The Life of a Star
No, this is not a TLC featurette on some Hollywood actor, nor is this a biography of a Rockstar, this is a story of how the sparkles in the night sky come to be, and how they go away.
The story begins in a nursery, a “star nursery”. The nurseries, also called stellar nebulae (nebulae is plural for nebula), are giant clouds of hydrogen gas. These clouds are really big, stretching across trillions of kilometres! The path that a star’s life takes depends on how much gas there is in these nebulae.
First, the hydrogen gas begins to get pulled together because of its own gravity. Once the gas gets pulled further, it forms a huge ball that starts spinning. As the ball of gas spins faster, it gets hotter. Now, a protostar is born.
Once the protostar gets hot enough, something really interesting happens. At 15 million degrees, the hydrogen atoms (atoms are really small particles that everything in the universe is made of) that are in the centre of the protostar begin to merge together to make atoms of Helium! This is called nuclear fusion, which is an incredible phenomenon that creates heat and light. The gas ball starts to glow brightly and is now called a star. It stays like this for billions of years, fusing its hydrogen atoms to create light. Yes, our very own sun is a star like this, the closest one to us.
Beginning of the end
The hydrogen in the star’s centre cannot last forever. Once it starts to run out, less and less heat and light is formed, and the centre starts to shrink. The outer layers of the star still have hydrogen in them, and so they start expanding, getting bigger and bigger, until the star is 5 times its original size. The star is now much cooler than it used to be (still really hot, though). It now gives out a reddish light and so is called a red giant.
Now, here is where the path diverges. There are two ways a star dies, one admittedly more spectacular than the other.
Stars, like our sun, that were born from smaller nurseries, continue to shrink in their core. There is no hydrogen left in the centre, so the helium starts to fuse to form other elements, like carbon. Once the core has shrunk to its limit, the outer layers are expelled from the star. The outer layers end up forming a different type of nebula called a planetary nebula. The inner core of the star becomes a white dwarf, that eventually cools to become a black dwarf. Black dwarfs don’t produce any heat or light.
Stars that were born in huge nurseries, take a different route. In the centre of these massive stars, fusion doesn’t stop at carbon, it keeps going. The centre continues to shrink until iron is produced through nuclear fusion. It takes a lot of energy to fuse iron. The star doesn’t have this amount of energy, so it begins to shrink drastically trying to push its iron atoms together. But this dramatic collapse makes the centre very unstable and it explodes in a massive, breathtaking supernova! The heat and energy that comes out of this explosion is so huge that it makes a whole lot more elements through nuclear fusion.
Sometimes, the remnants of the explosion get pulled together to form a neutron star. Neutron stars are known to give out X-rays. Other times, if the star that exploded was large enough then the remnants of the explosion get pulled together to form a black hole.