%\begin{table}
\begin{center}
\begin{small}
\begin{tabular}{p{0.32\textwidth} p{0.42\textwidth} p{0.26\textwidth}}


variable  & name & units (cgs) \\[.5em]
%---------------------------------------------------------------------------------------------------
\hline \\

$I_\nu (\x, \mathbf{n}, t)$ &
        specific intensity &
        $[I_\nu] = \frac{\text{erg}}{\text{cm}^2 \text{ rad Hz s}}$
\\[.5em]
$u_\nu (\x, \mathbf{n}, t) = \frac{I_\nu}{c}$ &
        radiation energy density &
        $[u_\nu] = \frac{\text{erg}}{\text{cm}^3 \text{ rad Hz}}$
\\[.5em]
$E_\nu (\x, t) = \int_{4 \pi} \frac{I_\nu}{c} \de \Omega$ &
        total energy density &
        $[E_\nu] = \frac{\text{erg}}{\text{cm}^3 \text{ Hz}}$
\\[.5em]
$N_\nu (\x, t) = E_\nu / h$ &
        photon number density &
        $[N_\nu] = \frac{1}{\text{cm}^3 \text{ Hz}}$
\\[.5em]
$E_{rad} (\x, t) = \int_{0}^{\infty} E_\nu \de \nu$ &
        total integrated energy density &
        $[E_{rad}] = \frac{\text{erg}}{\text{cm}^3}$
\\[.5em]
$N_{rad} (\x, t) = E_{rad} / h$ &
        integrated photon number density &
        $[N_{rad}] = \frac{1}{\text{cm}^3}$
\\[.5em]
% $J_\nu (\x, t) = \int_{4 \pi} I_\nu \frac{\de \Omega}{4 \pi}
%             = \frac{c}{4 \pi} E_\nu$ &
%             mean radiation specific intensity &
%             $[J_\nu] = \frac{\text{erg}}{\text{cm}^2 \text{ Hz s}}$
% \\[.5em]
$\mathbf{F}_\nu(\x, t) = \int_{4 \pi}  I_\nu \mathbf{n} \de \Omega$ &
        radiation flux &
        $[\mathbf{F}_\nu] = \frac{\text{erg}}{\text{cm}^2 \text{ s Hz}}$
\\[.5em]
$\mathbf{P}_\nu (\x, t) = \frac{\F_\nu}{c^2}$ &
        radiation momentum density &
        $[\mathbf{P}_\nu] = \frac{\text{erg}}{ \text{cm}^4 \text{ s}^{-1} \text{ Hz}}$
\\[.5em]
$\mathds{P}_\nu (\x, t) = \int_{4 \pi} \frac{I_\nu}{c} \mathbf{n} \otimes \mathbf{n} \de \Omega$ &
        radiation pressure tensor &
        $[\mathds{P}_\nu ] = \frac{\text{erg}}{\text{cm}^3 \text{ Hz}}$


\\
%---------------------------------------------------------------------------------------------------
\hline\\


$\eta_\nu(\x, t)$ &
        source function (of $I_\nu$)&
        $[ \eta_\nu ] = \frac{\text{erg}}{\text{cm}^3 \text{ rad Hz s}}$
\\[.5em]
$n(\x, t)$ &
        number density &
        $[ n ] = \text{cm}^{-3}$
\\[.5em]
$\mathcal{J}_\nu(T)$ &
        (specific intensity of a) spectrum &
        $[\mathcal{J}_\nu (T)] = \frac{\text{erg}}{\text{cm}^2 \text{ rad Hz s}}$
\\[.5em]
$\Gamma_{\nu, j}$ &
        photoionization rate of species $j$ &
        $[\Gamma] = \text{s}^{-1}$
\\[.5em]
$\mathcal{H}_{\nu,j}$ &
        (photo)heating rate of species $j$ &
        $[\mathcal{H}_{\nu,j}] = \frac{\text{erg}}{s\ cm}^{3}$
\\[.5em]
$\alpha_\nu = \frac{1}{\lambda_\nu} = \rho \kappa_\nu = n \sigma_\nu$ &
        absorption coefficient &
        $[\alpha_\nu] = \text{cm}^{-1}$
\\[.5em]
% $\alpha_E = \frac{\int_0^\infty \alpha_\nu E_\nu \de \nu}{\int_0^\infty E_\nu \de \nu}$ &
% 	 energy mean of abs. coeff. &
% 	 $[\alpha_E] = \text{cm}^{-1}$
% \\[.5em]
% $\alpha_F = \frac{\int_0^\infty \alpha_\nu F_\nu \de \nu}{\int_0^\infty F_\nu \de \nu}$ &
% 	 flux mean of abs. coeff. &
% 	 $[\alpha_F] = \text{cm}^{-1}$
% \\[.5em]
% $j_\nu $ &
%         emission coefficient &
%         $[j_\nu] = \frac{\text{erg}}{\text{cm}^3 \text{rad Hz s}}$
% \\[.5em]
$\lambda_\nu $ &
        mean free path &
        $[\lambda_\nu] = \text{cm}$
\\[.5em]
$\kappa_\nu $ &
        opacity &
        $[\kappa_\nu] = \frac{\text{cm}^2}{\text{g}}$
\\[.5em]
$\tau_\nu(s) = \int_0^s \alpha_\nu(x) \de x$ &
        optical depth &
        $[\tau_\nu] = 1$
\\[.5em]
$\sigma_{j\nu}$ &
        interacton cross section (of species $j$)&
        $[\sigma_{j\nu}] = \text{cm}^2$
\\[.5em]
$\sigma_{ij}^N = \int_{\nu_{i}} N_\nu \ \sigma_{j\nu} \de \nu / \int_{\nu_i} N_\nu \de \nu$ &
        number weighted average cross section &
        $[\sigma_{ij}^N] = \text{cm}^2$
\\[.5em]
$\sigma_{ij}^E = \int_{\nu i} E_\nu \ \sigma_{j\nu} \de \nu / \int_{\nu i} E_\nu \de \nu$ &
        energy weighted average cross section &
        $[\sigma_{ij}^E] = \text{cm}^2$
%

\end{tabular}
\end{small}
\end{center}
%\caption{Common quantities appearing in the context of radiative transfer, and their units.}
%\label{tab:rt-variables}
%\end{table}