Step 1: Select a Gardient tool 
and
then press a Reflected Gradient button:
Step 2: Then make a gradient of your choice:
Step 3: Go to Filter > Pixelate > Mosaic:
Step 4: Then set these options:
Step 5: Go to Filter > Distort > Polar Coordinates:
Step 6: Set these options:
Step 7: Now it should look like this:
Your Starburst is finished.
