Employing Bohr's quantum theory, the author deduces three limits, which correspond to the magnetic fields of white dwarfs, neutron stars and the strongest in the universe. The author discusses the possible origins of magnetic fields due to collapse of stars, which produces a magnetic field of 10[superscript 8] T. Although the complete analysis involves quantum mechanics, with the limitation of Bohr's theory, it is very useful for students (in high school or as undergraduates) to understand magnetic fields in the universe.