We live in a Universe that is filled with diverse celestial objects, and on a clear night, we can look up and see thousands and thousands of those adorning the beautiful night sky. We can see the stars, the planets, and even our own Milky Way galaxy and they are absolutely magnificent.
According to our current understanding, the Universe was dark and featureless for a long while, quite unlike what we see today. First stars did not appear until at least 100 million years after the Big Bang. Since we are inquisitive by nature, we often find ask ourselves wondering: how did this dramatic transition from darkness to light and everything we see come about?
Even though astronomers and cosmologists have made great strides towards unraveling various mysteries, the study of the early universe is hampered by a lack of direct observations. Scientists have devised a lot of simulations and have shown that how density fluctuations left over from the Big Bang coalesced into giant gas clouds and eventually into the stars we know. The studies also suggest that the stars in the early universe were quite massive and luminous than the stars that form today. Their formation was an epochal event because they paved way for a lot of transformations.
First Stars in the Universe
The early stars ionized the gases around them and the supernovae resulting from their deaths helped disperse heavier elements around the Universe. Being much more massive than the stars today, their collapse also led to the formation of supermassive black holes, similar to the ones in the galactic cores today. Since they were massive, their life cycle was also short and was quite violent during that lifespan.
In the early universe, the matter was distributed quite uniformly throughout the entire expanse of space. We call this expanse of hot gases the Primordial Soup. However, there were some density fluctuations; meaning there were some areas that had more matter than the others and this helped kick-start the process of the formation of stars. The gravity started pulling the cloud towards itself, slowly at first and then at a rapidly increasing rate. This increased the temperature in the location and gradually, the clouds coalesced into extremely hot and dense gravitationally bound structures. This is a positive feedback loop and the pressure kept on rising. At some point, due to the increasing temperature and pressure, the hydrogen atoms (which were much more abundant in the early universe) began to fuse together to form helium.
This fusion of hydrogen nuclei creates a net outwards pressure due to the tremendous energy output of the process. This pressure is what prevents further collapse of the cloud into itself and makes a stable celestial body. In fact, the size of a star is determined by this same balance between the inwards pressure generated by gravity and the outwards pressure generated by fusion. This balance is what the Scientists call hydrostatic balance. In the present day, the gas clouds or nebulae contain a significant amount of heavier molecules which lead to a lower ambient temperature in the area. The size of a star-forming clump is inversely proportional to the square root of the temperature and since the early universe did not contain larger molecules, the higher temperature meant much larger clumps were needed to form a star and hence, the earlier stars were quite significantly larger than present-day ones.
Stars, as mentioned, primarily generate energy by nuclear fusion of the hydrogen atoms. This leads to the release of a large amount of energy. This happens in the cores of the stars where both the temperatures and pressure are sufficiently high. The helium fuses into lithium by absorbing neutrons, which releases more energy and so on until we form carbon or oxygen. They are difficult to fuse in the cores of average-sized stars and hence collect in the center.
However, this core still has a gravitational pull. Since it does not contribute to any energy release, the rest of the star goes into a somewhat overdrive to balance the gravitational pull. The rate of consumption of hydrogen is increased and more and more iron keeps accumulating in the center. Slowly and slowly, the star is now running out of fuel in its core to burn. After it is all consumed, the gravity begins to pull the outer layers of the stars into itself. The layers thus pulled inwards bring along the hydrogen with them and the fusion continues in this ‘shell’.
Formation of a Red Giant
The hydrogen pulled into the core results in something known as ‘mirror principle’, which basically says that if an inner layer is pulled into the core, the outer layer must expand. The exact mathematics of the mirror principle is quite complex, but in a nutshell, it dictates that the outer layers must expand to conserve thermal and gravitational potential energy. The outer layers soak up energy in the form of heat from the shell undergoing fusion and start expanding into what we call a red giant.
A typical red giant has a radius of around a hundred times that of the initial star. The temperature is significantly lower though and hence it appears hazy-red or orange in appearance. This is what gives it the name. Stars that are much more massive transform into what we call super-giants. Their cores are at a much higher temperature and hence carbon nuclei fuse together to form heavier elements.
The End of Red Giants
Red giants are not the final state of a star. After the giant has consumed the majority of the hydrogen and helium left around the core, the core starts to collapse under its gravity. However, electron degeneracy comes into play and the star explodes into what we call a planetary nebula. It was thought in the past that these nebulae were responsible for planetary formation but we now know that it isn’t so. However, the name has stuck.
The outer layers of the red giant dissipate into space, while the core of the star forms what we call a white dwarf. They are hot, but not big in size. Our sun is expected to collapse into a white dwarf at the end of its life. It is estimated that its radius after the collapse would be twice that of the earth. A white dwarf is extremely dense; with its mass almost the same as the original star but the radius many many times smaller. No nuclear fusion takes place in them, and the luminosity of the white dwarfs is simply due to stored thermal energy.
It is expected that white dwarfs, after they have cooled off turn into what we call black dwarfs. The period of time it takes a white dwarf to cool down is longer than the age of the universe, and hence none are expected to exist presently.
Death of a Super-Giant
In the case of super-giants, the core is hot enough to continue fusion well after carbon has formed, and they lead to the formation of iron-peak elements. They are exceptionally stable and hence start accumulating in the core. The fusion of iron is in fact a net endothermic process and hence it has no contribution to energy production. To add, it is dense. The core accretes the hydrogen from surrounding layers until it runs out of it. The core begins to collapse and the star goes out in a massive explosion known as a supernova.
Supernovae are extremely luminous and can shine even brighter than entire galaxies for a while. However, the core is largely intact and it is out of fuel to counteract the gravity. The core starts to collapse into itself. There are two pathways this can take. If the star is not extremely heavy, the intense gravitational pull leads the collapse and the pressure gets so high that electrons, that are circling the nucleus now fuse with the protons to form neutrons. This exotic form of matter is what constitutes a neutron star. Further collapse is prevented by something known as neutron degeneracy which is a consequence of Pauli’s Exclusion Principle.
A neutron star is extremely dense. In fact, a chunk of a neutron star the size of a matchbox would weight around eight billion tonnes. A chunk of earth that occupies half a cubic kilometer of space would weigh around the same. The surface of the neutron stars is also extremely hot and they have really strong magnetic fields around themselves and ionize gases floating around themselves.
However, in extreme cases, the stars are so massive that even neutron degeneracy is insufficient to stop the further collapse. This process leads to the formation of what we call a Blackhole. A black hole is so dense that even light cannot escape the gravitational pull. It bends the space around it and at its core is what we call a singularity. They are some of the most extreme yet fascinating objects in the universe that we know of.
Black Dwarfs, Neutron Stars, and Black Holes are what astronomers call the corpses of the stars. It is strange, indeed. From a simple cloud of gases that was just floating around in space to some of the densest objects we can imagine, the life-cycle of a star is a truly magnificent journey through vast expanses of space and eons.