Stellar Life Cycle
Jan. 1, 2024 - Written by Kevin Jie
Similarly to many other objects in the universe, stars have a cycle. This perpetual cycle that exists in every star in the cosmos is called the stellar life cycle. The stellar life cycle begins in a stellar nebula, a nebula that harbors all the necessary elements to create a star. However, not all stars walk the same path, many diverge and become objects of their own, unique to most other stars. In order to understand how we ended up at this point, the star, we will have to trace it back to where it first began.
All stars start out as a protostar, a celestial body that looks like a star but isn’t hot enough for its core to begin hydrogen fusion. A protostar will only begin fusion when it reaches 10 million kelvin. This absurd temperature can be achieved by the collection of small particles around the protostar. As the protostar gathers more particles, they slowly add to its mass, creating a stronger gravitational pull over time. This gravitational energy is what will eventually be converted into thermal (heat) energy. Once the protostar finally reaches 10 million kelvin, fusion will take place and thus a main-sequence star is born. This entire process can take anywhere from 1 to 100 million years. Though, it should be noted that this time alters based on the mass of the star, the higher the faster.
Proto Star
You may wonder where the paths diverge? Well in fact, it can happen right after this stage. When a star does not have the right, or enough, elements, it will essentially run out of fuel. The star will fizzle out as it cannot continue fusion. As the star loses its ability to create fusion, it will also lose large amounts of heat and its former capacity of emitting light. The celestial body will then fall into a near dormant state, gathering more debris as its surface becomes terrestrial. Going from once a radiating object in the sky to a dense planet-like wasteland with only a faint glow under its surface to represent what it once was. This is called a brown dwarf, often referred to as a failed star and you can probably see why. Once in this stage, it will remain as such till the end of time, as there is no time limit to a brown dwarf’s existence.
Although a sad story, the tale of a brown dwarf isn’t common. A more common type of star is the low-mass star. Low mass-stars stem out from the place of successful protostars. Low-mass stars far outnumber every other type of star in the universe and also happen to be the longest living ones. Due to the low-mass of this star, it is usually cooler and much smaller. It should be pointed out that they are also primarily red. All of these features can be explained by the Hertzsprung-Russell Diagram. The low-mass star will remain in its main-sequence stage for tens of billions of years. Some of them have even been predicted to live for a trillion years, a truly incomprehensible amount of time.
Following the main-sequence stage, the star will become a giant star, growing in size, luminosity, and mass. This is caused by the core running out of hydrogen to burn as it begins to use the hydrogen further out from its core. This process causes the star to grow, allowing its radius to expand up to 400 times larger than its original size. This growth occurs across a timeline of around 1 billion years before peaking in size. While at this peak, being a giant star, it will burn far more hydrogen than it used to while also gaining more mass. The giant will remain at this peak for no more than a few million years. This may seem like a lot, but when comparing it to the time it spent as a main-sequence star, it’s the blink of an eye.
Main-Sequence
Brown Dwarf Render
After these million years, the giant will shed its skin, expelling all of its extra layers surrounding the core into space. These layers will become a cloud surrounding what once was its core. This cloud, under select circumstances, will become one of two things: a planetary nebula or a stellar nebula. However, the lonely core of the star still remains. The core stays as a white object around the size of the Earth, yet 200,000 times as dense. This lonely floating object is what we call a white dwarf. A star that isn’t really a star with a lifetime that will outlast the universe. It’s predicted that a white dwarf will stay a white dwarf for, once again, tens and possibly hundreds of billions of years. After this immense amount of time, the light of the dwarf will fade, the energy will be lost, and the surface will harden, becoming an unilluminating and cold celestial body called a black dwarf.
Planetary Nebula
Illustration made by Astron Analytics
A supergiant is stable, however, when it comes close to the end of its life, it’s quite the opposite. The instability leads to the outer layers collapsing and ejecting themselves into space at incredible speed. This is much less of an ejection and more of a massive explosion. This explosion is rightfully named, the supernova. The supernova leaves behind a beautiful cosmic art called a nebula. Just like a giant star’s nebula, it too can become either a planetary or stellar one. Dissimilar to the white dwarf core of a giant’s nebula, a supernova’s center is one of two: a neutron star or a black hole. Neutron stars look like white dwarfs but are far more dense. The density of a neutron star is unimaginable. Some neutron stars with a high enough rotation can even create extreme amounts of gamma radiation, creating two beams of light from both poles. This is called a pulsar.
Pulsar Render
Although a site to behold, if we take the density a step further, we can make a black hole. An object with a gravitational pull so strong that not even light can escape its pull. Bending light and space itself, black holes suck in objects around them, growing in size with higher-mass bodies. A black hole will wander, eating whatever lies in its path. From here, it will persist in this perpetual life it has created for itself. Slowly evaporating away as it loses energy. The lifespan of a black hole is possibly infinite, devouring every object before sputtering out into nothingness, blank space in the universe that once was a floating light in the sky.
Astron Original Document -
Graphic Made by Astron Analytics
Planetary Nebula With White Dwarf Visible
As depressing as this may be, we still have the high-mass star. A high-mass star, unlike low-mass, does not excel in longevity. After a successful high-mass star is formed, it will be catapulted into a massive star as it enters its main-sequence stage. Already consuming huge amounts of hydrogen, the massive star will remain a massive star for no more than a 100 million years. Keep in mind that this time is shorter the bigger the star. The growth isn’t stunted at its “massive” stage, as following the main-sequence it will grow even further, becoming a supergiant star. During the growth to a supergiant, the star’s radius will expand anywhere from 1.5 to 7.5 times its original size. It may not seem like a lot until you realize that a massive star is anywhere from 200 to 800 times the size of the sun, the enlargement to a supergiant makes it around 1500 times larger than the sun. The lifetime of the supergiant stage is around a few million years but no longer than 30 million.
Stellar Life Cycle
Jan. 1, 2024 - Written by Kevin Jie
Similarly to many other objects in the universe, stars have a cycle. This perpetual cycle that exists in every star in the cosmos is called the stellar life cycle. The stellar life cycle begins in a stellar nebula, a nebula that harbors all the necessary elements to create a star. However, not all stars walk the same path, many diverge and become objects of their own, unique to most other stars. In order to understand how we ended up at this point, the star, we will have to trace it back to where it first began.
All stars start out as a protostar, a celestial body that looks like a star but isn’t hot enough for its core to begin hydrogen fusion. A protostar will only begin fusion when it reaches 10 million kelvin. This absurd temperature can be achieved by the collection of small particles around the protostar. As the protostar gathers more particles, they slowly add to its mass, creating a stronger gravitational pull over time. This gravitational energy is what will eventually be converted into thermal (heat) energy. Once the protostar finally reaches 10 million kelvin, fusion will take place and thus a main-sequence star is born. This entire process can take anywhere from 1 to 100 million years. Though, it should be noted that this time alters based on the mass of the star, the higher the faster.
Proto Star
Main-Sequence
You may wonder where the paths diverge? Well in fact, it can happen right after this stage. When a star does not have the right, or enough, elements, it will essentially run out of fuel. The star will fizzle out as it cannot continue fusion. As the star loses its ability to create fusion, it will also lose large amounts of heat and its former capacity of emitting light. The celestial body will then fall into a near dormant state, gathering more debris as its surface becomes terrestrial. Going from once a radiating object in the sky to a dense planet-like wasteland with only a faint glow under its surface to represent what it once was. This is called a brown dwarf, often referred to as a failed star and you can probably see why. Once in this stage, it will remain as such till the end of time, as there is no time limit to a brown dwarf’s existence.
Brown Dwarf Render
Although a sad story, the tale of a brown dwarf isn’t common. A more common type of star is the low-mass star. Low mass-stars stem out from the place of successful protostars. Low-mass stars far outnumber every other type of star in the universe and also happen to be the longest living ones. Due to the low-mass of this star, it is usually cooler and much smaller. It should be pointed out that they are also primarily red. All of these features can be explained by the Hertzsprung-Russell Diagram. The low-mass star will remain in its main-sequence stage for tens of billions of years. Some of them have even been predicted to live for a trillion years, a truly incomprehensible amount of time.
Following the main-sequence stage, the star will become a giant star, growing in size, luminosity, and mass. This is caused by the core running out of hydrogen to burn as it begins to use the hydrogen further out from its core. This process causes the star to grow, allowing its radius to expand up to 400 times larger than its original size. This growth occurs across a timeline of around 1 billion years before peaking in size. While at this peak, being a giant star, it will burn far more hydrogen than it used to while also gaining more mass. The giant will remain at this peak for no more than a few million years. This may seem like a lot, but when comparing it to the time it spent as a main-sequence star, it’s the blink of an eye.
Illustration made by Astron Analytics
After these million years, the giant will shed its skin, expelling all of its extra layers surrounding the core into space. These layers will become a cloud surrounding what once was its core. This cloud, under select circumstances, will become one of two things: a planetary nebula or a stellar nebula. However, the lonely core of the star still remains. The core stays as a white object around the size of the Earth, yet 200,000 times as dense. This lonely floating object is what we call a white dwarf. A star that isn’t really a star with a lifetime that will outlast the universe. It’s predicted that a white dwarf will stay a white dwarf for, once again, tens and possibly hundreds of billions of years. After this immense amount of time, the light of the dwarf will fade, the energy will be lost, and the surface will harden, becoming an unilluminating and cold celestial body called a black dwarf.
Planetary Nebula
Planetary Nebula With White Dwarf Visible
As depressing as this may be, we still have the high-mass star. A high-mass star, unlike low-mass, does not excel in longevity. After a successful high-mass star is formed, it will be catapulted into a massive star as it enters its main-sequence stage. Already consuming huge amounts of hydrogen, the massive star will remain a massive star for no more than a 100 million years. Keep in mind that this time is shorter the bigger the star. The growth isn’t stunted at its “massive” stage, as following the main-sequence it will grow even further, becoming a supergiant star. During the growth to a supergiant, the star’s radius will expand anywhere from 1.5 to 7.5 times its original size. It may not seem like a lot until you realize that a massive star is anywhere from 200 to 800 times the size of the sun, the enlargement to a supergiant makes it around 1500 times larger than the sun. The lifetime of the supergiant stage is around a few million years but no longer than 30 million.
Graphic Made by Astron Analytics
A supergiant is stable, however, when it comes close to the end of its life, it’s quite the opposite. The instability leads to the outer layers collapsing and ejecting themselves into space at incredible speed. This is much less of an ejection and more of a massive explosion. This explosion is rightfully named, the supernova. The supernova leaves behind a beautiful cosmic art called a nebula. Just like a giant star’s nebula, it too can become either a planetary or stellar one. Dissimilar to the white dwarf core of a giant’s nebula, a supernova’s center is one of two: a neutron star or a black hole. Neutron stars look like white dwarfs but are far more dense. The density of a neutron star is unimaginable. Some neutron stars with a high enough rotation can even create extreme amounts of gamma radiation, creating two beams of light from both poles. This is called a pulsar.
Pulsar Render
Although a site to behold, if we take the density a step further, we can make a black hole. An object with a gravitational pull so strong that not even light can escape its pull. Bending light and space itself, black holes suck in objects around them, growing in size with higher-mass bodies. A black hole will wander, eating whatever lies in its path. From here, it will persist in this perpetual life it has created for itself. Slowly evaporating away as it loses energy. The lifespan of a black hole is possibly infinite, devouring every object before sputtering out into nothingness, blank space in the universe that once was a floating light in the sky.
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