Start representing the sun. Create firstly a round marking with Elliptical Marquee Tool, pressing the shift button and paint it out with the Gradient Tool, changing the gradient's type. Create after that a new layer again and represent on it an oval which must be painted too. Make many copies of the layer in case: Layer ? Duplicate Layer (ctrl+j). Use then the next option Edit ? Free Transform (ctrl+t) to place the rays around the sun.



Insert the flares, applying the same instruments.



Create a new layer again and group it out with the layer the sun is situated on: Layer ? Create Clipping Mask (ctrl+g). Create one more marking on the sun's surface and paint it out in white color. Change the layer's Blending Mode on Soft Light.



Combine all the layers composing the sun: Layer ? Merge Down (ctrl+e), and make a copy of the new layer (ctrj+j) and then make the lowest copy look totally white, using the next option: Image ? Channel Mixer:



Blur it out with Filter ? Blur ? Gaussian Blur.



Then make two copies of the layer just to make the luminescence more intensive: