Can Two Stars of One Same Type Orbit Each Other? This is a simple, textbook definition of a binary star orbit.  One low-mass star orbits a higher-mass star in a narrow but definitive center.  For example, the high-mass star would be our own sun--a G-type main sequence star--being orbited by a smaller dwarf, like a red dwarf or a brown dwarf.  But what about binary orbits of two identical stars--like our sun orbiting another G-type main-sequence star?  Is this sort of orbit possible?
Certainly, the figure only uses different masses because that's the more general possibility (after all, the two masses won't be exactly the same).  In fact, if you look at the distribution over mass ratio q in binary systems in Figure 1b in https://arxiv.org/pdf/1304.3123.pdf, it seems that the cumulative number of binaries with mass ratio less than q is more or less equal to q (where q is smaller mass over larger mass).  That means the probability of mass ratio between any q and q+dq is about equal to dq and nearly independent of q.  So the chance of the mass ratio being between 0.9 and 1 is about the same as being between 0 and 0.1, for example.  In some sense, that's like saying all q are equally likely.  And of course if q is near 1, then the stars will have the same age and be in the same evolutionary state, so will be the same stellar type as well.