Stellar Evolution

The life cycle of a star







Stars are huge luminous balls of plasma held together by their self-gravity. Our sun is also a type of main sequence star with average mass. Stars are the source of all light which makes us see every object during daytime. Without the presence of the sun, life on Earth or any other planet is nearly impossible. Our ecosystem is made up in a such way that primary organisms in our system get energy from the sunlight. If the sun disappears in just only one minute. it will result in the collapse of our ecosystem and in the end extinction of all organisms including human beings. Like our solar system, every solar system has at least one star. Sometimes, there may be two or more stars in one solar system called a binary system. Approximately, there are around a billion stars present in our galaxy. The only difference between them is their diversity in masses and sizes. Therefore, stars are classified on the basis of their spectra (the elements that they absorb) and their temperature. There are seven main types of stars. In order of decreasing temperature, O, B, A, F, G, K, and M.

O and B stars are uncommon but very bright; M stars are common but dim. Our sun is a G-type, yellow dwarf main sequence star. Here, in this article, we will be going to know about the life cycle of stars. We will try to understand How a star is born or what happens in the last stage of a star.

Nebula the womb of a baby star:


All the stars begin their journey from the giant interstellar cloud of dust and gas which we called a nebula. A nebula is a vast luminous region in space made up of cosmic dust and gases like hydrogen, helium, and other ionized gas roughly 100 light-years across and containing up to 6,000,000 solar masses. They are often the region where stars are born in space. Nebulae exist in interstellar space (space between the stars). the closest known nebula to the Earth is Helix Nebula which is 700 light years far from the Earth. Nebulae are often formed from the gas and dust thrown out from dying stars or from the gases that are already present in interstellar space.

There are four major groups:

1. HII (nebula that contain ionized hydrogen)

2. Planetary nebulae

3. Supernova remnant

4. Dark nebula







Formation of baby star inside nebula:






Inside the nebula, due to an increase in gravitational force all surrounding molecule and particles begin to collapse with each other. This collapsing of surrounding molecules and particles divide the giant gas cloud into a number of small fragments called clumps. In these clumps, the increasing gravitational force makes molecules come closer and collide with each other. This collision of molecules produces heat counter to collapsing gravitational force and achieves an equilibrium state. This stage of the star which achieves stability by heat energy generated due to the collision of gas molecules(accretion) is called a protostar. This protostar continues to grow by accretion of gas and dust from the molecular cloud, becoming a pre-main-sequence star as it reaches its final mass. Further development is determined by its mass.

Brown dwarfs and sub - stellar objects:






The protostar does not have enough mass to initiate nuclear fusion to achieve this stage of the star which we called a brown star. Scientists initially theorized the existence of brown dwarf in 1960 and in the mid-1990 it was first discovered. Due to very low surface temperatures, we can't observe them with the visible region but with advanced infrared detecting devices, we can observe them. The nearest -known brown dwarfs are located in the Luhman 16 system, a binary of L- and T - type brown dwarfs about 6.6 light-years away from the Sun. Scientists believe that the size of brown dwarfs could be roughly the same mass as Jupiter, while the boundary between the most massive brown dwarfs and true stars is thought to be around 90 Jupiter masses. However, the Brown dwarf is massive enough to fuse deuterium and live for millions of years emitting radiation and light in the infrared region. Despite their name, brown dwarf can be found with different colors depending on the surface temperature. Other which are smaller than 13 solar masses are classified as sub - brown stars and if they are found revolving around other stars then we can consider them as a planet.

Main - sequence star:









Once a protostar has accreted all of the gas and dust that it can from the cloud from which it was born, then this protostar becomes massive enough to generate a temperature of nearly 10 million in its core able to initiate the proton-proton chain reaction and make hydrogen fusion to helium possible in the core. this fusion of hydrogen produces heat energy to oppose gravitational collapse and finally achieve hydrostatic equilibrium and this phase of the star is called the Main-sequence star. In the main-sequence star, energy is generated through a nuclear fusion reaction. Stars spend almost 90% of their life in the main-sequence stage. There are many types of main-sequence stars depending on their size, mass, and luminosity. this classification is done by Hertzsprung–Russell diagram. The Hertzsprung-Russell diagram shows the relationship between a star's temperature and its luminosity. It is also often called the H-R diagram or color-magnitude diagram. It is a very useful graph because it can be used to chart the life cycle of a star. According to this diagram. The low-mass stars fuse hydrogen slowly and will remain on the main sequence for hundreds of billions of years or longer, whereas massive, hot O - type stars will leave the main sequence after just a few million years. As time passed, a time will come when the core consumes it whole fuel then it will begin to collapse again because now there is no outward force due to After this stage, further stages of the main - sequence star depend on their mass.




Star having mass lower than the sun:

As we know that the inward gravity of these stars is not much high due to their low mass and therefore these stars consume their fuel gradually. Stars of that small size will consume their fuel gradually. it can take trillions of years for evolving off its main-sequence star. Our universe has only passed 13 billion which is a very short period of time in comparison to the time taken for low masses of stars to exhaust their fuel and therefore scientists are still not able to observe this star's next stage. The astrophysical model suggests that these stars will take several hundred billion years to exhaust it hydrogen in the core and finally collapse into white dwarfs under gravitational force. Such stars are not able to reach the red giant phase as the whole star is convective, it will exhaust all hydrogen until the whole star becomes made up of helium.

Stars having mass equal to our sun or smaller than 8 solar masses:

As time elapsed, a time comes when the core of the main-sequence star of this group consumes all of its fuel. then, fusion will stop temporarily in the core. At that moment star will again begin to contract under the gravitational collapse because there is no outward force to oppose this inward gravitational force. But there is still some hydrogen left in the outer shell of the core. Now, this hydrogen of the outer shell starts to fuse. the energy generated by this fusion will expand the outer shell to a large extent. but the core is still contracting. this stage of the star is Red Giant. Our sun will also gain this stage in nearly 4-5 billion years. when our sun becomes a red giant. its size will increase much greater than it will consume mercury.

The contraction of the core still continues until a phase came when the pressure inside the core is enough to initiate helium - helium fusion to carbon. when this fusion begins then it generates a vast amount of energy which will lead to the stability of the star again for 1 - 2 billion years (depending on the mass of the stars). This stable stage remains until the core exhaust all its helium. After the depletion of helium again core began to contract under gravitational collapse. But now the kinetic energy generated due to collapse will not be high enough to cause the fusion of carbon atoms. and now the end star will start.






Death of star:

After the depletion of all helium, the core will continue to contract without any thermal force. But there is still some amount of helium in the outer layer of the core. Now, this helium will start fusion to yield carbon in the outer layer. the energy generated due to this fusion will again start the expansion of the outer layer. Now the expansion of the outer layer is much larger than the red giant phase and due to this expansion star begins to lose matter from the outer layer in the form of a planetary nebula (A planetary nebula is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.)and Inner core will continue contraction until electron degeneracy pressure stop this contraction. At this stage, the core of the star gains stability again and this stable stage of the star is called a White dwarf. Now the density and temperature of the white dwarf are very high. The white dwarf will lose its brightness gradually in 10 billion years and finally become a cooled and black object which we called a Black dwarf (A black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently to no longer emit significant heat or light).

Star having mass greater than 8 solar masses:

Unlike other stars which in their red giant stop fusion after carbon, the temperature due to contraction is very high so this heavier star in their red giant stage will be able to continue its fusion in the core until iron came which is the most stable nuclei. Fusion will not happen because after iron fusion will be endothermic. Now after iron fusion stop, and contraction continue due to the absence of any outward force. As time passed. a time comes when the star will not be able to stand its own gravitational collapse and become highly unstable. Finally, the star will do a supernova implosion (the type of explosion where matter goes inward) or we can say that star died.




Supernova:







A supernova is a powerful and brightest inward explosion of a massive star that occurs at the end stage of the star or when the white dwarf is going to run away its fusion. Supernova can expel several solar masses of material at velocities up to several percent of the speed of light. After the supernova implosion, the star will become either a neutron star or a black hole depending on its core mass. Supernovas are classified on the basis of their light curves and the absorption lines of different chemical elements that appear in their spectra. Some of the types of supernovae are Type I, Type II, Type III, Type IV, and Type V




Neutron Star:








A star whose core mass is lesser than 5 solar masses or higher than 1.4 solar masses will become a neutron star after a supernova, this is called the Chandrashekhar limit. The core of this star will collapse until the pressure inside core causes electrons and protons to fuse by electron capture and yield neutron and this continue till all proton converts into a neutron. this neutron nucleus will further collapse and resist contraction of the star core by the Pauli exclusion principle, in a way analogous to electron degeneracy pressure, but stronger. These stars are a package of very large amounts of mass in an extremely small volume-- on the order of radius, 9 km mean neutron is an enormous densely celestial object that if we take a spoonful of neutron star then it can be denser than entire Earth. Neutron can't be observed directly because it emits radiation in X-ray region instead of radiation in the visible region. But we can observe them in the form of Pulsar. When the magnetic poles of the neutron star aligned with the Earth, then it blasts out pulses of radiation at regular intervals ranging from seconds to milliseconds which we can detect on the Earth, and we called this neutron star pulsar. This pulsar is first discovered in 1967 by Jocelyn Bell, an astronomy graduate student working with Prof. Antony Hewish at Cambridge.




Black hole:








If the mass of the core of the stellar remnants exceeds five times of solar mass, then it will generate so much strong inward gravitation force even that the electron and neutron degeneracy pressure will be insufficient to prevent collapse below the Schwarzschild radius and core fall onto singularity region and we called them black hole. it will produce enormous gravitational force even though light can't escape from it once it enters the region of the black hole by mistake. let's assume a ship falls into a black hole and now this ship wants to escape then for this it will require an escape velocity greater than the speed of light. Now you have the question if light can't escape from it then how scientist observes them. Yes, it is true that we can't see black holes. But scientists can see how strong gravity affects the stars and gas around the black hole. Scientists can study stars to find out if they are flying around, or orbiting, a black hole. The black hole can be small in size equal to the size of an atom but have a mass greater than a mountain or can be a giant like a monster having millions of galaxies in them. The largest black hole is called a supermassive black hole. Scientists also found that Our galaxy also has a gigantic black hole at it center. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million sun's.