I want to draw this picture, can anyone help me complete it?
\documentclass[a4paper,11pt]{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\shade[ball color = brown, opacity=0.7] (0,0) circle (2cm);
\draw (0,0) circle (2cm);
\draw[thick, -latex] (-130:1cm) -- (-130:3.5cm) node [below] {$ x $}; % x axis
\draw[thick, -latex] (-20:1cm) -- (-20:3.5cm) node [right] {$ y $}; % y axis
\draw[thick, -latex] (90:1cm) -- (90:3.5cm) node [above] {$ z $}; % z axis
\draw (60:1.5cm) -- (60:3.5cm) node [anchor=south west, xshift=-2em]
{$ x^2 + y^2 + z^2 - a^2 = 0 $};
\end{tikzpicture}
\end{document}