These are special bottle caps that have a blue star underneath it.
There is a legend of a grand prize given to anyone who finds enough blue star bottle caps, the prize is being guarded by an animatronic cowboy named Festus. He is the one who gives you the quest called The Legend of the Star.
Anyone in New Vegas from cutthroats to simple wastlanders who have heard of the legend, search the Mojave Desert for these legendary caps for hope of a grand prize; some would even kill for these rare bottle caps.
You will be able to receive these special caps through drinking Sunset Sarsaparilla, off other collectors or by select locations.
After you start the game and set foot on your journey through the Mojave, one of the first times you sleep OUTSIDE, in a bed you have found (it's best to sleep 24 hours, just incase), a man will wake you up suddenly and tell you the story of the Sunset Sarsparilla Star Caps, and then tell you he has none. Kill him, search his body and it will contain 6-9 star caps. Of course, you lose a little karma for ruthless murder, but what's that at the cost of those precious Star Caps.