ABSTRACT
After the Big Bang, the Universe was composed of subatomic particles and radiation only. What happened next is still unknown – the first possibility is that small particles slowly get together and gradually formed stars, star clusters and eventually galaxies. The second possibility is that the Universe first organizes as large clumps of matter that later subdivided into galaxies. Milky Way galaxy is typical: It has hundreds of billions of stars and has enough gas and dust to make billions of more stars. Also, it has ten times as much dark matter as all the stars and gas put together. The Milky Way has a spiral shape, like more than two-thirds of all known galaxies. Other galaxies have elliptical shapes, and a few have unusual shapes such as toothpicks or rings. Magnetic fields are the most crucial player in the interstellar medium of many galaxies. Although the initial discovery of cosmic rays dates back to more than a century ago, their origin is still one of the most enduring mysteries in physics. Regardless of the details of the acceleration mechanism, cosmic rays and magnetic fields in the galaxy are crucial factors in the equilibrium of interstellar gas.
