Stars! Me, You, literally everything around and even deep space objects are all stardust. The leftovers of a star’s death will lead to the birth of new objects over a course of time. Thus whatever we see around us is all cosmic products. Much like living organism’s stars do have a finite lifetime. Death! Can stars escape? Nope! The way they die might vary but none of them can escape certain death.

There are roughly 300 billion stars in our own Milky Way Galaxy! Do you think that is a big number? There are galaxies that host nearly 1000 billion stars! Our sun is one of the tiniest of the stars in the observable Universe. There are stars that are 10^6 times more massive than our sun. UY Scuti is one of such stars which has volume nearly 5000000000 that of our Sun. Our Sun is in its prime and medieval age and it is roughly 4.6 billion years old. It has stable nuclear fusion taking place in its core where hydrogen atoms are continuously fused to produce helium atoms with outburst of nuclear energy. Sun will continue to do so for another 5 billion years before it ends as a red giant. Sun will end as a white dwarf, which is the most common ending point of stars. Stars which are a tenth the size of the Sun will end as a red dwarf (They are estimated to live for 10 trillion years) Thus no red dwarf is spotted till date as the Universe itself is just 14 billion years old.  On the other hand, Betelgeuse is 1000000 times bigger than our sun and is almost nearing to its death! More massive the star more is its gravitational pull at its core and faster is the rate of fusion and thus leading to a faster death. However, at all stars ending stages gravitational pull will win over the nuclear force and end up in a massive core. The degree of the massiveness of the may vary based on the star’s original size and its end is governed by Chandrashekar’s Limit and the Tolman– Oppenheimer– Volkoff limit.

Image showing the Nebula after a supernova explosion Source: Google

All-stars have an origin from the past remnant of a nebula which collapses to form a protostar. The mass that it gains at this stage decides the fate of its death! The protostar stabilizes and evolves as a main-sequence star. The most violent reactions happen inside a star’s core – The Nuclear Fusion. The most basic and fundamental element that make up any star is hydrogen. Stars have different layers of above the core, However, the fusion reaction takes place only inside the hot molten core. Thus it all begins with the fusion of hydrogen-hydrogen atoms (1 proton and 1 electron) leading to the formation of heavier elements. Helium (2 protons 2 electrons and 2 neutrons) is the 1st to appear after fusion of hydrogen atoms with the release of an enormous amount of nuclear energy once all the Hydrogen is completely fused it starts to ignite the stable helium nucleus. Massive stars undergo this process very fast and run out of hydrogen. Helium is a noble gas and is stable and inert to any reaction, However, the star’s core compresses these stable helium nuclei and forms heavier elements like carbon and oxygen during which the outer layers of the star expand and become red giants. As the outer layers of a star expand the core’s gravitational pull becomes enormous and the star collapses under its own gravity. Our sun will also become a red giant and will expand and consume Mercury and Venus and will come very close to Earth in 5 billion years. These outer layers will expand and move away from the star to form the planetary nebula leaving behind a supermassive core made up of mostly carbon and oxygen. This is the common way of stellar evolution for all main-sequence stars, but from now on the way stars end up will be governed by Chandrashekar’s limit. Chandrashekar’s limit states that stars whose core weigh 1.4 times the solar mass after the red giant stage will end up as white dwarfs. 97% of the stars fall in the category of lower medium massive stars to which our sun also belongs to. Stars whose core mass is more than Chandrasekhar’s limit will go on to form a neutron star owing to the fact that electron degeneracy pressure will be unable to support its weight against the force of gravity and the star explodes by the process of a supernova explosion. Stars of super masses are governed by the laws of Tolman– Oppenheimer– Volkoff limit which gives the mass of neutron star to fall within the range of 10 to 25 times the solar mass and any star above this range approximately greater than 30 solar masses will end up as a black hole.

Lifecycle stages of a star Source: Google

White dwarfs! The most common way of death of a star. These star cores are the size of Earth but will be more massive than the Sun. These super-dense star cores are plentiful in our own galaxy. Stars whose core are not massive enough to sustain fusion of heavier elements like carbon and oxygen and maintain the temperature of 1 billion Kelvin at the core will not undergo a supernova explosion. Instead, they will shred the outer layers and have a massive core of carbon and will continue to shine (but much dimmer than the parent star) owing to the scorching temperature of the core. It shines for millions of years till the core cools down and finally becomes a black dwarf. This core of carbon or cosmic diamond rather 😛 will remain as space junk. However, at less than 14 billion years old, the universe is still too young to have created any black dwarfs. The nearest white dwarf is Sirius B which is a twin star system in the Orion nebula along with its twin Sirius A.

Stars of more than 10 solar masses will result in a violent explosion called the Supernova! The core of the white dwarf left behind with carbon and oxygen will be massive enough and have temperatures beyond 1 billion Kelvin will start to fuse carbon atoms forming heavier and heavier elements like neon and silicon. The innermost core will finally have iron. Fusing iron will not release energy and fight against the inward pulling gravity rather suck in energy. Iron is the most stable element in the universe with the least binding energy of all the elements in nature. Iron core is the last possible element that any star can fuse and the moment the star starts to fuse iron it collapses under its own gravity and explodes in a violent and catastrophic explosion called the Supernova. Bigger the mass of the star more is the catastrophic explosion. The end product of this stellar explosion can be either a Neutron star or Black holes.

Comparison of the size of Sun and UY Scuti Source: Google

Neutron stars that can be observed are very hot and typically have a surface temperature of around 600000 K. They are so dense that a normal tennis ball-sized sphere of neutron star material would have a mass of approximately 3 billion tonnes. Their magnetic fields are between 108 and 1015 (100 million to 1 quadrillion) times as strong as that of the Earth. The gravitational field at the neutron star’s surface is about 2×1011 (200 billion) times that of the Earth. Newly formed neutron stars rotate at up to several hundred times per second. Some neutron stars emit beams of electromagnetic radiation ranging from Cosmic rays, Gamma rays, X rays, visible light, Microwaves, etc. that make them detectable as pulsars. The fastest-spinning neutron star known is PSR J1748-2446ad, rotating at a rate of 716 times a second or 43,000 revolutions per minute, giving a linear speed at the surface on the order of 0.24 c (i.e. nearly a quarter the speed of light). The magnetic field strength on the surface of neutron stars’ ranges from ca. 104 to 1011 tesla

The gravitational field is around 2.0×1012 m/s2. Such a strong gravitational field acts as a gravitational lens and causes an effect called gravitational lensing. This effect bends the space-time fabric around it and causes the electromagnetic radiation to bend and making an observer seeing from distant places like Earth to miscalculate its actual position and its actual distance. Everything about these neutron stars is just massive. Nothing much about these Neutron stars are known to scientists.

Depiction of a neutron star Source: NASA(Google)

Stellar masses which are even more massive than neutron stars will end up as black holes. Black holes are a new chapter that can be discussed in detail and is out of the scope of this article. Black holes are super dense objects which have a gravitational pull so strong that it won’t even let light escape its gravitational pull. Thus to escape a Blackhole you must travel with an escape velocity greater than the speed of light which is not possible even theoretically. Black holes also create the effect of gravitational lensing and distort space and time. There are primarily 3 types of black holes stellar, supermassive, and miniature black holes. A supermassive black hole is believed to exist in the center of the galaxy. Milky way’s supermassive black hole is called the Sagittarius A *.  Thus mass is the key in stellar evolution, from being a brown dwarf (failed star) to black holes, all are owing to their masses and all phenomenon from their birth as a protostar to death is primarily controlled by the mass they own.