Your Life as a Star

By Kay Yu

You are but dust and gas, floating in clouds that stretch as far as you can see. In fact, the

mass of the clouds, called molecular clouds, could be from 1000 to 10 million times the

mass of the Sun. But you are far, far away from the Solar System in which the Sun resides.

You are inside a clump of gas, cold and dense. The clump grows in mass as it collides with

other clumps and collects more matter. Suddenly, the clump collapses! You are compressed,

heated, and become a proper baby star. The actual name is protostar – and now you have

started your life as a star. Around you form other stars and planets.

You continue collapsing under the force of gravity. You grow hotter and hotter, until the

temperature rises enough for the hydrogen nuclei inside you to fuse together and make

helium. This nuclear fusion process is your energy source, and you stop collapsing due to the

released energy and heat. While nuclear fusion of hydrogen into helium continues inside you, you are called a main sequence star. You will remain in this phase for the longest time in

your life as a star. You are in something called hydrostatic equilibrium, which means that the

gravitational force from your mass and the gas pressure from the aforementioned energy

generation in the core is in balance.

Eventually, your core runs out of hydrogen. What now? The gravitational force is no longer

balanced out, so your core starts to collapse. Due to the heat energy produced by the

collapsing core, hydrogen converts into helium outside the core. So overall, your size

increases. What you become now depends on whether you have a lower mass or a higher one.

I. If you’re a low-mass star

Your atmosphere expands until you become a subgiant or a giant star. You may

pulsate, unstable, and periodically inflate and deflate, in order to remain in

hydrostatic equilibrium. In the end, your outer layers are ejected into space, and a

planetary nebula, made up of gas and dust, is created. Only your core is left,

collapsing, and is now a white dwarf, small and dense.

II. If you’re a high-mass star

Inside you continues nuclear fusion, producing heavier elements such as carbon,

silicon, and iron. When silicon fuses into iron in the core, you start collapsing

until the amount of energy created leads to a massive explosion. This is called a

supernova, and from it material is flung into the universe. Your core continues

collapsing and becomes a dense neutron star or an even denser black hole.

Materials from planetary nebulae and supernovae help form molecular clouds, leading to the

births of stars and planets. This is just one of the many beautiful cycles of the universe.