help me...(about a graviton)

N.J.R

I don't know the relation of graviton and closed string.
Why the graviton is closed string? And what conditions need that?
I really want to know about it in detail.(of course including formula)

selfAdjoint

The graviton in bosonic string theory comes from the interaction of the right moving excitations and the left moving ones on the string. This requires the string to be closed; on an open string the R and L excitations would just peter out at the endpoints.