Life and Death of Stars

Just like the ecosystems of our planet, stars have a life cycle. The death of one star can mean life for another as the materials created when a star dies can be used to form a whole new star. 


Stars are created in many different sizes, and many are born with companions, forming a binary star system. Measuring the orbits of binary stars can provide evidence to determine the mass of the stars. In some nearby systems, the stars are close enough that we can see their positions change on the sky as they orbit. Other binary systems can be detected by observing the changes in light as one star moves towards us and the other moves away from us. And some binary systems show eclipses where one star occasionally blocks the light of another star as they orbit. Studying binary systems helps us to build models for stellar evolution as well as determining properties of the individual stars. On August 17, 2017, NASA’s missions Fermi, Swift and Chandra worked together with NSF’s LIGO to confirm a pair of orbiting neutron stars that collided, creating a huge explosion in both light and gravitational radiation.


Existing in the universe are huge molecular clouds consisting of dust and gas where there is a high density of Hydrogen and Carbon Monoxide compared to the rest of outer space. When the density of gas and dust becomes high enough in a certain region of the cloud, the molecules will begin to fall into a gravity well. When the density of dust is large enough, its gravity will exert a force causing the cloud of dust to collapse in on itself. Star formation begins.

The core becomes so incredibly dense and hot that a process of nuclear fusion may begin. The atoms within the core are literally packed so tightly      that they begin to fuse together. Nuclear fusion starts off as two hydrogen atoms forming a helium atom releasing large amounts of energy as they merge. This huge energy released by nuclear fusion manifests itself as what we know as starlight . It is the process that powers our very own Sun. Nuclear fusion’s resulting energy prevents further gravitational collapse by providing an opposing outward force.


Stars in the middle of their lifespan are called main sequence stars meaning that the star is still fusing hydrogen to make helium. Main sequence stars come in all sorts of sizes, colors and temperatures. The hotter a star is, the more blue light it emits because that color is on the more energetic side of the visible spectrum. The cooler a star is, the more it tends towards the red side of the visible spectrum. 

This explains why when stars become older they become more red; as they cool they emit light at a lower energy because they are no longer as hot or energetic due to a slowing of nuclear fusion. As the star runs out of hydrogen, and the core is primarily helium, hydrogen fusion occurs primarily on the surface of the core. This outward force against the core causes the star to start to expand outwards, growing in size. 


Depending on the initial size of the star it will either become a red giant or a red super giant. When the rate of nuclear fusion is no longer enough to overcome the force of gravity on the star it will collapse in on itself, and depending on its size lead to different cosmic events. A red giant, such as Betelgeuse found in the constellation Orion, will eventually collapse and explode producing a planetary nebula that ultimately yields a whit dwarf. Red super giants are a bit of a different story. Since they have so much more mass, they experience more intense gravity making their collapse far more energetic. The death of a red supergiant leads to a type two super nova. Two unique remnants can be left behind from this huge explosion; neutron stars and black holes.