Astronomy Study Guide

Stars with a mass significantly larger than the Sun follow a different evolutionary path due to their greater gravitational forces and higher internal temperatures and pressures. These massive stars begin their lives, like all stars, from a collapsing cloud of gas and dust, forming a protostar. Once nuclear fusion of hydrogen into helium begins in their core, they enter a main sequence phase, burning fuel at a much faster rate than smaller stars. After exhausting the hydrogen in their core, they expand into a red supergiant. The core then contracts and heats up, enabling the fusion of heavier elements, such as carbon, oxygen, and eventually iron. Crucially, fusion beyond iron does not release energy but consumes it. This leads to a catastrophic core collapse. The outer layers of the star are then ejected in a spectacular event known as a supernova, leaving behind either a neutron star or, if the initial star was extremely massive, a black hole.

The discovery of Cosmic Microwave Background (CMB) radiation provided crucial evidence that solidified the Big Bang theory as the most widely accepted model for the Universe's origin. Before its detection, the Big Bang theory predicted that the early Universe was extremely hot and dense, and as it expanded, it would cool down. This cooling process would leave behind a faint glow of electromagnetic radiation, stretched to microwave wavelengths due to the Universe's continued expansion. In 1964, Arno Penzias and Robert Wilson, while working with a new antenna, detected a persistent background noise that they could not eliminate. This noise was isotropic, meaning it came from all directions, and had a temperature of about 2.7 Kelvin, corresponding to microwave radiation. This accidental discovery perfectly matched the predicted properties of the leftover radiation from the Big Bang. The existence of CMB radiation, acting as a 'fossil' of the early Universe, strongly supported the idea of an initial hot, dense state, offering compelling observational proof that the Universe indeed originated from a singular, extremely hot event and has been expanding and cooling ever since. This discovery provided a more robust explanation for observations like redshift and the abundance of light elements, integrating them into a cohesive cosmological model.