Life cycle of a star

There are all manner of different creatures and things that go through a cycle of birth and ultimately death. This lifetime is defined in certain steps and absolute events, but no two are ever exactly the same. Some are longer, some shorter, and still others will choose a different path than the rest.

The life cycle of a star, the sun in our sky for example, is defined, but it also flexible. A star is born, and a star “dies” in a sense of the term, but the way it leads this cycle is not always the same from star to star. They can fizzle out, explode in a spectacular display, or even turn into a black hole.

Read on as we discover the many ways that a star can shine, and what factors decide how and when then reach the end of their lives.

How a Star is born: The First Nurseries

We are all most familiar with the sun, our solar system’s one and only star, which the Earth orbits around, along with all of the other planets in our system. This star may shine bright each day, but it didn’t start that way. Stars begin their lives as nothing more than molecular clouds in interstellar space.

These areas are referred to “stellar nurseries” or “star-forming regions.” The formation of stars is a branch of astronomy that focuses on studying the interstellar medium and these giant molecular clouds (GMC). If we back out to a view of our entire galaxy, the Milky Way, we’ll see these long arms that spiral out from the center.

It is within these arms that there are thick clouds of hydrogen, helium, and other heavier elements left behind by dying or dead stars. These dense areas are referred to as Nebulae, and it is here that stars are born. The name nebula means “cloud” in Latin.

These regions of space can span hundreds of light years in size. One example of a famous nebula is the Eagle Nebula, which provided one of NASA’s most famous images: “The Pillars of Creation.”  Inside these dense clouds, Hydrogen is found in its molecular form, (H2) which is where the name “molecular cloud” comes from.

These clouds are unique because they don’t always form stars. They remain in a state known as hydrostatic equilibrium for as long as certain factors hold true. There must be a balance between the gas pressure and the internal gravitational force for it to remain a cloud. Both the temperature and the density of the cloud will also decide when and how it will collapse.

The result of this balance being upset is a gravitational collapse. In some cases, known as “triggered star formation” this collapse can occur as the result of several possible events. Two clouds may collide with one another, or a star nearby could go supernova and interact with the cloud. Finally, collisions in general can trigger starbursts as the gas clouds become compressed and increase in density.

As the gas cloud begins to collapse, the matter within breaks down into smaller and smaller pieces. This gravitational collapse releases heat and energy, which causes it to drastically increase in temperature. Everything becomes focused on the central area, which is where the first visible portion of the star can be seen.

This area is known as the Hydrostatic Core. Eventually, when the temperature surpasses 2,000 Kelvin, the hydrogen and helium gases begin to ionize and absorb the energy being released from the gravitational collapse. The energy from the core and the radiation from the exterior allows it to create a state where the internal pressure is enough to stop any further collapse.

This state, known as hydrostatic equilibrium leads into the formation of a protostar. The protostar continues absorbing dust and other materials until it reaches a temperature where deuterium fusion can begin. This process involves an isotope of Hydrogen which contains one proton and one neutron, making it deuterium or “Heavy Hydrogen.”

This fusion generates continued heat for the star until it reaches a stage known as a pre-main-sequence or PMS star. During this phase, the star’s final mass is decided. Solar mass is measured in comparison to the mass of our own sun. The mass of the star decides what happens next.

Sometimes though, the formation is never completed. Stars that have less than .08 solar mass are not able to reach a temperature where they can begin fusing hydrogen instead of deuterium. These are known as brown dwarfs and while they do reach the protostar stage, they shine dimly and die out very slowly before they can reach the main sequence.

We are close now, for stars that are large enough in mass, the temperatures within can reach 10 million Kelvin and a process known as the proton-proton chain reaction occurs. In this process, the star is able to fuse hydrogen molecules into helium. If the star is larger than 1 solar mass, an alternate process called the carbon-nitrogen-oxygen reaction occurs.

In either case, the nuclear fusion creates a hydrostatic equilibrium as we previously discussed, and this allows the star to become stable. The pressure from the radiation being emitted by fusion protects the star from further collapse. This is when the star enters the main-sequence and the bulk of its life.

The Main Sequence: A Star’s Life

Stars that have reached a stable state can appear on main sequence which is a measurement of their color and brightness. This measurement appears on a scatterplot known as the Hertzsprung-Russell diagram. Stars that fall on this main sequence are known as “dwarfs.”

Our sun is a yellow dwarf and falls within this spectrum, in fact, almost 90% of the stars in the universe fall on the main sequence spectrum. The lifespan of these stars is dependent on how massive they are. Stars remain on this phase until they’ve exhausted their supply of hydrogen to fuse into helium.

Our sun will spend roughly 10 billion years in the main sequence phase. Other stars which are far more massive will only maintain fusion for 20 million years or so. On the other side of things, stars half the size of our sun can remain in their main sequence for as long as 100 billion years which is much longer than the current age of our universe.

The stars that fall on the main sequence can be placed into several different categories based on their temperatures, their luminosity, and their mass. Let’s examine briefly each of these stellar classifications:

  1. Class O

Stars within this class are among the largest in the known universe and burn extremely hot and bright. The light they emit is mostly within the ultraviolet range of the spectrum. They are known as supergiants and burn through their supply of hydrogen much faster than other stars and leave the main sequence far more quickly.

  1. Class B

This type of star is extremely rare. Only about 0.125% of the stars in our solar neighborhood fall into this class. They are extremely right and give off a blue color but they, like O stars, burn through their hydrogen extremely quickly.

  1. Class A

While only about 1 in 160 stars in our area fall into this class, they are among the more common stars that can be seen by the naked eye. They shine with a blueish-white light and have a strong luminosity.

  1. Class F

These stars range in size from 1.0 to 1.4 times the mass of our own sun and shine with a similar yellow-white hue. These are more abundant and account for about 1 in 33 stars in the night sky. One example is Canopus, a supergiant that represents the second brightest star in the night sky.

  1. Class G

These are the most well-known stars, mostly because our own sun falls into this class. Roughly 7.5% of the stars in our area (1 in 13) are within this classification of main sequence stars.

  1. Class K

Even more prevalent are the Class K stars which are cooler than the sun and give off an orangish color. These account for almost one in eight of the stars in our neighborhood. There is also a giant form of this star class that includes supergiants such as the star Arcturus.

  1. Class M

While this class makes up a shocking 76% of the main sequence stars in our area, they are also too dim to see with the naked eye.

Hydrogen Runs out: The Possible Deaths of a Star

When the hydrogen in a star runs out, several possible things can occur based on the mass of the celestial body. Stars that have a lower mass are the ones where we have the least amount of knowledge. These types of stars are currently still in their main sequence despite the fact that the universe is 13.8 billion years old.

Some of these stars will live for much longer before their ending phases can be observed. It is thought that stars like red dwarfs which are 0.1 solar mass will burn for six to twelve trillion years and slowly increase in both temperature and luminosity. After this, several billion more years will go by before it collapses into a white dwarf. Stars that are slightly larger but their helium cores are not big enough to reach the temperatures required for helium fusion. These will move toward the red giant phase but ultimately will become a white dwarf.

Stars that range from .5 to 10 solar masses will become red giants when their hydrogen supply runs out. These stars leave the main sequence are classified as K or M. These stars have cores that have ceased to fuse and instead have layers on top of the core called hydrogen-burning shells that continue the process of fusing hydrogen into helium.

From here, we have two phases of red giants that exist after the main sequence phase:

  1. Red-Giant-Branch Stars

Once the main sequence has finished, the core of the star collapses under the internal pressure and releases a burst of energy which triggers the hydrogen shells around it to begin fusing themselves. The core of a mid-sized star up to the size of a few solar masses continues collapse until it reaches another state of hydrostatic equilibrium.

This balance is achieved as a result of electron degeneracy pressure which balances the gravitational pressure on the core. This results in the first layer above the core being compressed by gravity. This layer then begin fusing hydrogen much faster than before.

The star’s brightness is multiplied by as much as 10,000 and it expands rapidly, decreasing the overall temperature of the star. The resulting helium from the outer layers fusing hydrogen is added to the core until the star is hot enough to ignite helium fusion. The time it takes for this to occur is again, dependent on the star’s mass.

  1. Asymptotic-Giant-Branch Phase

Once the helium has been exhausted in the core, the fusion moves up into shells around a dense core of carbon and oxygen. This next phase runs parallel to the original red giant phase, but it happens much faster. While helium continues to be burned in a shell, the majority of the star’s energy comes from hydrogen being burned near the surface.

There are a number of classes within this phase, but in the end they eventually run out of fuel for any further burning in the outer shells. They cannot sustain temperatures that allow for carbon fusion so they collapse again. The gases and elements escape from the core as it cools to a white dwarf. The surrounding area forms a planetary nebula around the still hot star in the center.

These phases apply directly to mid-sized stars, but what about the ones that are massive in size? What happens to them?

The Fate of Massive Stars

For more massive stars, the core is is already large enough to burn helium before the electron degeneracy pressure is able to take hold. They expand and cool like mid-sized stars, but they do not do so with as much change in their brightness. These types of stars very rarely survive to become red supergiants, instead they are destroyed in type II supernovas.

Incredibly large stars which are more than 40 solar masses tend to lose their mass extremely fast from stellar winds that strip off their outer layers. They have extremely high surface temperatures. In cases where the solar mass is below 1.4 the star will most likely end as a white dwarf surrounded by a planetary nebula.

For those that are larger (above 2.5 solar masses) the inner temperature can reach 1.1 gigakelvins, at which point neon starts to break down into oxygen and helium. The helium here immediately fuses with the remaining neon to form magnesium. The oxygen also fuses into sulfur, silicon, and other elements.

At a certain point, the temperature becomes so high than any nucleus or element will partially break down and fuse into something heavier. In these cases, the core mass may become too high to form a white dwarf. In addition, it also does not have the ability to continue converting the neon to oxygen and magnesium.

This results in a core collapse which is sudden, violent, and catastrophic in nature. This results in a supernova of various types, in most cases a type II supernova. The brightness of such an event outshines an entire galaxy, releasing as much as energy as some stars do in their entire lifespans! The star’s material is sent outward in a shock wave that travels as 10% of the speed of light.

This mixture of violent and quiet ends results in a final stage known as the stellar remnant of a star. Let’s examine the possible ends to such a beautiful and incredible phenomenon.

A Final Monument: 3 Possible Stellar Remnants

The final fate of a star’s life span comes in three separate forms. Below are the three major classifications and a description of each:

  1. White and Black Dwarfs

When a star finally ceases to burn, the remaining mass can potentially be roughly the size of the Earth and is known as a white dwarf. This mass is still extremely hot and takes billions of years to cool completely. When the mass has fully cooled, all that remains is a dark mass called a black dwarf.

No black dwarfs exist yet though because the universe isn’t old enough for any white dwarves to have cooled fully. While this state can be altered by the addition of new matter, resulting in supernovas or smaller explosions called a nova, for the most part the white dwarf marks the end of most stars.

  1. Neutron Stars

In certain cases, when the core collapses the pressure results in many of the protons in the star being converted into neutrons in a process called electron capture. The result is a star which is incredibly small, about the size of a large city. The star rotates incredibly fast, in some cases over 600 revolutions per minute.

In some instances, the magnetic poles of these stars align with the Earth. This causes a pulse of radiation to be sent out every second that we can detect. These are known as pulsars, and it was how we first discovered neutron stars existed.

  1. Black Holes

The mass of a stellar remnant can be high enough to outweigh the neutron degeneracy pressure. This will result in a collapse that falls below something called the Schwarzschild radius. At this point, a black hole is formed where the gravitational pull is so great that not even light can escape the pull.

We don’t know for certain if a black hole is formed without a supernova. Some theories suggest that neutron stars must be formed first, then collapse into a black hole. Until we can observe more supernova events, we won’t know for certain. As it stands, this is one possible fate of a star.

From Dust to Dust: The Life Cycle of a Star

Stars are incredible and extremely prevalent aspects of our universe. The life cycle of a star, and its death are something we can glimpse, but not something we can witness entirely. They are seemingly immortal, with lifespans that, in some cases, have yet to be reached since the beginning of the universe. Thanks as always for reading, and remember to check out our other in-depth articles.


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