Start a new document about 800x800. We will intially make it large, then scale it down. It looks better that way.

- Create a new layer.

- Then use the eliptical marquee tool, and while holding down shift, create a circle selection. Make the selection almost as big as your canvas.

Then go to Image->Adjustments->Levels.
Move the center and right slider towards the left, something similar to mine below:

Now go to Filter->Distort->Spherize:
Amount: 100%

Then Filter->Distort->Spherize again, but this time:
Amount: 50%

Then go to Image->Adjustments->Color Balance

Apply the following color levels:
Shadows: +100 | 0 | -100
Midtones: +100 | 0 | -100
Highlights: +70 | 0 | -15

Alternatively, you can use the Hue & Saturation to colorize your sun.

Now sharpen the image again. Filter->Sharpen->Unsharp Mask.
Amount: 300%
Radius: 3
Threshold: 15

CTRL+D to deselect

If you want to learn how to create stars, See the SIMPLE STARS tutorial