1. Open a new Document

2. Use the Elliptical Marquee Tool to create a circular selection (keep the Shift key pressed to draw a circle).

3. Now use a 2-color linear Gradient Tool, with Mode parameter set to Difference, and play with it until you are satisfied with the result.

4. Create a new layer, reset the swatches (press 'D'), keep the previous selection and go to Filter > Render > Clouds.

Set the blending mode of this layer to Color Burn and its opacity to about 80%.

5. Create a new layer, keep the circular selection, fill it with black, then modify the layer style by adding a inner shadow effect.

Click here to see the settings

Set the blending mode of this layer to Soft Light

6. If you want to add some lighting effect, create a new layer, select all the canvas, fill it with black and go to Filter > Render > Lens Flare.

Set the blending mode to Lightnen

7. In order to create an additional soft light effect, create a new layer and apply a radial gradient (see picture on the right).

Set the blending mode to Lighten and the Opacity to about 50%.

8. That's it! If you like you can add a backgorund layer full of stars...