On visual inspection M94 appears to be a series of ring like structures. As one of the closest starburst-ringed galaxies it possesses one of the highest optical surface brightness nuclei known. At its center is a 1400 light year stellar bar which has been an important influence on the overall morphology of the galaxy. Surrounding the central bar is an inner stellar disk with a radius of about 2300 light years. Further out at a radius of about 3500 light years is an almost perfectly circular starburst ring. The bright ring hosts a variety of prominent HII regions, young supernova remnants, and bright young stars formed during the latest starburst epoch which began some 10 million years ago. The starburst ring is surrounded by an oval shaped massive disk of older yellowish stars and an outer ring of very faint disk material that has formed some 18000 light years from the center. The faint outer ring is visible in very deep images and is clearly present in the accompanying image.
Astronomers now understand that the association
of ring structures and nuclear bars is more than just coincidence.
The physics and evolution of bars in galaxies suggest they may
be the driving force for the formation of inner and outer rings
via the phenomenon of "bar mediated resonances". A
nuclear bar apparently creates a powerful torque on the disk components
of a galaxy which in turn alter the internal dynamics of mass
distribution within the galaxy. A redistribution of matter occurs
with the formation of ring like structures called resonance rings.
Gas inflows from elsewhere in the disk tend to concentrate within
inner and outer resonance rings. The inner nuclear starburst region
of M94 was presumably formed as a result of this process. Resonant
ring phenomena is likely the driver as well for the unusual oval
shape of the galaxy and the faint outer ring noticeable on deep
exposures. In addition to its starburst properties, M94 is thought
to have a weakly active galactic nucleus. It is also classified
as a LINER type galaxy (Low-Ionization Nuclear Emission-line Region),
a lower luminosity cousin of the Seyfert type active galaxy. LINER
type galaxies are a heterogeneous class of galaxies, and as a
group represent the most common type of galactic nucleus activity.
In some LINERs the photoionization is due to nuclear starbursts
but in others it occurs from a nonstellar source, presumably an
active galactic nucleus.