yes, well the stars can be one because you can see it with a naked eye
To date, none have been observed.
Hydra is found in the constellation called Phoenix
M83, also known as the Southern Pinwheel Galaxy
"Magnitude" is usually indicated for individual stars, not for an entire constellation like Eridanus. You could add the magnitude of the 100 or 1000 brightest stars or so, for any constellation (after converting from the magnitude scale, which is logarithmic, to a linear scale), but since this serves no real purpose, this is not something that you find published anywhere.
The river Nile does not have any special features.
no there are no special features found on mars...
yes it does have special features
There is no star by that name. There is a constellation with the name Eridanus. That constellation, like any constellation, contains many stars, at different distances.---ANSWER---Just a sub note.I believe what you may be refering to in the question is the star Epsilon Erandi. At 10.5 ly away i believe it is the closest star to have confirmed extra solar planets orbiting it.
no
no there are not.
yes
I was expecting for a answer