Large intense white bright light in the late evening of May 29, 2012, near Hamilton, MT

On the night of May 29, 2012, around 11:30 PM, I witnessed a bright light in the sky near Hamilton, MT, the Bitterroot Valley.  I went outside to have a cigarette, and I looked up at the stars, turned my head down and looked up again, and an extremely white light was shining-for example, say the stars are a size of a dime, and this light was the size of a grapefruit. 
It lasted only about 10-15 seconds, and then slowly diminished.  It did not move right or left or up and down; after it disappeared an aura remained that was rather light bluish.  
Was it an exploding star, I have no idea, other than it was remarkable-the brightness was crystal white, brightness I have never seen before.  It was round with feathered light near the outside; hard to explain.  I was stunned and shocked.
I checked out images of exploding stars on the web and there are a few that resemble what I saw-the bright white intense light.  I just have to report this, since I have never seen anything like this before.
Witness elects to remain totally anonymous; provides no contact information.  PD