Giants

As a main sequence star extinguishes its supply of hydrogen it doesn’t just simply turn off and dissipate as one would expect, it changes and becomes a whole new type of star. A giant.

This is a stage that is reached very quickly by high mass stars, which fuse their supplies of hydrogen much faster than their low mass counterparts. The giants, often red in nature, and sometimes blue, can be considered ‘old aged’ in the life cycle of the celestial bodies.

To attain this stage a star, as we’ve said, near depletes its stores of hydrogen that it has been fervently using to fuse and maintain its huge size. At this point the internal pressure being generated to counteract the contractive gravitational force begins to falter. As a result the star rapidly shrinks in size until the heavier elements in the outer layers of the star, collide together and release a huge wave of radiation. This sudden outburst of energy allows the star to rapidly expand up to potentially hundreds times its original size. For context, when this happens to our sun in 5 billion years the surface will be where Mars is, and remember our sun is small compared to other stars.

This process occurs in stars that have masses between 0.4 and 8 times the mass of our sun, whereas for super mass stars, with masses exceeding 8 times the mass of our sun grow even bigger and become supergiants. Supergiants are exactly what they sound like. They are much, much, larger versions of giants, and fuse their material at far higher rates.

Within a giant of any kind hydrogen fusion stops and helium fusion begins. Soon after this process has begun the star can even become hot enough to begin fusing carbon, if the mass is high enough, if not it will move on to the next stage of its life.

At the end of this stage one of several things will happen, depending on the mass of the star. For low mass stars that became red giants, they will finish fusing helium in their core and collapse even further, shedding their outer layers into a planetary nebula. The stars will then collapse further and form into a white dwarf.

For high mass stars that have become supergiants, they will utilise the majority of the material within their core until they collapse so quickly that a supernova will occur. A supernova is essentially a huge explosion releasing as much energy as our sun would do in its main sequence life time. At this point the star will either form a neutron star, a black hole, or be completely eradicated and become on with the ISM, or a brand new nebula.