On July 4, 1934, a significant event occurred in the history of physics and astronomy. On this day, American astronomer Walter Baade and Swiss astronomer Fritz Zwicky published a paper in the Proceedings of the National Academy of Sciences, proposing the existence of neutron stars. This groundbreaking idea came just two years after the discovery of the neutron by James Chadwick.

In their paper, Baade and Zwicky argued that when a massive star exhausts its nuclear fuel and undergoes a supernova explosion, the remaining core could collapse into an extremely dense, compact object composed primarily of neutrons. They suggested that this "neutron star" would have a diameter of only about 10 kilometers but a mass comparable to that of the Sun.

The concept of neutron stars was initially met with skepticism, as the idea of such dense objects seemed far-fetched at the time. However, their existence was confirmed in 1967 with the discovery of pulsars by Jocelyn Bell Burnell and Antony Hewish. Pulsars are rapidly rotating neutron stars that emit beams of electromagnetic radiation, which appear as pulses due to the star's rotation.

The discovery of neutron stars has had a profound impact on our understanding of the universe. These objects provide valuable insights into the behavior of matter under extreme conditions, such as incredibly high densities and strong magnetic fields. They also play a crucial role in the study of gravitational waves, as the merger of two neutron stars can produce detectable ripples in spacetime.

Baade and Zwicky's proposal of neutron stars on July 4, 1934, was a major milestone in the history of science, paving the way for future discoveries and a deeper understanding of the cosmos. Their work demonstrates the power of theoretical predictions and the importance of pushing the boundaries of scientific knowledge.