Stars
The sentinels of light, the bringers of warmth and undisputedly one of the greatest marvels and phenomenons in the Universe. What could it be? Shining millions of light-years away, lighting up the day, and forever glistening in the night sky, stars, as we know them, could possibly be the entire reason we exist, making them the guardians of life itself as no known form of life can exist in a world without the Sun. Although they may be a necessity for existence, little is known about the life of a star itself, which is what we are going to do, following the lifecycle of a star from the clouds of nebula in which they are born, to when they are no longer able to distribute light to their celestial neighbours, fulfilling their life's purpose.
The Lifecycle of a Star
Stellar Nursery
A star's life begins in a stellar nursery, also known as a molecular cloud. A stellar nursery is unique in that it is the only place in the Universe that permits the formation of a star, through the fact that it allows the growth of molecules, typically that of hydrogen gases and various other particle, the main ingredients in the creation of a star.
Protostar
A protostar is an early stage in the formation of a star, which is basically a large mass formed through the compression of gases in a stellar nursery, the birthplace of star. The protostellar phase lasts typically 100 000 years, although this period of time can vary depending on the mass of the protostar, and the abundance of the gases it so craves.
Main Sequence
After a protostar comes a main sequence star. By this time and stage in the star's lifecycle, nuclear fusion will have begun in its core, and the process of exchanging hydrogen to helium should be well underway, fuelling its fiery ambitions as a star. The rate of which they do ths and the amount of fuel available ultimately decides the mass of the star. Mass, for a star, is the key to the following factors; its luminosity as a star, its size as a star, and finally, its lifespan as a star. A star will spend most of its existence as a main sequence star, although this factor varies on a large scale, according to its mass. For example, a star approximately the size of our sun will spend 10 billion years as a main sequence star, stars half the size of our sun, however, will spend 80 billion years as a main sequence star, and stars 10x the mass of our sun, will only spend 20 million years as a main sequence star.
Red Giant or Red Supergiant
The next stage of a star's lifecycle is dependent upon the mass of the star, for example, a star up to 1.5x the mass of the Sun will transform into a Red Giant, whereas a star 1.5x to over 3x the mass of the Sun will evolve into a red Supergiant, basically a larger form of Red Giant in everything but mass. This stage occurs when a star has used up its supply of hydrogen in its core, forcing thermonuclear fusion of hydrogen in the shell surrounding the core to occur, greatly increasing the radius and size of the star, and consequently engulfing surrounding planets and other celestial bodies. Due to their increased size, red giants/red supergiants are many times more luminous than they were as main sequence stars, some as much as 100x the luminosity of their main sequence forms. But even being some of the largest stars in the galaxy has its drawbacks, as their surface temperature is inferior to that of main sequence stars, possibly due to the fact they are so massive. How massive? Well, to give you the picture red giants can be 100x that of the Sun, and red supergiants can be up to 8x larger than red giants, or 800x the diameter of our own Sun. That's big.
Supernova or Planetary Nebula
The next stage in evolution for a star is also based on mass. For stars that became a Red Giant in the previous stage will undergo a process known as Planetary Nebula, whereas stars that became Red Supergiants will experience something quite explosive, known as a Supernova. A Supernova is a stellar explosion which expels most or even all of a star's material, and is caused by the collapse of the massive star's core. Supernovae are a rare phenomenon, occurring only up to three times a century in the Milky Way Galaxy, and can briefly outshine entire galaxies and produce as much energy as our Sun is expected to within its entire lifetime, typically lasting a few weeks or even months on end, before fading from the skies altogether. Planetary Nebula is a process which begins at the end of the Red Giant phase, where the outer layers of the star are removed, most commonly through that of pulsations and strong stellar winds until all that remains is the core itself. Planetary Nebula are relatively short-lived (for a star), lasting approximately tens of thousands of years. During this phase, the exposed core of the star emits ultraviolet radiation, somewhat solidifying to some extent the outer layers of the star, previously ejected in the Red Giant phase of the star's lifecycle. These outer layers then reradiate the absorbed ultraviolet at visible frequencies, producing the spectacular phenomenon known as Planetary Nebula.
Neutron Star or White Dwarf or Black Hole
Now at the end of its life, a star with a low mass (up to 1.5x the mass of the Sun) will evolve into that of a White Dwarf, a star of particularly high mass, however, will convert into the similar Neutron Star, and the most massive of stars transform into something... else. A White Dwarf is the remnants of a stellar being, and is thought to be the final phase of evolution for most of the stars in the Milky Way (97%). A White Dwarf is merely what a star has left behind, with its faint source of light coming from the emission of stored thermal energy it once had in life. Despite having a size comparable to that of the Earth, its mass is still equivalent to that of our Sun, making it extremely dense in matter. When a White Dwarf has sufficiently cooled (13.8 billion years), it will turn into a Black Dwarf, a star that no longer emits light. A Neutron Star is a star that undergoes a gravitational collapse during a Supernova event, and are renowned to be the tiniest stars in the Universe, no longer than 10km in diameter. Regardless of this, Neutron Stars have a mass several times that of the Sun, and are thought to be the final evolutionary stage of life for a star with a high amount of mass, making them big in life, big in death. For stars with a mass of over 25x of the Sun (an extremely large star) however, have no means of withstanding their own gravity as they die, turning on those it once nourished and protected and turning into the dreaded and feared Black Hole, eating stars and devouring entire solar systems to sustain themselves.
Stellar Nursery
A star's life begins in a stellar nursery, also known as a molecular cloud. A stellar nursery is unique in that it is the only place in the Universe that permits the formation of a star, through the fact that it allows the growth of molecules, typically that of hydrogen gases and various other particle, the main ingredients in the creation of a star.
Protostar
A protostar is an early stage in the formation of a star, which is basically a large mass formed through the compression of gases in a stellar nursery, the birthplace of star. The protostellar phase lasts typically 100 000 years, although this period of time can vary depending on the mass of the protostar, and the abundance of the gases it so craves.
Main Sequence
After a protostar comes a main sequence star. By this time and stage in the star's lifecycle, nuclear fusion will have begun in its core, and the process of exchanging hydrogen to helium should be well underway, fuelling its fiery ambitions as a star. The rate of which they do ths and the amount of fuel available ultimately decides the mass of the star. Mass, for a star, is the key to the following factors; its luminosity as a star, its size as a star, and finally, its lifespan as a star. A star will spend most of its existence as a main sequence star, although this factor varies on a large scale, according to its mass. For example, a star approximately the size of our sun will spend 10 billion years as a main sequence star, stars half the size of our sun, however, will spend 80 billion years as a main sequence star, and stars 10x the mass of our sun, will only spend 20 million years as a main sequence star.
Red Giant or Red Supergiant
The next stage of a star's lifecycle is dependent upon the mass of the star, for example, a star up to 1.5x the mass of the Sun will transform into a Red Giant, whereas a star 1.5x to over 3x the mass of the Sun will evolve into a red Supergiant, basically a larger form of Red Giant in everything but mass. This stage occurs when a star has used up its supply of hydrogen in its core, forcing thermonuclear fusion of hydrogen in the shell surrounding the core to occur, greatly increasing the radius and size of the star, and consequently engulfing surrounding planets and other celestial bodies. Due to their increased size, red giants/red supergiants are many times more luminous than they were as main sequence stars, some as much as 100x the luminosity of their main sequence forms. But even being some of the largest stars in the galaxy has its drawbacks, as their surface temperature is inferior to that of main sequence stars, possibly due to the fact they are so massive. How massive? Well, to give you the picture red giants can be 100x that of the Sun, and red supergiants can be up to 8x larger than red giants, or 800x the diameter of our own Sun. That's big.
Supernova or Planetary Nebula
The next stage in evolution for a star is also based on mass. For stars that became a Red Giant in the previous stage will undergo a process known as Planetary Nebula, whereas stars that became Red Supergiants will experience something quite explosive, known as a Supernova. A Supernova is a stellar explosion which expels most or even all of a star's material, and is caused by the collapse of the massive star's core. Supernovae are a rare phenomenon, occurring only up to three times a century in the Milky Way Galaxy, and can briefly outshine entire galaxies and produce as much energy as our Sun is expected to within its entire lifetime, typically lasting a few weeks or even months on end, before fading from the skies altogether. Planetary Nebula is a process which begins at the end of the Red Giant phase, where the outer layers of the star are removed, most commonly through that of pulsations and strong stellar winds until all that remains is the core itself. Planetary Nebula are relatively short-lived (for a star), lasting approximately tens of thousands of years. During this phase, the exposed core of the star emits ultraviolet radiation, somewhat solidifying to some extent the outer layers of the star, previously ejected in the Red Giant phase of the star's lifecycle. These outer layers then reradiate the absorbed ultraviolet at visible frequencies, producing the spectacular phenomenon known as Planetary Nebula.
Neutron Star or White Dwarf or Black Hole
Now at the end of its life, a star with a low mass (up to 1.5x the mass of the Sun) will evolve into that of a White Dwarf, a star of particularly high mass, however, will convert into the similar Neutron Star, and the most massive of stars transform into something... else. A White Dwarf is the remnants of a stellar being, and is thought to be the final phase of evolution for most of the stars in the Milky Way (97%). A White Dwarf is merely what a star has left behind, with its faint source of light coming from the emission of stored thermal energy it once had in life. Despite having a size comparable to that of the Earth, its mass is still equivalent to that of our Sun, making it extremely dense in matter. When a White Dwarf has sufficiently cooled (13.8 billion years), it will turn into a Black Dwarf, a star that no longer emits light. A Neutron Star is a star that undergoes a gravitational collapse during a Supernova event, and are renowned to be the tiniest stars in the Universe, no longer than 10km in diameter. Regardless of this, Neutron Stars have a mass several times that of the Sun, and are thought to be the final evolutionary stage of life for a star with a high amount of mass, making them big in life, big in death. For stars with a mass of over 25x of the Sun (an extremely large star) however, have no means of withstanding their own gravity as they die, turning on those it once nourished and protected and turning into the dreaded and feared Black Hole, eating stars and devouring entire solar systems to sustain themselves.
The Hertzprung-Russel Diagram
The Hertzprung-Russel (H-R) diagram plots the temperature of a star on the horizontal axis of the chart and absolute magnitude (brightness) on the vertical axis. The table gives values for over 50 stars and can be seen to the left of this text.
The Hertzprung-Russel (H-R) diagram plots the temperature of a star on the horizontal axis of the chart and absolute magnitude (brightness) on the vertical axis. The table gives values for over 50 stars and can be seen to the left of this text.