  The nuclear spectrum of M87 covering the Ly_a-H_a wavelength range was
obtained with the HST Faint Object Spectrograph (FOS) trough a 0.21 arcsec
aperture. Contrary to some previous claims, a single power law (F(nu)~nu^(-a))
can not reproduce the observed continuum shape and at least a broken power law
is require for a good fit (a = 1.75 and 1.41 shortward and longward of the
break at ~4500 A). We detect a set of broad (FWHM ~ 400 km/s) absorption lines
arising in the gas associated with M87. These are only lines from neutral and
very low ionization species blueshifted by ~150 km/s relative to the M87
systemic velocity, indicating a net gas outflow and turbulence. The excitation
sensitive emission line ratios suggest that shocks may be the dominant energy
supplier.
  The nuclear source in M87 is significantly variable. From the FOS target
acquisition data, we have established that the flux from the optical nucleus
varies by a factor ~2 on time scales of ~2.5 months and by as much as 25% over
3 weeks, and remains unchanged (<2.5%) on time scales of ~1 day. These
timescales limit the physical size of the emitting region to a few hundred
gravitational radii. The variability, combined with other observed spectral
properties, strongly suggest that M87 is intrinsically of BL Lac type but is
viewed at an angle too large to reveal the classical BL Lac properties.
