Space Physics and Astrophysics - SS2 Physics Past Questions and Answers - page 3

Stars are formed from the gravitational collapse of giant molecular clouds consisting of gas and dust. As the cloud collapses, it forms a protostar, where nuclear fusion reactions begin in the core. These reactions convert hydrogen into helium, releasing immense amounts of energy in the form of light and heat. This energy counteracts the gravitational force, leading to a stable star.

During its evolution, a star goes through various stages depending on its mass. For low- to medium-mass stars, like the Sun, the main sequence stage is the longest, where hydrogen fusion occurs in the core. As hydrogen is depleted, the star expands into a red giant, fusing helium into heavier elements. The final stages depend on the star's mass, with low-mass stars forming white dwarfs and high-mass stars undergo supernova explosions, leaving behind remnants like neutron stars or black holes.

Galaxies are vast systems of stars, gas, and dust held together by gravitational forces. They come in different shapes and sizes and can be broadly classified into three main types: spiral, elliptical, and irregular.

Spiral galaxies, like the Milky Way, have a central bulge surrounded by spiral arms. They contain a mix of young and old stars, as well as dust and gas. Elliptical galaxies have a more rounded shape and are typically composed of older stars. They have less interstellar matter and are often found in galaxy clusters. Irregular galaxies lack a defined shape and may result from gravitational interactions or mergers.

Galaxies play a crucial role in the structure of the universe. They are the building blocks of large-scale structures, such as galaxy clusters and superclusters. Gravity acts on these structures, driving the formation of filaments and voids in the cosmic web. Understanding the properties and distribution of galaxies helps astronomers unravel the large-scale structure and evolution of the universe.