We present analysis of MACHO Alert 95–30 , a dramatic gravitational microlensing event towards the Galactic bulge whose peak magnification departs significantly from the standard point–source microlensing model .
Alert 95–30 was observed in real–time by the Global Microlensing Alert Network ( GMAN ) , which obtained densely sampled photometric and spectroscopic data throughout the event .
We interpret the light–curve “ fine structure ” as indicating transit of the lens across the extended face of the source star .
This signifies resolution of a star several kpc distant .

We find a lens angular impact parameter \theta _ { min } / \theta _ { source } = 0.715 \pm 0.003 .
This information , along with the radius and distance of the source , provides an additional constraint on the lensing system .
Spectroscopic and photometric data indicate the source is an M4 III star of radius 61 \pm 12 { { \em R } _ { \odot } } , located on the far side of the bulge at \sim 9 { kpc } .
We derive a lens angular velocity , relative to the source , of 21.5 \pm 4.9 { km s ^ { -1 } kpc ^ { -1 } } , where the error is dominated by uncertainty in the source radius .
Likelihood analysis yields a median lens mass of 0.67 ^ { +2.53 } _ { -0.46 } { { \em M } _ { \odot } } , located with 80 \% probability in the Galactic bulge at a distance of 6.93 ^ { +1.56 } _ { -2.25 } { kpc } .
If the lens is a main–sequence star , we can include constraints on the lens luminosity .
This modifies our estimates to M _ { lens } = 0.53 ^ { +0.52 } _ { -0.35 } { { \em M } _ { \odot } } and D _ { lens } = 6.57 ^ { +0.99 } _ { -2.25 } { kpc } .

Spectra taken during the event show that the absorption line equivalent widths of H \alpha and the TiO bands near 6700Å vary , as predicted for microlensing of an extended source .
This is most likely due to center–to–limb variation in the stellar spectral lines .
The observed spectral changes further support our microlensing interpretation .
These data demonstrate the feasibility of using microlensing limb crossings as a tool to probe stellar atmospheres directly .