NGC 3166 & NGC 3169 -- Spiral Galaxies in Sextans
Two Different Identical Galaxies in the Sextant
With just my eyes the constellation Sextans (the Sextant) is a no starter from my suburban
backyard. With its brightest star shining at only magnitude 4.5 the entire constellation is a
back void in the night sky between Leo and Hydra, showing no stars from the deck of my
house. However, it does have a couple deep sky galaxies that are within reach of my
155mm refractor. For the interacting galaxy pair, NGC 3166 and NGC 3169, I had to start
my star hoping from the bright star Regulus in the Lion. It was a long star hop but was not
difficult and took less time than I expected to reach the two galaxies.
What was most interesting to me was that the two galaxies (based on the data) should
appear in a mid-sized amateur telescope as identical. Both are approximately at the same
distance, size, galaxy type, surface brightness and magnitude. But for some reason, NGC
3166 appeared far brighter than NGC 3169. Both did appear around the same size and
shape. And both showed a slightly brighter nucleus and a dimmer surrounding disk, but
NGC 3166 (magnification 10.4 -- surface brightness 12.9) was seen easily while NGC 3169
(magnification 10.2 -- surface brightness 12.9) took more effort to tease out at the eyepiece.
The only explanation that I can think of, for the apparent differences in brightness, was that
NGC 3169 had a star just to its east and NGC 3166 had no nearby star. My eye may have
confused the brightness of the star with the adjacent galaxy and my brain interpreted the
galaxy as being less bright than it really was. Anyway, that is my best guess.