The Milky Way contains several thousand giant molecular clouds. Describe a giant molecular cloud and the role it plays in star formation.

Stars are born in vast, cold regions called giant molecular clouds (GMCs). GMCs are the primary sites for star formation in galaxies. Here, dense regions within these clouds begin to collapse under their own gravity. This collapse initiates a chain reaction.
First, small fragments form within the larger cloud. These fragments, known as clumps, continue to shrink and heat up. Over time, the core of these collapsing fragments becomes hot and dense enough for nuclear fusion to begin. This core is now a protostar, which will gather mass from the surrounding cloud material as it evolves into a full-fledged star. This process explains how massive clouds, spread over hundreds of light-years, turn into stellar nurseries.

Inside giant molecular clouds, the primary component is molecular hydrogen (H₂). Molecular hydrogen forms under the extremely cold temperatures present in these clouds, ranging from 10 to 30 Kelvin.
Cold temperatures protect the hydrogen molecules from being broken down by ultraviolet radiation. These temperatures allow hydrogen to bond into molecular form which is more stable. Molecular hydrogen is crucial because it acts as the raw material for forming new stars. Without this molecular hydrogen, the dense regions that collapse into stars could not form.

Gravitational collapse is the driving force behind star formation. In giant molecular clouds, certain regions become denser than others. Such dense regions pull in surrounding material due to gravity, starting the collapse process.
As collapse continues, these regions fragment into clumps that keep contracting. The gravitational energy gets converted into heat energy, causing the core to heat up. When the core temperature and pressure rise sufficiently, nuclear fusion begins, creating a protostar. Gravitational collapse explains how giant clouds can initiate and sustain the formation of stars.

The interstellar medium (ISM) is the matter that exists between stars in a galaxy. It consists of gas, dust, and cosmic rays. GMCs are part of this vast ISM, differing due to their higher density and cooler temperatures. The ISM plays several roles:
- Provides material for star formation.
- Distributes heavy elements produced in stars.
- Affects galactic dynamics and evolution. The interplay between GMCs and the ISM drives the cycle of star birth, death, and the creation of new molecular clouds. This cycle keeps the galaxy evolving and contributes to its structure and composition.

Giant molecular clouds and their star-forming activity greatly influence the structure of galaxies. New stars formed from these clouds emit energy and light, shaping the galactic appearance. Additionally, the explosions of dying stars, such as supernovae, scatter heavy elements into the ISM, enriching future generations of stars.
These stars, clouds, and elements reside in various parts of the galaxy, forming spiral arms, halos, and other galactic features. The continuous process of star formation and destruction drives the dynamic evolution of these structures, contributing to the ongoing growth and complexity of galaxies like the Milky Way.