Stars: their life and death

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Our Sun, also a star. (image courtesy: Wikipedia)

Let me first show you the general idea of the formation of a star. Let’s start with an imagination of an enormous cloud of hydrogen both in size and mass in space. We know that two bodies attract each other due to gravity so the hydrogen atoms will be attracted towards each other. After a million years, those clouds are going to get denser which will result in increase in temperature. Most of you are wondering that how will an atom attract each other? Suppose there are two protons of hydrogen and both will repel each other according to Coulomb’s law but if the pressure is too high and somehow those protons are brought too close, then a strong force will be activated, which would be greater than Coulomb’s force and the protons will attract each other and thereby allowing the hydrogen cloud to get condensed. So these protons will fuse together. High temperature and pressure over comes the coulomb’s force and then fusion occurs and the resulting mass of the fused proton is less than that of its each parent. The dense region in cloud collapses and forms protostars. During fusion, large amount of energy is released and there is an outward pressure which balances it so that it doesn’t keep collapsing. Hydrogen is being fused into Deuterium also known as the heavy hydrogen. Once the star contracts enough that its central core can burn hydrogen to helium, it becomes a main sequence star.

LIFE OF A STAR.

Star undergoes an evolution known as the stellar evolution. In this process, star goes through a sequence of evolution. Life of the star depends on the mass of the star which ranges from only a few million years. Massive stars may have trillion years.

PROTOSTAR

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Birth of a star within a dense molecular cloud. (Image courtesy: NASA)

It forms by the contraction out of the gas of a giant molecular cloud which is also known as a GMC  in the interstellar medium. Once the Jeans mass is achieved contraction starts. The GMCs have turbulent velocity which compress the gas in shocks. The cloud breaks up into smaller and denser  areas which may again break into still smaller areas. They form a cluster of protostars. The cloud becomes opaque to radiation in the infrared which makes it difficult for us to observe directly what is happening. As long as the surrounding matter is falling onto the central condensation, it is considered to be in protostar stage. When the surrounding gas/dust envelope disperses and the accretion process stops, the star is considered as a pre-main sequence star.

The Jeans mass

The Jeans mass is a theoretical concept by the British physicist Sir James Jeans. He was able to show that, under appropriate conditions, a cloud, or part of one, would become unstable and begin to collapse when it lacked sufficient gaseous pressure support to balance the force of gravity. Remarkably, the cloud is stable for sufficiently small mass (at a given temperature and radius), but once this critical mass is exceeded, it will begin a process of runaway contraction until some other force can impede the collapse. He derived a formula for calculating this critical mass as a function of its density and temperature. The greater the mass of the cloud, the smaller its size, and the colder its temperature, the less stable it will be against gravitational collapse.

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Drawing of a T-Tauri star with a circumstellar accretion disc.

PRE-MAIN SEQUENCE STAR

This is a stage just before the main sequence. There are three possibilities or types of a pre-main sequence star. They generate energy through gravitational contraction.

­ T Tauri Star: Found near molecular clouds and identified by their optical variability and strong chromospheric lines.

­ FU Orionis Star: Displays an extreme change in magnitude and spectral type.

­ Herbig Ae/Be Star: They are still embedded in gas-dust envelopes and may be surrounded by circumstellar disks.

MAIN SEQUENCE STAR

When a star starts generating its energy using the core instead of generating energy through gravitational contraction, it is known as main sequence star. The generation of energy is begun at the core using a nuclear fusion process (two or more atomic nuclei collide at very high speed and join to form a new type of atomic nucleus) that converts hydrogen into helium. The proportion of helium in a core will steadily increase thus the rate of nuclear fusion at the core will slowly increase, which will result in increase in the star’s temperature and luminosity. Stars spend about 90% of their lifetime doing it. Our Sun is a main sequence star which reached this stage 4.6 Billion years ago. It is estimated to increase its luminosity by 40%.

POST-MAIN SEQUENCE STAR

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Betelgeuse, a post-main sequence star (image courtesy: Hubble Telescope).

They are the main sequence stars which consume their supply of hydrogen at their core. Their outer layers expand greatly and cool to form a Red Giant. The core is compressed enough to start helium fusion and the star now gradually shrinks in radius and its surface temperature increases. If the solar mass of a star is too high (more the 10M☉)then the star becomes a red supergiant. These are the largest stars in the universe in terms of volume. The luminosity of a red supergiant can exceed 500,000 times that of the Sun. Once the fuel is exhausted at the core, they continue to fuse elements heavier than helium. Later the fusion continues to stages where Carbon, Oxygen and Silicon is used as a fuel. The stars then start producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, any fusion beyond iron does not produce a net release of energy — the process would, on the contrary,  consume energy. Since a large core of inert iron will accumulate in the center of the star. The heavier elements in these stars can work their way up to the surface, forming evolved objects known as Wolf-Rayet stars that have a dense stellar wind which sheds the outer atmosphere.

DEATH OF A STAR

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A white dwarf star in orbit around Sirius. (image courtesy: NASA).

Its core collapses and it explodes into a supernova. After the supernova explodes, it leaves behind a black hole, a neutron star or a white or black dwarf. All of the material that explodes outward from the star continues outward forever as it slowly dissipates. After the explosion of the star, the expanding material is what creates new stars. The waves created by an exploding star begin a chain reaction with other particles in space that in the end forms new stars. Once they have come to the end of their life, they explode violently into a supernova. Then, the entire mass of the star is squeezed into a ball of 10 – 16 km of diameter. This ball is made of only neutrons, and hence called neutron star. If the star is really large, its gravity may pull it further inside, and they cross a particular radius known as the Schwarzschild radius and form a black hole.  Stars of size about 1.4 times of our Sun or less size off without much fuss. Before it dies, it expands to a red giant, coming in its form for the last time. Then it releases its outer layer, which is enriched by heavier atoms. Eventually, its deeper layers are releases, and only the core remains. The star has exhausted almost all its nuclear fuel, gives up to its own gravity, which pulls the mass of the star towards its center, and compressing the atoms, making the star really small and hot. This is the white dwarf. Once the white dwarf reaches the size of earth, it cannot contract anymore, and the gravity too gives up, leaving the star as a dead, dense black dwarf, in the graveyard of the universe.

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Simulated view of a black hole (image courtesy: Wikipedia).

SCHWARZSCHILD RADIUS

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Any object of any density can be large enough to fall within its own Schwarzschild radius,

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WHY ARE STARS SPHERICAL IN SHAPE?

Now we arrive to one of the most common questions people ask which is – why are stars spherical? Well, everything always tends to move to the lowest potential that it can, be it gravitational potential, electromagnetic, mechanical, or what have you. With gravitational forces, things will be attracted to the center of mass of an object. So you can imagine, when stellar bodies form, they begin as a collection of only a few particles. However, the gravitational force is very weak (at least compared to the other fundamental forces in nature). At first, this force is not enough to do anything. But as more and more particles and groups of particles collide, they will form larger and larger objects. Eventually, they have enough mass that gravity starts influencing how the body forms. A sphere geometrically has the highest volume/surface area ratio. This means that you can pack the most stuff as close to the center as possible. Since gravity acts towards the center of mass, the shape that allows everything to be as close to the center as possible (and thus at the lowest gravitational potential) is a sphere. And this is why large stellar objects are spherical.

CONCLUSION

Stars sure are something where there’s more than meets the eye because stars are not just tiny points of light in the sky. They have different nuclear fusions that happen inside them. Sometimes the smallest of stars are the hottest and emit the most radiation. There are huge colourful nebulas that have been left over by old dying stars. There are some stars that are only due to dye at 55 trillion years old. Some stars are weird because they are so big and massive. Even as technology improves we keep on learning more and more about stars. The stars will continue to shine on. They live and they die, so will humanity. We shouldn’t fear death and destruction. We should reflect on the good that has influenced our lives. One day our light will die, and hopefully someone, somewhere will remember us for the beauty that lived inside of us.

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Life cycle of a star (image courtesy: NASA).

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