The Big Bang- 3

Origin of stars.

By the time the universe was 200 million years old and its temperature had dropped so fast that it was around -220⁰C. (H atoms bond together to form H₂ molecules now.) When the early universe emerged from the Big Bang it was uneven. It contained cracks which were filled in with swelling clouds of dust and gases such as H₂ to form nebulae. Let's see what nebulae are; A huge cloud of gases and dust in the intergalactic space is called a nebula. There are various types of nebulae such as;

• Ring Nebula

• Reflection Nebula

• Emission Nebula
  

• Planetary Nebula
  

• Stellar Nursery

These stellar nurseries give birth to proto stars. As these huge dusty gaseous clouds pull over together due to gravitational forces the clouds shrink and make a proto star. The temperature rises when the rate of collision increases as the clouds get denser. The contraction stops when the pressure is equal to the gravitational pull. Then it has two options: one is to become a brown dwarf star if the temperature needed to accelerate nuclear fusion is not reached. If it does, nuclear fusion would start over in the core. H₂ is the most abundant of all gases, so two ionized H⁺ s { Due high temperature of the core, H₂ molecules would be ionized to H⁺/ protons} would fuse together to form He. With the emission of the heat and radiation (energy) from the nucleosynthesis (nucleosynthesis: the process by which nuclei of chemical elements are formed) a star is born.

The formation of the alpha particle

(H) 4 p⁺ → 4 He²⁺ + 2e⁺ + energy( E=mC²)

This process of burning hydrogen is known as the main sequence. A star occasionally spends 90% of its life time in main sequence. When the primary burning of H₂ is over the star reaches its older age.

The duration of the death of the star depends on the initial mass of the star. Larger stars have more fuel, but they have to burn it faster in order to maintain equilibrium. Because thermonuclear fusion occurs at a faster rate in massive stars all of their fuel is used during a comparatively shorter period of time. This means that bigger is not better with respect to how long a star will live. A smaller star has less fuel, but its rate of fusion is not as fast. Therefore, smaller stars live longer than larger stars because their rate of fuel consumption is not as rapid.

When a star has used up all its H₂ then He comes to picture. As most of the H₂ is used up there’s not enough pressure to balance the gravitational force and the core starts shrinks again. When the star regains enough pressure to start fusion of He atoms production of carbon and other light elements begin. Once ignited, helium burns much hotter than hydrogen. The additional heat pushes the outer layer of the star out much further than it used to be, making the star much larger.

3 ⁴He → ¹²C + energy

With the entire He gone the lower mass stars (<.5 Solar Mass) can’t get C atoms to fuse together. They exit creating explosions called planetary nebulae and collapse into white dwarf stars.

Massive stars (> 3 Solar masses) utilize carbon burning. It was Sir Fred Hoyle who determined that stars acted like nuclear reactants producing elements heavier than H and He. Fusion reactions inside these stars release enormous amounts of energy and heat which force more atoms to fuse and create new heavier elements one after the other. Three He nuclei combine to create C, two carbon nuclei fuse to form Mg, Mg formed Ne and so on. This continues until silicon finally fuses to form iron. And the process is called stellar nucleosynthesis.

Iron is a very special atom because ⁵⁶Fe has the most stable nucleus of all. Within the nucleus there are energy levels, it’s considered stable when the no of protons and neutrons are equal inside the nucleus and they are tightly bound together so that the extreme temperatures within the stars couldn’t get it to fuse together to form newer elements. Production of elements would shut down when they reach iron.

²H⁺ + ⁴He²⁺ → other elements { ¹²C, ¹⁶O, ²⁰Ne, ⁵⁶Fe} All are multiples of He

Still some of the vital elements were missing; elements that are heavier than Fe. A higher mass core in medium mass stars would create neutron stars while bursting into supernovas. When the giants stars, which had already made the lighter elements, run out of fuel their cores collapse on themselves creating incredible amount of energy in enormous explosions called supernovas. These are so powerful that they are able to fuse elements heavier than iron. This is called explosive nucleosynthesis. This is the point where most heavy elements such as uranium origin.

Basically stars can be categorized as below:

• First generation stars: Mostly red dwarf stars which are still in the main sequence burning its primary H and He. They consist of only H and He (perhaps Li) as elements.

• Second generation stars: giant and super giant stars which consist of lighter elements from C to Fe. Our sun is a second generation star.

• Third generation stars: The stars which undergo supernova explosions and later turn into either neutron stars or black holes. Most blue giants are in this category.

Together these elements formed smaller bodies which orbit around the sun that later developed into planets and asteroids.

Earth is the next post to come :) TC