The star means to take the complex conjugate of the variable. So since , .
Here it is in its full glory:
The problem with leaving the expression like this is that there is a radical in the denominator, , which is typically removed. So if you multiply the denominator by the conjugate of the denominator (in this case rechristened as the "complex conjugate") we get:
of the electrical/electronic persuasion use j for the imaginary unit to avoid
confusion with the use of i for current (I had to go through the last paper I
sent for publication changing all the i's to j's as it was going to a
nominally electronic engineering journal, though as most of the stuff
published in it is related to DSP I think they are living in the past).
2. Often * is used to denote complex conjugate plain in ASCII maths.