  The nucleus of M81 was observed at 8.3 GHz with VLBI at 20 epochs over
4.5yrs, with a linear resolution at the source of about 2000 AU or 0.01 pc.
Phase-referenced mapping with respect to the geometric center of supernova
1993J enabled us to find, with a standard error of about 600 AU, a stationary
point in the source. We identify this point as the location of the core and the
putative black hole at the gravitational center of the galaxy. The upper bound
on the core's average velocity on the sky is <730 km/s. A short, one-sided jet
extends from the core. The orientation of the jet varies smoothly, with
timescales of about 1yr and an rms of 6degrees about the mean of 50degrees.
Occasionally the jet appears to bend to the east. The length of the jet is only
about 1 mas (3,600 AU), and varies with an rms of about 20%. The inferred
speeds are below 0.08c. The total flux density of the core-jet varies
erratically, changing on occasion by a factor of two over a few weeks, without
any significant changes in the source size and orientation. The inferred
velocity of the plasma flow is >0.25c. The results are consistent with a model
in which plasma condensations with short lifetimes are ejected relativistically
from the core on a timescale of less than a few weeks, and then travel along a
tube whose pattern and geometry are also variable but only on a timescale of
about one year. The central engine of M81 has qualitative similarities to those
of powerful the AGN of radio galaxies and quasars, and may also represent in
power and size a scaled-up version of the largely hidden nucleus in our own
Galaxy.
