simplifying expressions - how to take complex conjugate of a real variable

I have a function defined as follows:

g[θ_] := 1/k (Exp[-I δ0] Sin[δ0] + 3 Exp[I δ1] Sin[δ1 Cos[θ] + 
            5/2 Exp[I δ2 (3 (Cos[θ])^2 - 1))

and I compute the product of g with its complex conjugate as follows:

f[θ] Conjugate[f[θ]]

which gives me:

4.18986*10^-30 ((0.453154 - 0.288691 I) + (0.443562 - 0.0670825 I) Conjugate[Cos[θ]] + 
 (2.49994 - 0.0174532 I) (-1 + 3 Conjugate[Cos[θ]]^2)) ((0.453154 + 0.288691 I) + 
  (0.443562 + 0.0670825 I) Cos[θ] + (2.49994 + 0.0174532 I) (-1 + 3 Cos[θ]^2))

However I want theta to be real so that Conjugate[Cos[θ]] is just Cos[θ] and likewise for Conjugate[Cos[θ]]^2.

How can I do this?

EDIT

I should say my end goal is to plot and integrate the product of g with its product. I've tried what Rashid Zia suggested like so:

Plot[
 Simplify[g[θ] Conjugate[g[θ]], θ ∈ Reals], {θ, 0, 2 Pi}, 
 AxesLabel -> {rad, m^2/sr}
]

but the plot doesn't look like what I'm told it should, and the integral:

Integrate[2 Pi Simplify[g[θ] Conjugate[g[θ]], θ ∈ Reals], {θ, 0, Pi}]

doesn't seem to return the expected value.

Is there something I am missing here?