8. A figure composed of some vertices and edges is called a graph. Coloring the vertices of a graph with different colors, requiring that two vertices connected by the same edge be colored differently, is called vertex coloring of a graph. The usual problem studied in graph vertex coloring is the minimum number of colors needed to complete the coloring, which is called the chromatic number of the graph. The graph below is called the Petersen graph, and the chromatic number of the Petersen graph is _
$3$