8. If the real numbers $\alpha, \beta, \gamma$ form a geometric sequence with a common ratio of 2, and $\sin \alpha, \sin \beta, \sin \gamma$ form a geometric sequence, then the value of $\cos \alpha$ is
$\qquad$ .
$$
\begin{array}{l}
\sin ^{2} 2 \alpha=\sin \alpha \cdot \sin 4 \alpha=2 \sin \alpha \sin 2 \alpha \cos 2 \alpha \Rightarrow \cos \alpha=\cos 2 \alpha=2 \cos ^{2} \alpha-1 \\
\Rightarrow 2 \cos ^{2} \alpha-\cos \alpha-1=0 \Rightarrow \cos \alpha=-\frac{1}{2} .
\end{array}
$$