‘Life Styles’ of Stars (An Intro to Stars’ life-cycles and main-sequence)

“I love the stars too fondly to be fearful of the night!”
— -Sarah Williams

When you gleam at the night sky pockmarked with shiny dots called stars… do you ever realize that we have such a shiny candidate waiting just 150 million kilometers away from us — its called the ‘Sun’.

Source: Pinterest

But — what we actually do not know is that even stars have ‘lifestyles’ (though not life) even!

So, lets start from the very inception of a star.
So, how are stars actually born? Well, our cosmos is full of clusters of gases (mainly hydrogen and helium) and dust called nebulae.

The Crab Nebulae (Credits: ESA,NASA and Allison Loll/Jeff Hester)

Whenever any supernova explosion happens near a nebula or any big object like another star passes near it — it pulls a lot of nebula matter towards one side. Whenever that cluster is denser on one side, it starts to cluster and get heated up and soon start the nuclear fusion reactions. So, what is this nuclear fusion reaction? Well, its the very reaction that gives energy to our Sun and all other stars. Its a reaction in which hydrogen is forcefully converted into helium in extremes of pressure an temperature normally at the center of a star.

The nuclear fusion reactions turn hydrogen to helium and even produce a lot of energy which is emitted by stars.

A visual representation of the bombardment of deuterium and tritium in the nuclear fusion reaction

Back to the lifetime — when the cluster starts to form, it slowly starts to possess mass. And where mass is -gravity no question make! Gravity even follows and the cluster slowly starts to attract the very immediate gases and dusts and adds them up to its own body. Its now a new-born star, a nascent star - but not as innocent as nascent human babies! The nuclear fusion reactions at its core become its powerhouse.

Now, star after this formation can be classified into two categories:

  • Medium- sized stars or average sized stars. Our Sun is a medium sized star.
  • Massive sized stars. Examples include NGC 3603-A1,WR21a, and WR20a. This comparison of our sun and a massive star — -> LBV 1806–20 will tell you clearly how massive are massive stars!
A vivid depiction of the supposed ‘Life Cycle’ of a star. (Source: Futurism)

But there must be a limit which separates them! Sure we have it! Its called the Chandrashekhar Limit! Named after the Indian astrophysicist Subhramanyan Chandrashekhar, the Chandrashekhar Limit is about 1.4 times our sun’s mass (or solar mass). Though physicists even tell that average stars can be from 0.5 to 8 solar masses.

Any star below this mass will follow the upper route of the picture given above. Any star above this mass will follow the lower route of the above picture. Well, this limit destines whether a star would die off like a blaze in the universe or die a very silent honorable death! Well, yes!

Subrahmanyan Chandrasekhar was awarded the 1983 Nobel Prize for Physics with William A. Fowler for “…theoretical studies of the physical processes of importance to the structure and evolution of the stars” (Source: India Today)

When a star is at that stage of its hypothetical life, when it can do standard nuclear fusions at its core and has a majority of hydrogen over helium… its said to be in the main sequence of its life.

First, let’s talk of the average sized stars. According to astrophysicists — average stars can range from 0.5 to 8 solar masses. They have a steady rate of nuclear fusion reactions. Our Sun being one will currently support the reactions for about 4.5 or 5 billion years more.

In an average sized star, when the hydrogen fuel gets nearly exhausted and helium dominates over the majority of the star, there is no more an equilibrium acting between the pulling force of the gravity and the pushing force of the nuclear fusion reactions but the net gravitational force pulling everything to its center becomes larger . Average pressure of the average sized star becomes so high that fusion reactions start to take place on the outskirts of the star. That increases the volume of the star. Hence, it forms a red — giant and nears at the later stages of its life. One very popular example of a red giant is Aldebaran.

Aldebaran (Alpha Tauri)

​The surface temperatures of a red giant become as cool as 5000 K (not as cool as we wanted though!). Soon nuclear fusions with the remaining hydrogen again, producing more and more energy and helium! An average red giant has a diameter of about 10 or 12 times the diameter of our Sun.

​Now, the pressures at the core multiply by a huge amount when the upper layers starts to convert from hydrogen to helium and get heavier. The pressure becomes so much that the then helium core becomes a solid carbon core! Hydrogen takes the outermost layer of the star since it is lightest while helium takes the middle layer — and carbon forms the core.

​Slowly, when the red giant reaches the last stages of its expansion… the outer layers of the red giant drift so much that they are no more affected by the pull of the gravity of the star. This is called the planetary nebula stage of the average sized star. As the outer layers drift away — what remains is a silent carbon and helium core which starts to cool off. What was once a massive object gets shrunk to some mere thousands of kilometers in diameter.

Another Depiction of the Two forms of ‘Life-Cycles’
Hubble image shows striking details of the famed planetary nebula NGC 2818 (Pyxis)

Then it proceeds to its very near to death phase called the white dwarf phase where the star no longer does nuclear fusion reactions but emit its remaining potential energy in the form of light and heat for billions of year more.
Then, slowly, it proceeds to its death phase called the black dwarf phase. At this phase it has no more energy remaining and is cold and dark. It becomes nearly invisible and is like a dead body in space.

Now we will discuss on the life of a massive star.
Any star above the mass of 8 solar masses will develop to be massive stars. Since massive stars are bigger in comparison to average star… they have more hydrogen fuel - the nuclear fusion reactions are more rapid in them and hence they have a shorter lifetime. Its said — ‘The more you have… the faster you die!’ Some very good examples of massive stars are Rigel and Betelgeuse in the Orion Constellation.
Now, massive stars like other stars are formed from nebula. Even they have that phase when helium takes over and the pressures become so much that they convert into a carbon core. It is called the red supergiant phase. But the only difference is that they don’t stop there! They continue fusing into more and more elements from helium to carbon to oxygen to neon to magnesium until the core becomes pure inert ferum (iron) by fusing. This model of a red supergiant shows the layers of different elements formed inside a red supergiant due to continued fusion reactions:

A depiction of the concentrated fusion cores of a Red Supergiant Star (Source: phys.uaf.edu)

​Up to the production of iron in the most massive stars, the nuclear fusion process is able to create extra energy from the fusion of lighter nuclei. But the fusion of iron nuclei absorbs energy. The core of the massive stars implodes and the density gets so great that protons and electrons are combined to form neutrons + neutrinos and the outer layers are ejected in a huge supernova explosion, something like this -

An artist’s depiction of a supernova explosion

​The supernova explosion blows off the outer layers of the star and the gases flow off the star. But, there is a dense remnant core that is left behind -
The state of the remnant just after the supernova explosion becomes something like this:

An artist’s depiction of the remnant after Supernova explosion

A dense core remnant and the gases all around — oh, its so magnificent!
The remnant then makes the difference! If the remnant ranges from a mass of 1.5 to 3 times our solar mass — then it becomes a neutron star. A neutron star is a dense pulsating star, where one teaspoon of matter has a mass equivalent to our Mt. Everest!

An artist’s depiction of a neutron star

As written in NASA’s website:

Most known neutron stars belong to a subclass known as pulsars. These relatively young objects rotate extremely rapidly, with some spinning faster than a kitchen blender. They beam radio waves in narrow cones, which periodically sweep across Earth like lighthouse beacons.

What if the remnant is more than 3 solar masses?
It will give rise to the devil — the black hole!
When the remnant core is more than 3 solar masses… the gravity collapses from within to give rise to a black hole. Black holes are practically so dense objects that they curve the space-time curvature infinitely!

The interaction of celestial components with the space-time continuum

I mean its sooo dense that not even light can escape from it. It pulls everything! A typical black hole has the following parts:

The parts of a typical black hole (Source: NASA)
  • Event Horizon → It is that border of the black hole from where not even light with the enormous speed of 299,792,458 km/s can escape — its literally the point of no return.
  • Accretion Disk → This is a disk mainly composed of dusts and gases — that revolves around the black hole. It has extremes of temperatures — nearly about some millions of kelvins
  • Singularity — -> This is a place which no physicists can define till now. The physics of matter here is unknown. Physicists think that its the deepest curvature in a black hole and time actually stops for an observer inside it.

With this, we come to a end of this explanation. I hope this has been fruitful! :)
I will gratefully accept any errors done by me — since all this research is a product of my sole enthusiasm, please do comment.

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Spandan Mallick

Spandan Mallick

An active astronomy enthusiast. Pursuing B.E in Electronics