Astro2110 – The Solar System

When I was a kid, I thought stars were mysterious, distant, beautiful little things in the sky. I often pictured them as little beacons far out into the universe where the “spacemans” – a cute figment of my imagination – use to locate themselves once they get lost in the dark. A polar opposite of the tranquil stars, I thought, was the fiery Sun. I always imagined the Sun to be a giant ball of red fire – a fire-breathing dragon perhaps. I was completely shocked the first time I was told that they are actually the same thing – that both the Sun and the stars were spheres of extremely hot and dense gas emitting extremely hot light.

In their initial formation, the prototypes of stars accrete matter, faster and faster as its mass increases and gravitational forces become stronger. Gravity pulls matter closer to its center and the dense core begins to get hotter and hotter – at some point the inner pressure of the star cannot counter the gravitational force, and the protostar collapses. In this process, the core gets even denser until it is hot enough to perform hydrogen fusion, at which point the heat pressure inside the star is enough to counter the effect of gravitational collapse, and the star officially enters its main-sequence life stage.

Depending on its mass, the star can evolve into different astronomical bodies after it uses up its hydrogen. For small mass stars (like our sun), it will become hot enough to perform helium fusion but not more than that. When its core turns into a carbon ball, the star will officially be dead. Its outer layer will be carried out by stellar wind and become a planetary nebula where the matter will be reused, and its carbon core will be exposed, called a white dwarf.

For high mass stars, the star will continue to fuse heavier elements after helium – all the way until iron, where fusion does not create more energy anymore. The star will collapse and form a big explosion – supernova – and either turn into a neutron star or a black hole.