Stellar Evolution
“The universe is a pretty big place. If it's just us, seems like an awful waste of space.”
— Carl Sagan,
The universe is full of beauty, The milky way arching around the sea of blue is no doubt a sight to behold but while the sea is immortal the characters are not. Stars live and die like the rest of us, as the stoics say everything which has taken birth has to end one day, this does not mean they do not blind us in their dazzling beauty. The life of a star is as varied as beautiful as the night sky itself, let's go on an adventure into the cosmos
PROTOSTAR:
Under the watchful eyes of nebulae, stars are born. More than Dust clouds, nebulae contain enough mass on average to give birth to several thousand stars. Most nebulae are made up of hydrogen and helium molecules, but the occasional nebulae are blessed with heavier atoms from their ancestors. When irregularities in the density of gasses are observed, a net gravitational force begins to cause gases to collapse in on themselves which causes them to lose potential energy and increase their temperature.
The core of the cloud collapses faster than the outer parts, and the cloud begins to rotate faster and faster to conserve angular momentum. When the core reaches a temperature of about 2,000◦K, the molecules of hydrogen gas break apart into hydrogen atoms. Eventually, the core reaches a temperature of 10,000◦ K, and finally, fusion reactions begin and it results in becoming a Protostar.
T-Tauri Phase:-
Beginning its life as a protostar rested in the womb of its mother nebula, it slowly nourishes itself with new materials, eventually developing a proto-planetary disc around itself. Stellar Winds and radiation clear the shell of gas and dust surrounding it until we can finally see the T-Tauri phase in all its beauty.
Main sequence stars:-
A main-sequence star is any star that fuses hydrogen in its core and maintains the delicate waltz of outward pressure from core nuclear fusion with the gravitational forces pushing inwards. The amount of time a star stays in the main sequence depends on its size, mass, and temperature. Large hot stars consume hydrogen very quickly and thus can only sustain themselves to remain stable on the main sequence for a few thousand years. Some tiny red dwarf stars have been on the main sequence since the beginning of the universe. Once the star's core or protostar exhausts its supply of hydrogen, the star begins to evolve off the main sequence.
1.)Massive Stars:-
Any star larger than eight solar masses during its main sequence is considered a massive star. When these stars expand and cool down, their Luminosity does not change but their baseline Luminosity is still greater than that observed in the main sequence.
They typically don't last long thanks to their large masses due to which they burn through their fuel in a short period. They then have a rapid main sequence phase progressing into a short red supergiant phase, and finally a spectacular death via a supernova explosion which results in them either turning into a black hole or a neutron star.
1.1.)Red Supergiant Star Phase (RSG):-
A red supergiant occurs when a massive star exhausts its hydrogen fuel, evolves off of the main sequence, and transitions to fusing helium within its core. As this occurs, the star’s radius expands, causing its temperature to plummet. Red supergiants are among the coldest and biggest stars observed.
Examples:- Antares. 119 Tauri, Betelgeuse, Mu Cephei, and VV Cephei, etc.
1.2.) Supernova Phase:-
When the stars run out of fuel, the mass flows to its core due to which the core becomes heavy and can’t handle its gravitational force. The core collapses which results in a giant explosion known as a supernova.
After the explosion of a supernova, it either forms into a neutron star or a black hole.
a.) Blackhole:-
A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.
b.) Neutron Star:-
A neutron star is the collapsed core of a massive supergiant star. Neutron stars are the smallest and densest currently known class of stellar objects.
2.)Low-mass stars:-
Low mass stars are the smallest, coolest, and dimmest stars which have masses less than eight solar masses. Main Sequence stars and orange, red or brown. Low mass stars use up their hydrogen slowly and consequently have long lives. In their lifetime they go through their red giant phase, planetary nebula phase, progress to a white dwarf, and finally into a black dwarf. Our Sun is currently a low mass star.
2.1.) Red Giant Star Phase (RG):-
A red giant is a luminous giant star of low or intermediate mass that occurs in the latter half of its stellar evolution when the star has exhausted the supply of hydrogen in its core and has begun a thermonuclear fusion of hydrogen in a shell surrounding the core.
Examples:- Aldebaran (Alpha Tauri) and Mira (Omicron Ceti), etc.
2.2.) Planetary Nebula:-
A planetary nebula is an emission made up of expanding, glowing shells of ionized gas ejected from red giant stars late in their lives. A typical planetary nebula is roughly one light-year across, and consists of an extremely rarefied gas, with a density ranging from 100 to 10,000 particles per cm3.
Examples:- Helix Nebula, Tarantula Nebula, etc
2.3.) White Dwarf:-
A white dwarf, also called a degenerate dwarf, is a stellar core remnant composed mostly of
electron-degenerate matter. A white dwarf is theorized to be dense, with its mass comparable to that of the Sun and volume comparable to that of Earth. A white dwarf's faint luminosity comes from the emission of stored thermal energy; no fusion takes place in a white dwarf.
2.4.) Black dwarf:-
A white dwarf cooled to the extent that it no longer emits significant heat or light, a theoretical stellar remnant, the black dwarf hasn't been known to exist yet as for a white dwarf to reach this state it would take longer than the present age of the universe.
3.)Red Dwarf Stars:-
The most abundant type of star, with theoretic life spans ranging from 10 billion years for ones on the heavier to trillions of years for the lighter, nimbler ones, the red dwarfs will most likely outlive bigger stars and maybe even humanity itself. These immortal giants have not yet exhausted their internal supplies of hydrogen, Having the distinction of being the coolest and smallest type of star. The mighty red dwarf will not evolve into its red giant phase instead Convection observed throughout the entire mass of the star efficiently uses hydrogen from the outer region of the core to sustain itself until it burns through the entire supply of hydrogen causing it to become hotter and smaller eventually turning into a blue dwarf and finally a white dwarf.
4.)The Brown Dwarf: A failed star?
Nicknamed as "Failed Stars", the International Astronomical Union defines Brown dwarfs as those stars that are massive enough to fuse deuterium at some point in their lives but do not have enough mass to reach, let alone sustain the large temperatures required for the nuclear fusion of hydrogen to begin. These stars shine dimly at best, slowly fading away, getting eaten up by hundreds of years of gradual cooling.
