], to thoroughly analyze the stability of uniform-density, self-gravitating spheres. While uniform-density configurations present an overly simplified description of real stars, the stability analysis presented by {{ Sterne37 }} &#8212; hereafter, {{ Sterne37hereafter }} &#8212; is an important one because it presents a complete spectrum of radial pulsation eigenvectors &#8212; eigenfrequencies ''plus'' the corresponding eigenfunctions &#8212; as closed-form analytic expressions.  Such analytic solutions are quite rare in the context of studies of the structure, stability, and dynamics of self-gravitating fluids.

As has been explained in an [[SSC/Perturbations#The_Eigenvalue_Problem|accompanying introductory discussion]], this type of stability analysis requires the solution of an eigenvalue problem.  Here we begin by re-presenting the governing 2<sup>nd</sup>-order ODE (the [[SSC/Perturbations#2ndOrderODE|''Adiabatic Wave Equation'']]) as it was derived in the accompanying introductory discussion, along with the specification of two customarily used boundary conditions; and we review the properties of the equilibrium configuration &#8212; also derived in a [[SSC/Structure/UniformDensity#Isolated_Uniform-Density_Sphere|separate discussion]] &#8212; that are relevant to this stability analysis.  Interleaved with this presentation, we also show the governing wave equation as it was derived by {{ Sterne37hereafter }} &#8212; and a table that translates from Sterne's notation to ours &#8212; along with his corresponding review of the properties of the unperturbed equilibrium configuration.  Finally, we present the solution to this eigenvalue problem that was derived by {{ Sterne37hereafter }} and discuss the properties of his derived radial pulsation eigenvectors.

==The Eigenvalue Problem==
===Our Approach===
As has been derived in [[SSC/Perturbations#Eigen_Value_Problem|an accompanying discussion]], the second-order ODE that defines the relevant Eigenvalue problem is,

<div align="center">
<math>
\frac{d^2x}{d\chi_0^2} + \biggl[\frac{4}{\chi_0} - \biggl(\frac{\rho_0}{\rho_c}\biggr) \biggl(\frac{P_0}{P_c}\biggr)^{-1} \biggl(\frac{g_0}{g_\mathrm{SSC}}\biggr) \biggr] \frac{dx}{d\chi_0} + \biggl(\frac{\rho_0}{\rho_c}\biggr) \biggl(\frac{P_0}{P_c}\biggr)^{-1} \biggl(\frac{1}{\gamma_\mathrm{g}} \biggr)\biggl[\tau_\mathrm{SSC}^2 \omega^2 + (4 - 3\gamma_\mathrm{g})\biggl(\frac{g_0}{g_\mathrm{SSC}}\biggr) \frac{1}{\chi_0} \biggr]  x = 0 .
</math><br />
</div>
where the dimensionless radius,
<div align="center">
<math> 
\chi_0 \equiv \frac{r_0}{R} ,
</math><br />
</div>
the characteristic time for dynamical oscillations in spherically symmetric configurations (SSC) is,
<div align="center">
<math> 
\tau_\mathrm{SSC} \equiv \biggl[ \frac{R^2 \rho_c}{P_c} \biggr]^{1/2} ,
</math><br />
</div>
and the characteristic gravitational acceleration is,
<div align="center">
<math> 
g_\mathrm{SSC} \equiv \frac{P_c}{R \rho_c} .
</math><br />
</div>

The two [[SSC/Perturbations#Boundary_Conditions|boundary conditions]] are,
<div align="center">
<math>~\frac{dx}{d\chi_0} = 0</math>&nbsp; &nbsp; &nbsp; &nbsp; at &nbsp; &nbsp; &nbsp; &nbsp; <math>~\chi_0 = 0 \, ;</math>
</div>

and,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>~ \frac{d\ln x}{d\chi_0}</math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~\frac{1}{\gamma_g} \biggl( 4 - 3\gamma_g + \frac{\omega^2 R^3}{GM_\mathrm{tot}}\biggr) </math>&nbsp; &nbsp; &nbsp; &nbsp; at &nbsp; &nbsp; &nbsp; &nbsp; <math>~\chi_0 = 1 \, .</math>
  </td>
</tr>
</table>
</div>

===The Approach Taken by Sterne (1937)===
{{ Sterne37 }} begins his analysis by deriving the

<div align="center" id="2ndOrderODE">
<font color="#770000">'''Adiabatic Wave''' (or ''Radial Pulsation'') '''Equation'''</font><br />

{{Math/EQ_RadialPulsation01}}
</div>
in a manner explicitly designed to reproduce [[SSC/Perturbations#Eddington_.281926.29|Eddington's ''pulsation equation'']] &#8212; it appears as equation (1.8) in {{ Sterne37hereafter }} &#8212; and, along with it, the [[SSC/Perturbations#Ensure_Finite-Amplitude_Fluctuations|surface boundary condition]],

<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right" >
<math>r_0 \frac{d\ln x}{dr_0}</math>
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>\frac{1}{\gamma_g} \biggl( 4 - 3\gamma_g + \frac{\omega^2 R^3}{GM_\mathrm{tot}}\biggr) </math>&nbsp; &nbsp; &nbsp; &nbsp; at &nbsp; &nbsp; &nbsp; &nbsp; <math>~r_0 = R \, ,</math>
  </td>
</tr>
</table>
</div>
which appears in {{ Sterne37hereafter }} as equation (1.9).  Then, as shown in the following paragraph extracted directly from his paper, {{ Sterne37hereafter }} rewrites both of these expressions in, what he considers to be, "more convenient forms."


<div align="center">
<table border="2" cellpadding="10" width="80%">
<tr>
  <td align="center" colspan="1">
Reprint of the last paragraph of &sect;1 (p. 585) from &hellip;<br />
{{ Sterne37figure }}
  </td>
<tr>
  <td colspan="1">
<!-- [[File:Sterne1937B.png|600px|center|Sterne (1937)]] -->
<font color="darkgreen">
"Before proceeding further we write equations (1.8) and (1.9) in more convenient forms.  Let <math>\xi_0 = Rx</math>.  Then (1.8) becomes
<table border="0" cellpadding="5" align="center" width="100%">

<tr>
  <td align="center" >
<math>\xi_1^{''} + \frac{4-\mu}{x} \xi_1^' 
+ \frac{R\rho_0}{P_0}\biggl( \frac{n^2 R}{\gamma} - \frac{\alpha g_0}{x} \biggr)\xi_1 = 0 \, ;</math>
  </td>
  <td align="right" width="10%">
(1.91)
  </td>
</tr>
</table>
and the boundary condition (1.9) becomes, from (1.5),
<table border="0" cellpadding="5" align="center" width="100%">

<tr>
  <td align="center" >
<math>\biggl( \frac{n^2 R}{\gamma g_0} - \alpha \biggr) \xi_1 = \xi_1^'</math>
  </td>
  <td align="right" width="10%">
(1.92)
  </td>
</tr>
</table>
at <math>x = 1</math>.  Here dashes denote differentiation with respect to <math>x</math>, a convention henceforth to be followed."
</font>
  </td>
</tr>
<tr><td align="center">
 
  <table border="0" width="75%" cellpadding="4">
  <tr>
  <td align="center" colspan="3">
'''Notation:'''
  </td>
  </tr>
  <tr>
<th align="center" width="40%">Sterne's<p></p>
----</th>
<td width="20%">&nbsp;</td>
<th align="center">Ours<p></p>
----</th>
  </tr>
  <tr>
<td align="center" width="40%"><math>~\xi_0 = Rx</math></td>
<td>&nbsp;</td>
<td align="center"><math>~r_0</math></td>
  </tr>
  <tr>
<td align="center" width="40%"><math>~\xi_1</math></td>
<td>&nbsp;</td>
<td align="center"><math>~x</math></td>
  </tr>
  <tr>
<td align="center" width="40%"><math>~n^2</math></td>
<td>&nbsp;</td>
<td align="center"><math>~\omega^2</math></td>
  </tr>
  <tr>
<td align="center" width="40%"><math>~\alpha</math></td>
<td>&nbsp;</td>
<td align="center"><math>~3-4/\gamma_g</math></td>
  </tr>
  <tr>
<td align="center" width="40%"><math>~\mu</math></td>
<td>&nbsp;</td>
<td align="center"><math>~g_0 \rho_0 r_0/P_0</math></td>
  </tr>
  <tr>
<td align="center" width="40%"><math>~g_0 R^2</math></td>
<td>&nbsp;</td>
<td align="center"><math>~GM_\mathrm{tot}</math></td>
  </tr>
  </table>

</td></tr>
</table>
</div>

==Properties of the Equilibrium Configuration==

=== Our Setup===
From our derived [[SSC/Structure/Polytropes#n_.3D_0_Polytrope|structure of a uniform-density sphere]], in terms of the configuration's radius <math>R</math> and mass <math>M</math>, the central pressure and density are, respectively,
<div align="center">
<math>P_c = \frac{3G}{8\pi}\biggl( \frac{M^2}{R^4} \biggr) </math> , 
</div>
and
<div align="center">
<math>\rho_c = \frac{3M}{4\pi R^3} </math> .
</div>
Hence the characteristic time and acceleration are, respectively,
<div align="center">
<math> 
\tau_\mathrm{SSC} = \biggl[ \frac{R^2 \rho_c}{P_c} \biggr]^{1/2} = 
\biggl[ \frac{2R^3 }{GM} \biggr]^{1/2} = 
\biggl[ \frac{3}{2\pi G\rho_c} \biggr]^{1/2},
</math><br />
</div>
and,
<div align="center">
<math> 
g_\mathrm{SSC} = \frac{P_c}{R \rho_c} = \biggl( \frac{GM}{2R^2} \biggr) .
</math><br />
</div>

The required functions are,
* <font color="red">Density</font>: 
<div align="center">
<math>\frac{\rho_0(r_0)}{\rho_c} = 1 </math> ;
</div>

* <font color="red">Pressure</font>: 
<div align="center">
<math>\frac{P_0(r_0)}{P_c} = 1 - \chi_0^2 </math> ;
</div>

* <font color="red">Gravitational acceleration</font>: 
<div align="center">
<math> 
\frac{g_0(r_0)}{g_\mathrm{SSC}} = 2\chi_0  .
</math><br />
</div>

So our desired eigenfunctions and eigenvalues will be solutions to the following ODE:
<div align="center">
<math>
\frac{1}{(1 - \chi_0^2)}  \biggl\{ (1 - \chi_0^2) \frac{d^2x}{d\chi_0^2} + \frac{4}{\chi_0}\biggl[1 -  \frac{3}{2}\chi_0^2 \biggr] \frac{dx}{d\chi_0} +  \frac{1}{\gamma_\mathrm{g}} \biggl[\tau_\mathrm{SSC}^2 \omega^2 + 2 (4 - 3\gamma_\mathrm{g}) \biggr]  x \biggr\} = 0 .
</math><br />
</div>

===Setup as Presented by Sterne (1937)===

In &sect;2 of his paper, {{ Sterne37 }} details the structural properties of an equilibrium, uniform-density sphere as follows.  (Text taken verbatim from {{ Sterne37hereafter }} are presented here in green.)  Given that <font color="green">the undisturbed density is constant and equal to the mean density</font>, <math>~\bar\rho</math>, the <font color="green">mass within any radius is</font>,
<div align="center">
<math>M_r = \biggl( \frac{4\pi}{3} \biggr) \bar\rho \xi_0^3 \, ;</math>
</div>
<font color="green">the undisturbed values of gravity and the pressure are</font>, respectively,
<div align="center">
<math>g_0 \equiv \frac{GM_r}{\xi_0^2} = \biggl( \frac{4\pi}{3} \biggr) G\bar\rho R x \, </math>
</div>
and 
<div align="center">
<math>P_0 = \biggl( \frac{2\pi}{3} \biggr) G R^2 \bar\rho^2(1 - x^2) \, ;</math>
</div>
and the quantity,
<div align="center">
<math>\mu \equiv \frac{g_0 \bar\rho \xi_0}{P_0} = \frac{2x^2}{(1-x^2)} \, .</math>
</div>

Hence, for this particular equilibrium model, the wave equation derived by {{ Sterne37hereafter }} &#8212; his equation (1.91), as displayed above &#8212; becomes,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>~0</math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~\xi_1^{ ' ' } + \biggl[\frac{4-\mu}{x} \biggr]\xi_1^' + \frac{R\bar\rho}{P_0} \biggl( \frac{n^2 R}{\gamma} - \frac{\alpha g_0}{x} \biggr) \xi_1</math>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~\xi_1^{ ' ' } + \frac{1}{x}\biggl[4 -\frac{2x^2}{(1-x^2)} \biggr]\xi_1^' + 
\frac{3}{2\pi G R \bar\rho (1 - x^2)}\biggl[ \frac{n^2 R}{\gamma} - \biggl( \frac{4\pi}{3} \biggr) \alpha G\bar\rho R \biggr] \xi_1</math>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~(1-x^2) \xi_1^{ ' ' } + \frac{1}{x}\biggl[4(1-x^2) - 2x^2 \biggr]\xi_1^' + \biggl[ \frac{3n^2 }{2\pi \gamma  G \bar\rho} - 2 \alpha \biggr] \xi_1</math>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~(1-x^2) \xi_1^{ ' ' } + \frac{1}{x}\biggl[4 - 6x^2 \biggr]\xi_1^' + \mathfrak{F} \xi_1 \, ,</math>
  </td>
</tr>
</table>
</div>
where,
<div align="center">
<math>~\mathfrak{F}  \equiv \frac{3n^2 }{2\pi \gamma  G \bar\rho} - 2 \alpha \, .</math>
</div>

Note that, once the value of the parameter, <math>~\mathfrak{F}</math>, has been determined for a given eigenvector, the square of the eigenfrequency will also be known via the inversion of this last expression.  Specifically, in terms of <math>~\mathfrak{F}</math>,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>~n^2</math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~\frac{2\pi \gamma  G \bar\rho}{3} \biggl[ \mathfrak{F} + 2 \biggl(3-\frac{4}{\gamma}\biggr)\biggr]</math>
  </td>
</tr>

<tr>
  <td align="right">
<math>~\Rightarrow ~~~~~ \frac{n^2}{4\pi G \bar\rho}</math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~\biggl(1 + \frac{\mathfrak{F}}{6} \biggr)\gamma -\frac{4}{3}  \, .</math>
  </td>
</tr>
</table>
</div>

As a reminder, in these terms the inner boundary condition is
<div align="center">
<math>~\xi_1^'\biggr|_{x = 0} = 0 \, .</math>
</div>
And the outer boundary condition becomes,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>~\xi_1^'</math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~\xi_1 \biggl( \frac{n^2 R}{\gamma g_0} - \alpha \biggr)</math>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~\xi_1 \biggl[ \frac{3}{x}\biggl( \frac{n^2 }{4\pi \gamma G\bar\rho } \biggr) - 3 + \frac{4}{\gamma} \biggr]</math>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~3\xi_1 \biggl\{ \frac{1}{x}\biggl[ \biggl(1 + \frac{\mathfrak{F}}{6} \biggr) -\frac{4}{3\gamma} \biggr] - 1 + \frac{4}{3\gamma} \biggr\}</math>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; at &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 
<math>~x = 1 \, ,</math>
  </td>
</tr>

<tr>
  <td align="right">
<math>~\Rightarrow~~~~~\biggl[ \xi_1^' - \frac{\mathfrak{F} \xi_1}{2} \biggr]_{x=1}</math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~0 \, .</math>
  </td>
</tr>
</table>
</div>


<!--  OMIT NEXT TEXT
==Analytic Solution==

===First few lowest-order modes===
* <font color="purple">Mode 0</font>:
: <math>x_0 = \mathrm{constant}</math>, in which case,
<div align="center">
<math>
\omega_0^2 = - 2(4 - 3\gamma_\mathrm{g})\biggl[ \frac{2\pi G\rho_c}{3} \biggr] = 4\pi G \rho_c \biggl[ \gamma_\mathrm{g}- \frac{4}{3} \biggr]
</math>
</div>

* <font color="purple">Mode 1</font>:
: <math>x_1 = a + b\chi_0^2</math>, in which case,
<div align="center">
<math>
\frac{dx}{d\chi_0} = 2b\chi_0; ~~~~ \frac{d^2 x}{d\chi_0^2} =  2b;
</math>
</div>

<div align="center">
<math>
\frac{1}{(1 - \chi_0^2)}  \biggl\{ 2b (1 - \chi_0^2) + 8b \biggl[1 -  \frac{3}{2}\chi_0^2 \biggr]  +  A_1 \biggl(1 + \frac{b}{a}\chi_0^2 \biggr) \biggr\} = 0 ,
</math><br />
</div>
where,

<div align="center">
<math>
A_1 \equiv \frac{a}{\gamma_\mathrm{g}}\biggl[ \biggl( \frac{3}{2\pi G\rho_c} \biggr) \omega_1^2+ 2(4 - 3\gamma_\mathrm{g}) \biggr] .
</math>
</div>
Therefore,

<div align="center">
<math>
(A_1 + 10b) + \biggl[ \biggl(\frac{b}{a}\biggr) A_1 - 14b \biggr] \chi_0^2  = 0 ,
</math>
<br />
<br />
<math>
\Rightarrow ~~~~~ A_1  = - 10b ~~~~~\mathrm{and} ~~~~~ A_1 = 14a
</math>
<br />
<br />
<math>
\Rightarrow ~~~~~ \frac{b}{a} = -\frac{7}{5}  ~~~~~\mathrm{and} ~~~~~ \frac{A_1}{a} = 14 = \frac{1}{\gamma_\mathrm{g}}\biggl[ \biggl( \frac{3}{2\pi G\rho_c} \biggr) \omega_1^2+ 2(4 - 3\gamma_\mathrm{g}) \biggr] .

</math>
</div>
Hence,
<div align="center">
<math>
\biggl( \frac{3}{2\pi G\rho_c} \biggr) \omega_1^2 = 20\gamma_\mathrm{g}  -8
</math>
<br />
<br />
<math>
\Rightarrow ~~~~~ \omega_1^2 = \frac{2}{3}\biggl( 4\pi G\rho_c \biggr) (5\gamma_\mathrm{g}  -2)
</math>
</div>
and, to within an arbitrary normalization factor,
<div align="center">
<math>
x_1 = 1 - \frac{7}{5}\chi_0^2 .
</math>
</div>
<br />

END OMISSION -->

==Sterne's General Solution==

===Sterne's Presentation===
In what follows, as before, text presented in a green font has been taken verbatim from {{ Sterne37hereafter }}.  He begins by writing the unknown eigenfunction as a power series expanded about the origin, specifically,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>\xi_1</math>
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>\sum\limits_{0}^{\infty} a_k x^k \, ,</math>
  </td>
</tr>
</table>
</div>
with, <math>a_0 = 1</math>.  <font color="green">It is found by substitution that the terms in odd powers of <math>x</math> vanish, and that the coefficients of the even terms satisfy the recurrence formula</font>,

<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>a_{k+2}</math>
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>a_k \cdot \frac{k^2 + 5k - \mathfrak{F}}{(k+2)(k+5)} \, .</math>
  </td>
</tr>
</table>
</div>

The wave equation and attending boundary conditions will all <font color="green">be satisfied if we choose <math>\mathfrak{F}</math> so as to make the series solution terminate with some term, say the <math>2 j^\mathrm{th}</math> where <math>j</math> is zero or any positive integer.  This it will do</font> [via the above recurrence relation] if,
<div align="center">
<math>\mathfrak{F} = 2j(2j+5) \, .</math>
</div>

The first few solutions are displayed in the following boxed-in image that has been extracted directly from &sect;2 (p. 587) of {{ Sterne37 }}; to the right of his table, we have added a column that expressly records the value of the square of the normalized eigenfrequency that corresponds to each of the solutions presented by {{ Sterne37hereafter }}.

<div align="center">
<table border="2" cellpadding="5" width="70%">
<tr>
  <td align="center" colspan="1">
Table of exact eigenvector expressions extracted from &sect;2 (p. 587) of &hellip;<br />
{{ Sterne37figure }}
  </td>
  <td align="center" colspan="1">
<math>\frac{n^2}{4\pi G \bar\rho}</math>
  </td>
</tr>
<tr>
  <td colspan="1" rowspan="1">
<!-- [[File:Sterne1937SolutionTable1.png|600px|center|Sterne (1937)]] -->
    <table border="0" align="left">
    <tr>
    <td align="right"><math>j=0 \, ;</math>&nbsp; &nbsp; &nbsp;</td>
    <td align="right"><math>\mathfrak{F}=0 \, ;</math>&nbsp; &nbsp; &nbsp;</td>
    <td align="right">&nbsp;<math>\xi_1 = 1</math></td>
    </tr>
    </table>
  </td>
  <td align="center"><math>\gamma - 4/3</math></td>
</tr>

<tr>
  <td colspan="1" rowspan="1">
    <table border="0" align="left">
    <tr>
    <td align="right"><math>j=1 \, ;</math>&nbsp; &nbsp; &nbsp;</td>
    <td align="right"><math>\mathfrak{F}= 14 \, ;</math>&nbsp; &nbsp; &nbsp;</td>
    <td align="right"><math>\xi_1 = 1 - (7/5)x^2</math></td>
    </tr>
    </table>
  </td>
  <td align="center"><math>2(5\gamma - 2)/3</math></td>
</tr>

<tr>
  <td colspan="1" rowspan="1">
    <table border="0" align="left">
    <tr>
    <td align="right"><math>j=2 \, ;</math>&nbsp; &nbsp; &nbsp;</td>
    <td align="right"><math>\mathfrak{F}= 36 \, ;</math>&nbsp; &nbsp; &nbsp;</td>
    <td align="right"><math>\xi_1 = 1 - (18/5)x^2 + (99/35)x^4</math></td>
    </tr>
    </table>
  </td>
  <td align="center"><math>7\gamma - 4/3</math></td>
</tr>

<tr>
  <td colspan="1" rowspan="1">
    <table border="0" align="left">
    <tr>
    <td align="right"><math>j=3 \, ;</math>&nbsp; &nbsp; &nbsp;</td>
    <td align="right"><math>\mathfrak{F}=66 \, ;</math>&nbsp; &nbsp; &nbsp;</td>
    <td align="right"><math>\xi_1 = 1 - (33/5)x^2 + (429/35)x^4 - (143/21)x^6</math></td>
    </tr>
    </table>
  </td>
  <td align="center"><math>12\gamma - 4/3</math></td>
</tr>
</table>
</div>

===Validity Check===
Let's explicitly demonstrate that the first few eigenvectors derived by {{ Sterne37hereafter }} actually satisfy the governing adiabatic wave equation and the two boundary conditions.
====Mode j = 0:====
In this case,
<div align="center">
<math>~\xi_1 = 1</math>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; and &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
<math>~\mathfrak{F} = 0 \, .</math>
</div>
Hence,
<div align="center">
<math>\xi_1^' \equiv \frac{d\xi_1}{dx} = 0 \, ;</math>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; and &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
<math>\xi_1^{' '} \equiv \frac{d^2\xi_1}{dx^2} = 0 \, .</math>
</div>
So, the adiabatic wave equation gives,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>(1-x^2) \xi_1^{ ' ' } + \frac{1}{x}\biggl[4 - 6x^2 \biggr]\xi_1^' + \mathfrak{F} \xi_1 </math>
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>(1-x^2) (0) + \frac{1}{x}\biggl[4 - 6x^2 \biggr](0) + (0)(1) \, ,</math>
  </td>
</tr>
</table>
</div>
which properly sums to zero.  Next, because <math>\xi_1^' = 0</math> everywhere, we know that it is zero at the center of the configuration, which satisfies the inner boundary condition.  But, via the outer boundary condition, this also means that the product, <math>(\tfrac{1}{2} \mathfrak{F} \xi_1)</math> should be zero; which it is, because <math>\mathfrak{F} = 0</math> for this mode.

====Mode j = 1:====
In this case, according to {{ Sterne37hereafter }},
<div align="center">
<math>\xi_1 = 1 - \frac{7}{5} x^2 \, ,</math>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; and &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
<math>\mathfrak{F} = 14 \, .</math>
</div>
Hence,
<div align="center">
<math>\xi_1^' = - \frac{14}{5} x \, ;</math>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; and &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
<math>\xi_1^{' '} = - \frac{14}{5}  \, .</math>
</div>
So, the adiabatic wave equation gives,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>(1-x^2) \xi_1^{ ' ' } + \frac{1}{x}\biggl[4 - 6x^2 \biggr]\xi_1^' + \mathfrak{F} \xi_1 </math>
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>- \frac{14}{5}(1-x^2)  - \frac{14}{5}\biggl[4 - 6x^2 \biggr] + 14 \biggl( 1 - \frac{7}{5} x^2 \biggr) </math>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>\frac{14}{5}\biggl[- (1-x^2)  - (4 - 6x^2 ) +5 - 7 x^2  \biggr] \, , </math>
  </td>
</tr>
</table>
</div>
which properly sums to zero for all <math>x</math>.  Next, it is clear that the inner boundary condition is satisfied because,
<div align="center">
<math>\xi_1^'\biggr|_{x = 0} = - \frac{14}{5} (0)= 0 \, .</math>
</div>
And the expression for the outer boundary condition gives,

<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>\biggl[ \xi_1^' - \frac{\mathfrak{F} \xi_1}{2} \biggr]_{x=1}</math>
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>\biggl[ -\biggl(\frac{14}{5} \biggr) x - 7 \biggl( 1 - \frac{7}{5}x^2 \biggr) \biggr]_{x=1} </math>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>\biggl[ -\biggl(\frac{14}{5} \biggr)  - 7 \biggl( 1 - \frac{7}{5} \biggr) \biggr] \, ,</math>
  </td>
</tr>
</table>
</div>
which also properly sums to zero.

====Mode j = 2:====
In this case, according to {{ Sterne37hereafter }},
<div align="center">
<math>\xi_1 = 1 - \frac{2\cdot 3^2}{5} x^2 + \frac{3^2 \cdot 11}{5 \cdot 7} x^4 \, ,</math>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; and &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
<math>\mathfrak{F} = 2^2 \cdot 3^2 \, .</math>
</div>
Hence,
<div align="center">
<math>\xi_1^' = - \frac{2^2\cdot 3^2}{5} x + \frac{2^2 \cdot 3^2 \cdot 11}{5 \cdot 7} x^3 \, ;</math>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; and &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
<math>\xi_1^{' '} = - \frac{2^2\cdot 3^2}{5}  + \frac{2^2 \cdot 3^3 \cdot 11}{5 \cdot 7} x^2  \, .</math>
</div>
So, the adiabatic wave equation gives,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>(1-x^2) \xi_1^{ ' ' } + \frac{1}{x}\biggl[4 - 6x^2 \biggr]\xi_1^' + \mathfrak{F} \xi_1 </math>
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>
(1-x^2) \biggl[- \frac{2^2\cdot 3^2}{5}  + \frac{2^2 \cdot 3^3 \cdot 11}{5 \cdot 7} x^2 \biggr] 
</math>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
&nbsp;
  </td>
  <td align="left">
<math>
+ 2\biggl[2 - 3x^2 \biggr]\biggl[- \frac{2^2\cdot 3^2}{5}  + \frac{2^2 \cdot 3^2 \cdot 11}{5 \cdot 7} x^2\biggr] 
</math>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
&nbsp;
  </td>
  <td align="left">
<math>
+ 2^2 \cdot 3^2 \biggl[ 1 - \frac{2\cdot 3^2}{5} x^2 + \frac{3^2 \cdot 11}{5 \cdot 7} x^4 \biggr]
</math>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>
- \frac{2^2\cdot 3^2}{5}  + \biggl[\frac{2^2 \cdot 3^3 \cdot 11}{5 \cdot 7}  
+ \frac{2^2\cdot 3^2}{5} \biggr] x^2 - \frac{2^2 \cdot 3^3 \cdot 11}{5 \cdot 7} x^4 
</math>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
&nbsp;
  </td>
  <td align="left">
<math>
- \frac{2^4\cdot 3^2}{5}  +\biggl[ \frac{2^4 \cdot 3^2 \cdot 11}{5 \cdot 7}
+ \frac{2^3\cdot 3^3}{5} \biggr]x^2  - \frac{2^3 \cdot 3^3 \cdot 11}{5 \cdot 7} x^4 
</math>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
&nbsp;
  </td>
  <td align="left">
<math>
+ 2^2 \cdot 3^2  - \frac{2^3\cdot 3^4}{5} x^2 + \frac{2^2 \cdot 3^4 \cdot 11}{5 \cdot 7} x^4 
</math>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>
\frac{2^2\cdot 3^2}{5}\biggl[ -1 -4 + 5\biggr]  + \frac{2^2\cdot 3^2}{5\cdot 7}\biggl[33  + 7 + 44 + 42 - 2\cdot 3^2 \cdot 7\biggr] x^2  
+ \frac{2^2 \cdot 3^3 \cdot 11}{5 \cdot 7} \biggl[ 3 - 2 - 1\biggr] x^4 \, ,
</math>
  </td>
</tr>
</table>
</div>

which properly sums to zero for all <math>x</math>.  Next, it is clear that the inner boundary condition is satisfied because,
<div align="center">
<math>\xi_1^'\biggr|_{x = 0} = - \frac{2^2\cdot 3^2}{5} (0) + \frac{2^2 \cdot 3^2 \cdot 11}{5 \cdot 7} (0) = 0 \, .</math>
</div>
And the expression for the outer boundary condition gives,

<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>\biggl[ \xi_1^' - \frac{\mathfrak{F} \xi_1}{2} \biggr]_{x=1}</math>
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>\biggl[  - \frac{2^2\cdot 3^2}{5} x + \frac{2^2 \cdot 3^2 \cdot 11}{5 \cdot 7} x^3 
- 2\cdot 3^2 \biggl( 1 - \frac{2\cdot 3^2}{5} x^2 + \frac{3^2 \cdot 11}{5 \cdot 7} x^4 \biggr)  \biggr]_{x=1} </math>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>\frac{2\cdot 3^2}{5\cdot 7}\biggl[ -14 + 22  - \biggl( 35 - 126 + 99 \biggr) \biggr] \, ,</math>
  </td>
</tr>
</table>
</div>
which also properly sums to zero.

===Properties of Eigenfunction Solutions===

<table border="1" cellpadding="2" align="center">
<tr>
  <td colspan="1" align="center">
[[File:Sterne1937SolutionPlot1.png|350px|center|Sterne (1937)]]
  </td>
  <td colspan="1" align="center">
[[File:Sterne1937CritGamma1.png|350px|center|Sterne (1937)]]
  </td>
</tr>
</table>

==Stability==

The [[SSC/Perturbations#2ndOrderODE|Adiabatic Wave Equation]] that defines this [[SSC/Perturbations#The_Eigenvalue_Problem|eigenvalue problem]] has been derived from the fundamental set of nonlinear [[PGE#Principal_Governing_Equations|Principal Governing Equations]] by assuming that, for example, the radial position, <math>r(m,t)</math>, at any time, <math>t</math>, and of each mass shell throughout our spherical configuration can be described by the expression,
<div align="center">
<math>r(m,t) = r_0(m) [ 1 + x(m) e^{i\omega t} ] \, ,</math>
</div>
where, the fractional displacement, <math>|x| \ll 1</math>.  Switching to {{ Sterne37hereafter }}'s variable notation, this should be written as,
<div align="center">
<math>\xi(x,t) = \xi_0(x) [ 1 + A\xi_1(x) e^{i n t} ] \, ,</math>
</div>
with the presumption that the coefficient, <math>|A| \ll 1</math>, and the understanding that, in {{ Sterne37hereafter}}, the variable, <math>x</math>, is used to identify individual mass shells.  Specifically, given <math>R</math> and <math>\bar\rho</math>,
<div align="center">
<math>m \equiv M_r = \frac{4}{3}\pi \xi_0^3 \bar\rho = \frac{4}{3}\pi (R x)^3 \bar\rho </math>
&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; <math>\Rightarrow</math> &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 
<math>x = \biggl( \frac{3m}{4\pi R^3 \bar\rho} \biggr)^{1/3} \, .</math>
</div>

The [[#Sterne.27s_General_Solution|general solution of this eigenvalue problem]] presented by {{ Sterne37 }} describes mathematically how a self-gravitating, uniform-density configuration will vibrate if perturbed away from its equilibrium state; the oscillatory behavior associated with each pure radial mode, <math>j</math> &#8212; among an infinite number of possible modes &#8212; is fully defined by the polynomial expression for the eigenvector, <math>\xi_1(x)</math>, and the corresponding value of the square of the eigenfrequency, <math>n^2</math>. 


If, for any mode, <math>j</math>,  the ''square'' of the derived eigenfrequency, <math>n^2</math>, is positive, then the eigenfrequency itself will be a real number &#8212; specifically,
<div align="center">
<math>n = \pm \sqrt{|n^2|} \, .</math>
</div> 
As a result, the radial location of every mass shell will vary sinusoidally in time according to the expression,
<div align="center">
<math>\frac{\xi(x,t)}{\xi_0} - 1 \propto e^{\pm i \sqrt{|n^2|} t} \, .</math>
</div> 
If, on the other hand, <math>n^2</math>, is negative, then the eigenfrequency will be an imaginary number &#8212; specifically,
<div align="center">
<math>n = \pm i \sqrt{|n^2|} \, .</math>
</div> 
As a result, the radial location of every mass shell will grow (or damp) exponentially in time according to the expression,
<div align="center">
<math>\frac{\xi(x,t)}{\xi_0} - 1 \propto e^{\pm \sqrt{|n^2|} t} \, .</math>
</div> 
This latter condition is the mark of a dynamically unstable system.  It is in this manner that the solution to an eigenvalue problem can provide critical information regarding the relative stability of equilibrium configurations.

For any given mode number, <math>j</math>, then, the critical configuration separating stable from unstable systems occurs when the dimensionless eigenfrequency is zero.  Therefore, the critical state occurs when,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>0 = \frac{n_\mathrm{crit}^2}{4\pi G \bar\rho}</math>
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>\biggl(1 + \frac{\mathfrak{F}}{6} \biggr)\gamma_\mathrm{crit} -\frac{4}{3}  </math>
  </td>
</tr>

<tr>
  <td align="right">
<math>\Rightarrow ~~~~~ \gamma_\mathrm{crit}(j)</math>
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>\frac{4}{3}\biggl(1 + \frac{\mathfrak{F}}{6} \biggr)^{-1}  </math>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>\frac{4}{3}\biggl[1 + \frac{2j(2j+5)}{6} \biggr]^{-1}  </math>
  </td>
</tr>

<tr>
  <td align="right">
&nbsp;
  </td>
  <td align="center">
<math>=</math>
  </td>
  <td align="left">
<math>\frac{4}{3 + j(2j+5)}  \, . </math>
  </td>
</tr>
</table>
</div>

The plot titled, "Critical Adiabatic Index," that is [[#Properties_of_Eigenfunction_Solutions| presented above]] shows graphically how <math>\gamma_\mathrm{crit}</math> varies with mode number over the range of mode numbers, <math>0 \le j \le 11</math>.  All modes are stable as long as <math>\gamma > 4/3</math>.  As the adiabatic index is decreased below this value, the lowest order mode, <math>j = 0</math>, becomes unstable, first; then successively higher order modes become unstable at smaller and smaller values of the index.  A very similar explanation and enunciation of {{ Sterne37hereafter}}'s derived results regarding the stability of uniform-density spheres appears at the bottom of p. 338 of {{ Ledoux46full }}.  The relevant paragraph from {{ Ledoux46 }} follows:

<div align="center">
<table border="2" cellpadding="5" width="80%">
<tr>
  <td align="center" colspan="1">
Paragraph extracted<sup>&dagger;</sup> from the bottom of p. 338 of &hellip;<br />
{{ Ledoux46figure }}
  </td>
</tr>
<tr>
  <td colspan="1" rowspan="1">
<!-- [[File:Ledoux1946OnSterne01.png|700px|center|Ledoux (1946)]] -->
<font color="darkgreen">
"Another point brought out clearly by Sterne's analysis is that, while the fundamental mode becomes unstable for <math>\Gamma < \tfrac{4}{3}\, ,</math> the higher harmonics continue to be stable.  Indeed, as we may directly verify from equation (19), the first harmonic becomes unstable only if <math>\Gamma < \tfrac{2}{5}</math> and the second one if <math>\Gamma < \tfrac{4}{21} \, .</math>"
</font>
  </td>
</tr>
<tr>
  <td align="left">
<sup>&dagger;</sup>Our function, <math>~\gamma_\mathrm{crit}(j)</math>, is effectively the expression to which Ledoux is referring when he says, "&hellip; directly verify from equation (19) &hellip;"
  </td>
</tr>
</table>
</div>

==Numerical Integration==

In order to gain a more complete understanding of this type of modal analysis, let's attempt to obtain various eigenvectors by numerically integrating the governing LAWE from the center of the system, outward to the surface.  This will be done in a manner similar to our [[Appendix/Ramblings/NumericallyDeterminedEigenvectors#Numerically_Determined_Eigenvectors_of_a_Zero-Zero_Bipolytrope|numerical study of radial oscillations in zero-zero bipolytropes]].  Following our [[#Setup_as_Presented_by_Sterne_.281937.29|above review of Sterne's presentation]], the relevant LAWE is,

<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>~x(1-x^2) \xi_1^{ ' ' } </math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~- (4 - 6x^2 )\xi_1^' - x \mathfrak{F} \xi_1 \, ,</math>
  </td>
</tr>
</table>
</div>
where,
<div align="center">
<math>~\mathfrak{F}  \equiv \frac{\sigma_c^2}{\gamma} - 2 \alpha = \frac{\sigma_c^2 + 8}{\gamma} - 6\, ,</math>
&nbsp; &nbsp; &nbsp; &nbsp; and, &nbsp; &nbsp; &nbsp; &nbsp; 
<math>~\sigma_c^2  \equiv \frac{3n^2 }{2\pi G \rho_c}  \, .</math>
</div>

Following precisely the same logic as has been laid out in our [[Appendix/Ramblings/NumericallyDeterminedEigenvectors#Integrating_Outward_Through_the_Core|separate discussion]], if we set the central value of the eigenfunction to <math>~\xi_0</math>, then the eigenfunction's value at the first zone (distance <math>~\Delta</math>) away from the center will be,

<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>~
\xi_+  
</math>
  </td>
  <td align="center">
<math>~=</math>
  </td>
  <td align="left">
<math>~\biggl[ 1 - \frac{\Delta^2 \mathfrak{F}}{2} \biggr] \xi_0  \, .
</math>
  </td>
</tr>
</table>
</div>
While, for each successive coordinate location, <math>~a = x</math>, in the range, <math>~0 < x < 1</math>, we will use the general expression, namely,
<div align="center">
<table border="0" cellpadding="5" align="center">

<tr>
  <td align="right">
<math>~
\Rightarrow~~~\xi_+
</math>
  </td>
  <td align="center">
<math>~\approx</math>
  </td>
  <td align="left">
<math>~\frac{[4a (1 - a^2)  - 2\Delta^2 a \mathfrak{F} ]\xi_a
+ [ \Delta( 4 - 6a^2 )  - 2a (1 - a^2)]  \xi_-  }{[2a (1 - a^2)  +   \Delta( 4 - 6a^2 )  ] } \, .
</math>
  </td>
</tr>
</table>
</div>

{{ SGFfooter }}