[lunar11 (OP)]

I have heard that gamma rays that originate from the Sun or other stars does not penetrate the atmosphere. Why is this?

[Vern]

Most gamma rays are absorbed in the atmosphere. They interact with atoms in the atmosphere and can produce ions by dislodging electrons from the atoms. These secondary ions and electrons can rain down on earth.

Atmospheric penetration depends mostly upon the gamma ray frequency. Some frequencies do penetrate the atmosphere. The reason that most gamma rays do not penetrate the atmosphere is that they interact with atoms and molecules in the atmosphere. But I am not an expert on gamma ray atmospheric interaction. There is probably a better answer.

Nasa has a few observations about gamma rays.

Very few gamma-rays make it through the atmosphere. The atmosphere is as thick to gamma-rays as a twelve-foot thick plate of aluminum. Gamma-rays are very very unlikely to go through that much material. However, they can strike the material and produce 'secondary' particles which are more penetrating, and can go through the material.

Most of the cosmic rays which reach the Earth's surface are 'secondary cosmic rays', produced by gamma-rays or (much more commonly) 'primary cosmic rays' hitting the top of Earth's atmosphere. These primary cosmic rays are high energy particles (such are protons and the nuclei from iron atoms) moving at very close to the speed of light. These primary cosmic rays have a hard time even getting to the top of our atmosphere--the Earth's magnetic field deflects most of them away. If Earth didn't have a magnetic field, there would be many more primary cosmic rays hitting the atmosphere, and many more secondary cosmic rays hitting us.

[Mazurka]

This link

http://www.geo.oregonstate.edu/classes/geo444_544/LECTURES/lecture3.pdf

explains the background science very well.

For those who cannot be bothered with words, page 20 has a neat diagram showing which parts of the Electromagnetic scpectrum are absorbed by the atmsophere...