Stellar Lifecycle: Fusion

   Why do the stars shine? I have a very distinct and crisp memory of myself, at a mere age of seven, looking at the stars (and later, the Sun. By the way, that is not smart. My optometrist said it was likely my vision became so scarred because I looked at the Sun a bit too much.) and I would wonder What makes them shine? I knew the Sun was on fire, I knew it was flaming. But I also knew space lacked oxygen for fire to burn. This troubled me for a long time. Where did the Sun get its fuel from? When I was older, I received a book about stars. In the book, I learned stars were powered by a powerful force: fusion.

   As mentioned in the previous post, the early protostar first lights up when it becomes dense enough and under enough pressure, that the gas heats up and the hydrogen atoms and helium atoms start colliding into one another at super fast speeds. These collisions build up nuclei for heavier elements over time during stellar nucleosynthesis. The collisions also release huge amounts of energy. Stars are in a runaway state of constant fusion, they are basically one big hydrogen bomb. 

Deuterium and tritium (isotopes of hydrogen) combine into each other and form helium. In the process, energy is released, as well as a neutron

   

   So fusion creates energy for the star to shine. Not all stars burn their fuel in the same amount of time, it depends on the initial mass of the clump from which the star arises. Stars with higher mass burn their fuel more rapidly because they put more pressure at the core and fusion takes place at a more rapid pace. Lower mass stars last longer, usually billions of years (Like the Sun). The high mass stars in turn may only last a couple million years. During this time when hydrogen converts to helium, different stages of fusion must happen for the star to keep burning. Hydrogen starts to run low and different elements formed by early fusion slam into each other to make still heavier elements. Over time, stars forge these elements more and more. The higher mass stars get hot enough to sustain fusion up to the element iron. Then, iron begins to absorb the heat from the high mass star. All throughout its life, the star has been in a balance: it wants to blow apart from the heat released by fusion and it wants to collapse because of its own gravity. Iron absorbs that heat and suddenly the star begins to collapse. Gravity wins and the stars crushes down to an infinitesimally small point, a singularity. In this very short time, the star fuses heavier than iron elements, a last spurt of fusion. Then the implosion rebounds on itself and turns into an explosion. This explosion is known as a supernova. In it, the elements forged within the stars are thrown out into space, enriching it with elements that are heavier than hydrogen, helium, and lithium. Without this process, there would not be the necessary atoms to form people, planets, moons, comets, and all life in general. Carl Sagan was fond of saying "We are stardust." Truly, it seems we are children of the cosmos.

   In this post, I have outlined fusion. In the next post devoted to the stellar lifecycle, we will explore the deaths of stars and their stellar remnants. I have given a little hint of it in talking about supernovae, but I will be more detailed in the next post: Stellar Lifecycle: Death. Feel free to ask questions, share the blog with friends, or become a member. I'll post back soon, until then, salutations!