Life of Stars - Fate of stars

In the previous blog, we spoke about the birth of stars and how light was bought into our universe, which was merely an infant at that time. In this article we'll talk about the life of these stars and eventually, their fate after their long journey..

As you may remember, we discussed the birth and ignition of a star and we discussed nuclear fusion in detail as well. We also concluded with the sizes of stars, (small, medium and supermassive). What's next? How does different  size result in a different life and death of stars? We'll answer these questions and more in this blog.

Small sized stars

Small stars live a rather long and boring life. They live for hundreds of billions of years and are about quarter the mass of the sun (also called solar mass, which is approximately 2×10³⁰ kg). They start off as relatively cool and dim red dwarfs. As they age, their surface temperature increases and they become a blue dwarf. Eventually, as their light fades slowly, they cool down to become a white dwarf, which cools further to become a cold black dwarf. Small-sized stars thus die as cold black dwarfs.

Figure 1 - Black dwarf

Medium sized stars

For medium sized stars such as our Sun, the hydrogen keeps fusing to form helium until after a long time, the hydrogen in the core is exhausted and only helium is left in the core. Note that, stars exist because of the equilibrium of or balance between the outward force due to the fusion happening in the core, and the inward force due to the force of gravity that constantly tries to make the star collapse into itself. When the hydrogen is exhausted (and the fusion stops), the inward force due to gravity overwhelms the fusion-triggered outward force, and the the star's core collapses into itself and the star shrinks. As the star shrinks, it becomes hotter and denser.  Now something interesting happens. The hot and dense conditions allows the fusion of helium into carbon, the next heavy element. This fusion causes the star to rapidly expand again, and it becomes a red giant. 

The sun currently is in a perfect equilibrium state (between gravity and fusion). One day, the Sun reach a stage where its fusion stops, it shrinks before expanding again (due to reason I mentioned above). At some point, it will become big enough to swallow the inner planets, including earth. Well don't worry, this will happen after a very very long time and by then, humans would have found a habitable planet, (or become extinct but let's be optimistic). For a star, this is the beginning of the end. 

The outer layers of the star will move further and further from its core, and the gravitational force reduces gradually, to a point that the outer layers of the star completely separates itself and drifts away from its core. This results in the formation of a planetary nebula, which is one of the most beautiful objects in our universe. 

The core of the star becomes a white dwarf. It's approximately half the mass of the star but it's way tinier, in other words it's very dense. It's dead with no nuclear fusion taking place, and is very hot but will eventually cool down after millions of years to become a lifeless black dwarf (Figure 1).

Figure 2 - NGC 6326, an example of a planetary nebula

Figure 3 - NGC 2818, a beautiful planetary nebula in Pyxis

Supermassive stars

The death of supermassive stars is one of the most intriguing and fascinating events that occur in our universe. They live for the least time (compared to small and medium sized stars), few hundred million years, and are far larger than our sun. 

Like medium sized stars like our sun, the hydrogen in the core of these stars run out eventually, but, these stars, due to their sheer size and great amount of heat in them, can fuse heavier and heavier elements such as carbon, oxygen etc. 

 

 

Figure 4 - Core of a supermassive star 

Hydrogen is at the surface, still fusing to helium; a little further down, helium fusing to carbon and oxygen; further down we have silicon until the core, where silicon fuses to iron. Once the silicon burning phase has produced an iron core, the fate of the star is sealed. It triggers the death of the star.

Note that star expands because the outward force from fusion overpowers the gravitational force and the star keeps growing larger and larger as heavier elements fuse. However, once iron is reached, fusion is halted since iron is so tightly bound that no energy can be extracted by fusion. Iron can fuse, but it absorbs energy in the process and the core temperature drops. Iron is considered as nuclear waste. 

An inert iron core builds up at this time where successive layers above the core consume the remaining fuel of lighter nuclei in the core. The core is about the size of the Earth, compressed to extreme densities,  The outer regions of the star have expanded to fill a volume as large as Jupiter's orbit from the Sun. Since iron does not act as a fuel, the burning stops.

Within a fraction of a second, as the balance between the energy released by fusion and the inward force of gravity is broken, the core collapses due to the sheer gravity and the star collapses into itself and BANG! A massive explosion occurs and a massive amount of light and heat is released. This  explosion, called a supernova, releases so much light that it outshines the light of the entire galaxy of lakhs of millions of stars! 

  

Figure 5 - A supernova

This supernova forms two fascinating objects - black holes and neutron stars. 

The pressure causes protons and electrons to combine into neutrons forming a neutron star. The energy released in the process blows away the outer layers of the star far into space.

What happens next depends on the mass of the neutron star. If it is less than about three solar masses it remains as a neutron star.

If the neutron star is more than about three solar masses then the pressure exceeds the neutron degeneracy pressure. This causes the neutron star to collapse into a black hole. 

These will be elaborated in the upcoming blogs, where we will discuss theses fascinating bodies..