POST 4:: #astronomyatschool #astronomywithrahul #astronomy #sciencepopularization #popularscience #CosmosMusings
There are a few questions that are often asked by children during the interactive session after a popular lecture. The two most common are (1) How and when will the world come to an end and (2) Are we alone in the universe.
So, that is what I will be doing today – discussing in a popular way, the final scene in the drama of the solar system and leave the second question for a later post. And if any of you is saying “I know this answer”, wait! There are two competing events.
Our life on earth is intricately entwined to the governor of the solar system – the Sun. The day the Sun ends its life, will also be the day life on Earth will come to an end or at the most, begin to come to an end.
When we look up at the night sky, we see little specks of light as if studded on a hemispherical dark canvas. In truth, neither are the stars all at the same distance away from us, nor are they the same size. Nature really has made all varieties of stars possible. While there are stars as small as about one-third the mass of our Sun, the other extreme takes us to astonishingly large stars, some even a few thousand times bigger than our sun! Then again, many of the stars have planets revolving around them like our sun and then there are systems of double and multiple stars which have planets revolving around them! What fun it would be to have multiple sunrises and sunsets, and how beautiful it would be! But that’s a story for another occasion.
When a star initially forms from a collapsing molecular cloud due to mass accretion, it contains primarily hydrogen and a little bit of helium. As the star grows in mass, gravity crushes the gases tighter and as a result the temperature at the core rises. When the core reaches a temperature high enough to begin fusing hydrogen, which is a few million kelvin, it establishes hydrostatic equilibrium. This is a state when the inward pressure due to gravity is counter balanced by the outward pressure due to the hot gases. The star slowly converts the hydrogen in the core into helium through nuclear fusion, during its lifetime. Once all the hydrogen is fused into helium, its main-sequence life will end. This will take about 10 billion years for our Sun. Heavier stars burn much faster, converting more mass into energy per second and so, have a shorter lifetime than lighter stars.
Thus, the fate of a star is determined by its initial mass. The evolution and final death of a star takes a route depending on its initial mass. Stars that are 0.3 to 8 solar masses, and are in the final stages of stellar evolution are typically what we call Red Giant Stars. Our Sun will end as a Red Giant. In Red Giant stars, hydrogen has been used up and converted to helium in the core. The temperature not being high enough for the fusion of helium to take place, the reaction stops and the core begins to cool. Gravity had patiently been waiting for this opportunity. Throughout the life of a star, it is a dynamic tussle between gravity trying to pack the star into a tighter, denser ball and the hot gases in the core trying to prevent the crushing, thus achieving what is called the hydrostatic balance. When the fusion reaction stops, the outward pressure weakens and gravity crushes the star into a tighter mass. The layer just outside the core now has conditions to start fusion of hydrogen. But because the inner core has collapsed, the outer layers must expand in order to satisfy simultaneous conservation of gravitational and thermal energy in the star.
For the Sun and stars of less than about two solar masses, gravity will crush the core to become dense enough, so that electron degeneracy pressure will prevent it from collapsing further. This collapse will further heat the core to about 10^8 K, hot enough to begin fusing three helium nuclei to carbon. For stars of mass upto two solar masses, a beautiful phenomenon happens at this point. When the degenerate core reaches this temperature of 10^8 K the entire core will begin helium fusion nearly simultaneously in a phenomenon called helium flash. In heavier stars, the collapsing core reaches 10^8 K before it is dense enough to be degenerate and so helium fusion begins much more smoothly and there is no helium flash.
The obvious question that would come to your mind is whether the core will again collapse and the next higher order of reactions also start when helium is exhausted. A star below about eight solar masses will never start fusion in its degenerate carbon–oxygen core. It will eject its outer layers, setting off the red giant phase in all its glory. When it happens to our Sun, the expanding clouds of gas will consume the inner planets. The core of the star will become a white dwarf. The carbon rich core, crushed under pressure and heat converts into diamond over time and the sky is filled with such huge masses of pure diamond!
At this stage, the Sun will no longer produce heat and light at the rate it is now. The Earth will become a freezing dark place. Moreover, the Sun having lost a lot of its mass, will not have the same gravitational pull on the planets. The planets will fly tangentially off from their orbits into new orbits or may even be lost for ever.
Worried? Don’t be. This is going to take about another five to six billion years. But much before that, in about four billion years, there is another gigantic event that is going to happen to not only the solar system, but to the entire Milky Way Galaxy! The Andromeda Galaxy and the Milky Way Galaxy are hurtling towards each other at a tremendous speed of about 402,000 Km/hr! At present they are about 2.5 million light years apart, and even at that fantastic speed, their cores won’t meet in another four billion years, even though scientists believe the edges of the two galaxies are already almost touching! But when they do meet, what will happen to the stars of the Milky Way is difficult to predict. Though the possibility of head-on collisions of the outer regions is very small, given the large distances between stars in both systems, the possibility of stars and accompanying planetary systems being thrown about and tossed in space is not unlikely! But the cores of the two galaxies being very densely packed, pose a real possibility of collisions and mega “fireworks” in the sky. The tremendous pressures and temperatures that will be created will certainly not be conducive for any life as we know, to survive. Is this the only example of interacting galaxies? No. There are many interacting galaxies. The famous Stephen’s Quintet has five interacting galaxies. There are also examples of galaxies which have met and merged.
Are such events spoilers for life then? No! These are the very events along with supernova explosions that seed space with the right kind of material for life to start! Out of chaos comes order.
🙂 Think if there is any real logical connection between superstitious beliefs and the real science.
CosmosMusings 
