Birth of Stars

Stars are born in giant clouds of gas and dust scattered across the galaxy, called interstellar clouds. These clouds are enormous, spanning several light-years in size. To put this into perspective, a light-year is the distance light travels in one year, which is about 9.5 trillion kilometres (or 9.5 x 10¹² km). This means it would take several years for light to travel from one end of an interstellar cloud to the other, highlighting their immense size.

• Interstellar clouds are the birthplace of stars and can span several light years.
• Gravity causes these clouds to contract, increasing their density and temperature.
• When nuclear fusion begins, the star starts emitting light and becomes self-luminous.
• Stars are essentially gigantic spheres of hot, glowing gas powered by nuclear fusion.

A picture of interstellar clouds taken by the Hubble space telescope

1. Interstellar Clouds: These clouds, made mostly of hydrogen gas and tiny particles of dust, exist in the vast spaces between stars in a galaxy. They are cold and spread out, but something-a shockwave from a nearby star explosion or gravitational disturbances-causes parts of the cloud to start contracting.
2. Contraction of the Cloud: When a section of the cloud begins to collapse due to gravity, it pulls in more gas and dust. As it contracts, the material becomes denser, and the gas particles collide more frequently, causing the temperature to rise.
3. Formation of a Hot, Dense Sphere: The collapsing cloud continues to shrink, forming a dense, hot core at its centre. This core is made of gas that is heating up rapidly due to the compression caused by gravity.
4. Nuclear Fusion Starts: When the core becomes extremely dense and hot (millions of degrees), nuclear fusion begins. In this process, hydrogen nuclei (protons) combine to form helium nuclei, releasing a tremendous amount of energy in the process. This energy is what makes the star shine.
5. A Star is Born: Once nuclear fusion starts, the gas sphere stabilises and becomes self-luminous, emitting light and heat into space. This marks the birth of a star. The energy from fusion counteracts the force of gravity, preventing further collapse, and the star enters its stable phase.