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? :confused:
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.