The radio source M 81 ^ { * } at the core of the nearby spiral galaxy M 81 is a low-luminosity active galactic nucleus .
The close distance of 3.63 Mpc allows its morphology to be studied in great detail .
Here we present preliminary results from continuum \lambda 7 mm VLBI observations of its core , using phase-referencing techniques .
These observations set constrains on the size of M 81* at this frequency and enable us to test the frequency dependence on its physical properties .