[madbadgalaxyman (Robert)]

The small bluish knots at the end of one of the tidal tails are a really tough target. They are pretty obvious on some of the old blue-sensitive photographic plates, but not so obvious in CCD images.

The best I have seen them in an amateur image is probably at Rob Gendler's website.

The case for these knots being bona fide areas of very hot & very luminous OB stars is greatly strengthened when we consider the GALEX (Far Ultraviolet plus Near Ultraviolet) image of NGC4038/9 :



Blue in this image codes for Far-ultraviolet light.
(see the GALEX "how to" in the Observational Astronomy forum )
Note how there are star forming knots all along the tails!
They must be very faint indeed in visible light.....

Incidentally, the knots correspond well with those areas where the HI (cold neutral atomic hydrogen) in the two tidal tails is most dense....so there appear to be adequate raw materials (gas) to produce star formation in the tails.