What is the latest os I can run on my xserve

What is the NEWEST OS X version that I can install on my 2008 Xserve. Its currently running 10.6.8 and will not allow me to update to 10.10

Some Specs:

Model name: Xserve

Model Identifier: Xserve2,1

Quad core intel xeon 2.8 ghz

4GB ram