Cosmic ‘Star’-dom

The glowering sun affixed in the sky above enervates us, as beads of perspiration dwell on our murky skins. With a scornful look at the burning furnace, we often hurl out a couple of abuses at the sun and continue with our work. Rarely, do we care about how such a hot, dense piece of celestial object is present in the sky above us. While many eminent directors in the film studio are busy scripting down biography of humans, here I am, narrating the biography of a star. So, put on your 3-D classes and submerge yourself in this pool of information!

The interstellar medium consists of gas and dust that fills the spaces between the stars in any galaxy. A dark nebula is a type of interstellar cloud that is so dense that it obscures all the incoming background emissions coming from either reflection nebulas or other bright objects in the sky like stars. The other type of nebula that an interstellar medium can form is reflection nebula, which reflects the background emissions by nearby stars. To this list of nebulas, we add one more – the emission nebula- which is a cloud of ionized gas emitting light of various colors.

So, how exactly is a star formed from the vast expanses of a nebula? The gas clouds described above are stable if the internal pressure pushing outwards is balanced by the gravity. The minimum mass a star requires to balance gravity by internal pressure is often referred to as Jeans mass. At some point, the core of the collapsing clump is so dense that radiation being generated at the core is trapped, which causes the temperature of the core to surge rapidly. The core is now referred to as a proto-star. The temperature in the core of the proto-star is so high that thermal pressure becomes strong, slowing down the collapse such that the continual contraction of the proto-star transmogrifies gravitational potential energy into thermal energy, causing the object to radiate huge amounts of energy that is equivalent to 1000 times the energy our sun gives out.

Angular momentum is defined as the rotational equivalent of linear momentum and because the conservation of angular momentum dictates that spin of a rotating object is constant, unless acted upon by an external torque, the large amounts of energy produced makes the cloud of gas rotate more rapidly. This results in a disk forming around the proto-star that is known as the proto-planetary disk.

As the contraction continues, the proto-star undergoes a litany of veracious changes as the outer parts of the clump radiate an immense amount of light which varies over a span of short time scales. This period is known as the T-Tauri phase. Stars in this phase are known to emit bipolar outflows of materials (two continuous jets of gases dispelled out of either poles of the star). When temperatures reach 10 million Kelvins, protons fuse together to form helium, and generate energy in a process known as Proton-Proton chain. For massive cores, temperatures might reach 20 million Kelvins and they undergo a different type of fusion known as the CNO cycle (The Proton-Proton chain and the CNO cycle will be dealt later in another article. All you need to know now is that smaller stars like sun follow a specific type of fusion and those larger than sun follow another).

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The diagram above is known as the Hertzsprung-Russell Diagram, where the effective temperature of a star is plotted on X-axis and the luminosity on the Y-axis. All new-born members of the star family are categorized as main-sequence stars and an important property of all the stars in this band is that they are all chemically homogenous, converting hydrogen to helium in their cores. This band of Main sequence stars is otherwise termed as the zero-age main sequence (ZAMS). The most important factor determining the location of a star within the main sequence is its mass and hence one relation that we can derive out of this that the effective temperature is proportional to the square root of mass.

It must also be noted that high main sequence stars are more luminous and bluer in color compared to the low-mass main sequence stars. When nuclear fusion is going on in a star’s core, the pressure created pushes outward and is balanced by the inward pull of gravity. The Main Sequence part of star’s evolution dominates most of its life and the star surface temperature and luminosity changes slightly over the course of its Main Sequence. When the majority of hydrogen in the core is used up, the star is ready to leave the Main Sequence. The inert Helium core contracts due to weight of the other superfluous layers of star and as the star contracts, the gravitational energy released leads to the expansion of the outer layers.

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As the star expands, its surface temperature decreases and it metamorphoses into a red giant star. As the inert helium grows in mass, the luminosity of the hydrogen burning shell increases and the surface gravity of the core increases, squeezing the tin shell of hydrogen burning around the core, which in turn causes the density, temperature and luminosity of the shell source to burgeon. As the core contracts, it becomes a degenerate core.

What is degenerate matter? It is a state in which particles must occupy high states of kinetic energy to satisfy the Pauli exclusion principle (will be covered later in another article about quantum theory. In brief, the principle states that two or more identical fermions cannot occupy the same quantum scale in a quantum system at the same time.) As the mass of the degenerate core increases, its radius decreases.

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A contracting degenerate core does not heat up. The temperature of the core increases due to high thermal conductivity and it remains at the same temperature as the hydrogen shell around it. As the core’s mass increases, the luminosity and temperature also hike. At some point, the temperature is so high that conditions are favourable for helium to ignite.

When helium ignites, the inert core has a new source of energy and this makes the fusion go faster. The core cannot expand because it is a degenerate core. Thus the core is hotter now and the reaction rate is faster. As the temperature of the core increases dramatically, the core is no longer degenerated.

Once the core is nondegenerate, helium will fuse into the core. Till now the hydrogen is still burning in the hydrogen shell. Energy production in the core will now cause the core to expand, while the outer layers will contract. As the temperature of the core plummets, the star moves down the H-R diagram.

The mass of the helium in the core continues to grow and at the same time helium is converted to carbon-oxygen. Since the inert carbon-oxygen core is more compact and dense than a helium core, the gravitational potential energy released is much higher and. The star is now in the last stage of its life and is known as a supergiant star.

While this concludes the life cycle of a star, we must now focus on the death of a star. What happens when a star dies? Does it fade away into the dark abyss or is there something more fascinating that alludes our senses? Does it form a black hole? To know more about the death of a star, look out for the next articles. So, keep reading!!! And continue to speculate, innovate till you constipate!


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