Scientists say that a ten-second burst of gamma rays from a massive star explosion within 6,000 light years from Earth could have triggered a mass extinction hundreds of millions of years ago. In this artist&#39;s conception we see the gamma rays hitting the Earth&#39;s atmosphere. (The expanding shell is pictured as blue, but gamma rays are actually invisible.) The gamma rays initiate changes in the atmosphere that deplete ozone and create a brown smog of NO2. Credit: NASA