The New Astronomy One cannot perform laboratory experiments on stars and galaxies. For this reason, astronomy has always been a science of careful observation. Each kind of electromagnetic radiation moves at the speed of light, while cosmic rays are slightly slower.
Electromagnetic radiation is electrically neutral, but cosmic rays carry an electric charge.
As a result, light can travel through space in essentially straight lines, but cosmic rays spiral along the weak lines of magnetic force that permeate space. We can see where a beam of light or of X-rays has come from, but because of their spiralling, cosmic rays cannot be traced to their points of origin. Before the Space Age, all astronomy was performed on the ground, limited by the Earth's atmosphere. Cosmic rays could not be observed directly, but it was possible to study the showers of energetic charged particles that they produce when they strike the atmosphere. The atmosphere absorbs almost all of the radiation that reaches the Earth from space, so stars and galaxies could only be seen at the limited wavelengths to which the atmosphere is transparent, primarily visible light and radio waves.