  NGC 6946, known as the Fireworks galaxy because of its high supernova rate
and high star formation, is embedded in a very extended HI halo. Its northern
spiral arm is well detached from the galactic main body. We found that this arm
contains a large (~300 pc in size) Red Ellipse, named according to a strong
contamination of the H-alpha emission line on its optical images. The ellipse
is accompanied by a short parallel arc and a few others still smaller and less
regular; a bright star cluster is seen inside these features. The complicated
combination of arcs seems to be unique, it is only a bit similar to some SNRs.
However, the long-slit spectral data obtained with the Russian 6-m telescope
did not confirm the origin of the nebula as a result of a single SN outburst.
The emission-line spectrum corresponds to the photoionization by young hot
stars with a small contribution of shock ionization. The most likely
explanation of the Red Ellipse is a superbbuble created by a collective
feedback of massive stars in the star cluster located in the NE side of the Red
Ellipse. However, the very regular elliptical shape of the nebulae seems
strange.
