Stellar Evolution: What does the Life of a star look like?

13.8 billion years since the big bang, we are now left with the spectacle of the cosmos with the stars and galaxies slowly undergoing their life cycle.

Today on Earth we have the periodic table with 118 elements listed into various categories but all these came from the first element. Yes! When the universe had just been born, a few femto-femtoseconds after the big band, Hydrogen was the only element formed in the universe. So if you are wondering where did that silicon, used in building the very screen you are staring at, come from…. you will find the answers when you journey the evolution of stars.

Whatever your beliefs, all of them, including science says “whatever begins, has to end” and this applies to the stars as well. Lifetimes vary and what new comes is usually from that which was left behind by the old…

Like in ‘Seven Ages of Man’ by William Shakespeare, where he ponders about the roles played by person with the passage of time, Stars too go through various stages in their lifetime over a period of millions or billions of years.

Of course, human lifespan is nothing compared to theses stars, disabling us from observing the different stages of a single star during its lifetime, we can nonetheless observe millions of stars at one of the other points in their lifetime and deduce a whole lot of data from our observations.

The path that a star follows particularly depends mainly on its mass or how much gas is being collected which is basically the star’s fuel.

Like the two ends of a magnetic, like (positive and positive) poles repel each other. This also means two positively charge nuclei (that of hydrogen) cannot come close to each other. But at higher energies, they can overcome this repelling electromagnetic force just enough to activate the strong force which is 100 times stronger. When the two nuclei with single positive charges each get close enough we have a new nuclei with two positive charges. Et voilà! You have Helium!

But the mass of helium is less than the mass of two hydrogen leading to a difference in mass which is given out in the form of energy. (E=mc² … remember?)

The same process happens in the stars, the collision of nuclei gives out enormous amount of energy which forms an outward pressure, but these particles are so dense that they distort the space-time around them to have their own gravity which pulls back these particles.

It is the balance of this outward push by fusion and inward push by gravity, that forms stars. Forming from large amounts of gases, we have stars of large masses which help us determine their age. This means mass is going to be a key factor in this process, so let us consider stars of different masses and known what will happen in their life time.

Protostar: Baby Steps of Every Star

It took 17 minutes of the big bang for Nucleosynthesis to occur leading to the formation of Hydrogen and Helium. Large chunks of these elements (gases) condensed such that their density was large enough for them to have their own gravitational field.

This gravity brought together more chunks all of them getting squeezed, raising temperature and as mentioned above, at high temperature, due to strong nuclear force they fused to form heavier nuclei. The released energy pushing them outward and gravity pulling these nuclei inward. Equilibrium between the two forces leads to the formation of small stars. As the gaseous cloud in which they are in is smaller in size and mass, these small stars formed are called as Protostars.

If in case the forming-star cannot raise enough temperature to balance the gravitational pull the star fails to form a star and such objects are known as Brown Dwarfs.

The Protostars are red in colour, small in size with low mass and glows with all the energy they have from the collision and fusion that is happening inside it. This fusion reaction begins when two protons are colliding and fusing to have a β decay reaction where a proton and a neutron is formed which is also called as Deuteron ₁H², these Deuterons are involved in further fusion reaction to form isotopes of helium and then finally forms Helium atom ₂He⁴. These Protostars have their fuel as this collision.

Once the star has successfully completed the Protostar stage, it will now enter the main-sequence stage. At this point, its fate is decided by its mass.

Low mass star: “Life to Simplicity”

The stars with masses from 13 Jupiter masses to a mass of 1 M_☉, at first will slowly convert the Hydrogen into Helium in the core maintaining relatively steady size, temperature and luminosity. When most of the Hydrogen are converted into Helium, the core of the star will shrink and get hotter causing the remaining Hydrogen to fuse faster releasing extra energy which is radiated outwards. This makes the outer gaseous shell push away from the core as the outward pressure is more than the gravitational force holding it together. As these outer layers expand they cool and become more and more red. This can be understood by Wien’s displacement law,

λ_max = b/T .

The star becoming redder makes it a Red Giant. This causes a shift in its phase. The star will be in its red giant stage for several millions of years. After all the Hydrogen has fused into helium in the core, the core gets even small and even hotter leading it to enter into a helium flash stage where the helium nuclei fuses to form heavier nuclei like carbon and oxygen, this means that the star now runs on a new fuel, ₂He⁴ . From here the star starts pulsating as it has partially lost its equilibrium and entering into horizontal branch in the H-R diagram, with the star getting smaller, hotter and bluer.

When much of the ₂He⁴ is fused into heavier nuclei, once the core is mostly carbon with just a small quantity of helium around it, the amount of energy to exert and outward pressure is very low. Gravity wins, and the core of the star collapses and the star enters asymptotic giant branch. This causes the star to grow rapidly and become a giant star again until the last burst of energy eject the outer layer pushing it away from the core and back into the interstellar medium, leaving only a tiny very hot bare core around, which is about the size of the earth. This will gradually cool down as it has no more fuel left to burn and not being hot enough for further fusion of carbon or oxygen to form heavier nuclei. This leads to further contraction and this stage is called as White Dwarf star. The ejected hydrogen, helium gaseous shell is called as planetary nebulae. These planetary nebulae goes and joins up other heavy and big nebulae to form bigger nebulae to further form new stars and planets, these nebulae is very huge they are so vast that they are measured in light-years.

After spending billions of years being a white dwarf the star goes to a stage where the luminosity of the star decreases. We call this a black dwarf and their visibility becomes zero.

This is the end of life of small massed stars…

It was shown by Dr. Subrahmanyan Chandrasekhar , at the age of 20, who showed that stars with mass <8 M_☉ or lower are the stars that end up in the Red Giant stage. He also showed these Red Giants that leave behind a White dwarf will always have a mass that is less than or equal to 1.44M_☉

High mass stars: “Bigger stars goes out with a Bang!”

The stars with mass greater than our Sun ( or 8M_☉ to be precise) lead a not-so-simple life. Their end, will not be so quiet and simple; bigger stars goes out with a bang. While these stars start as Protostar itself, they are usually formed from nebulae that are much larger. More the mass, more is the gravity. This means the inward force is much stronger, and the star gets much hotter, so with increase in the temperature, the fusion is faster, which generates greater outward pressure to counter the gravity. This will make the star reach the main sequence. Main sequence star is hot, bigger, brighter and bluer.

This is the point of divergence from the low massed stars. Low massed stars takes billions and millions of years for using up all their fuel, whereas high massed stars which are much hotter burn their fuel very fast, that means they can burn up most of the hydrogen from their central core in a shorter duration (~100 million years or even less ). As the fuel starts running out, the core get smaller and hotter, producing more energy causing the star to swell up. As it continues to form carbon and oxygen the core gets even smaller and hotter; but, this core has so much energy that it can fuse carbon nuclei and oxygen nuclei into heavier nuclei. This continues till the central core has created the element Fe.. Iron.

When God calls you have to depart.. When Iron forms you have to fall apart.

The fusion of upper layers continues fusing until no fuel remains this happens till everything is converted into stable iron nuclei. Iron is a disease in stellar terminology as this is the point where the star falls apart. Iron unlike lighter elements requires a humongous amount of energy for fusion. While some of the stars can produce this energy, the bigger problem is that unlike the lighter elements, Iron does not release energy during fusion, but instead, absorbs energy during fusion.

When Iron fuses, the energy inside the star is absorbed and the outward pressure generated by the star decreases. At this point gravity wins and within a matter of seconds the star collapses with the outer layers bursting out and exploding. The explosion caused is one of the most violent, energetic and at the same time, beautiful events in the universe. We call this a Supernova. It is in this explosion that the Iron is ejected into space and the energetic ejection causes iron to fuse further and form all the elements [in the periodic table], that are formed naturally.

You are all starsdust!

All the elements that make up the material in our world, from Hydrogen in the water, to the Carbon that you breathe out, from the Iron gates outside your home to the Gold ornaments that are worn by you, everything comes from this supernova explosion. This is also the reason why the elements in the periodic table before Iron are in abundance while all the elements after Iron are rare.

One of the Three ways to go

Supernovae is the 2nd point of crossroad for the star which has had a mass to overcome the Red giant stage.

Fate #1 : 8 M_☉ to 10.5 M_☉

A main sequence star with mass less than 10.5 M_☉ will leave behind a white dwarf. Once the size of the core is reduced there is not enough gravity to overcome electron degeneracy pressure. This is why a white dwarf is a gigantic metallic solid with electron clouds around the nuclei pushing against each other and preventing further collapsing. Even though, this object is so dense that a teaspoon full of a white dwarf material would weigh about 15 tons. As stated by the Chandrashekar limit, the mass of these white dwarfs are always less than 1.44 M_☉.

If the white dwarf’s mass is greater than 1.44 M_☉, it will further collapse.

Fate #2 : 10.5 M_☉ to 29 M_☉

For stars with masses between 10.5 M_☉ and 29 M_☉, the gravity is so large that all the electrons of the atoms in the core collapses into the nucleus, which combines with protons to form neutrons

p⁺ + e^– → n⁰

What remains in the core is neutrons as all the protons and electrons have combined. With much of the matter gone, this leaves behind a sphere of radius about 10km. What a small sphere… Nope! Small may be the size, but the weight of such a Neutronstar is about 10 million tonnes.

The core of the star aka the neutron star weighs between 1.44 M_☉ to 3 M_☉.

Fate #3 : 10.5 M_☉ to 29 M_☉

When a star greater than 29 M_☉ collapses, it forms a neutron star greater than 3 M_☉ . The outward pressure of the neutrons pressing against each other (neutron degeneracy pressure) is not enough to stop gravity. The neutrons will be crushed together as the remaining mass collapses into a single point of infinite density. The entire mass of the star core contained within zero volume this unimaginable object is called as black hole.

The gravity of such an object is so strong that to escape its gravitational pull, one would have to move with a velocity greater than 3×10⁸m/s. This being the maximum velocity limit for matter, and light, not even light can escape the gravitational pull of such an object; matter as well just falls into such a point disappearing forever… earning it the name Black Hole. The gravity of such an object is so strong that light (which always travels in a straight line) passing nearby is bent.

Black holes formed by a star are called stellar blackholes. These stellar blackholes can have masses ranging from 3 M_☉ to millions of solar masses.

This marks the end of Stellar evolution. Each star is born as a Protostar or fails as a Brown Dwarf. If a star does succeed, it can (based on the mass) join any point in the main sequence of the HR diagram. Once the fuel has been used, (again based on mass) the star can end up as a White Dwarf through Red Giant, or cause a Supernova explosion ending up with a White Dwarf or a Neutron star or a Black Hole.

So amazing is the journey of the star that is is a constant battle between matter and gravity where one wins and the other loses. A process that takes millions of years.

More on black holes in a future post. Like this post to show your support and share it if you think someone will also like this post.

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Important cosmological or astronomical terms and concepts used here:

• Wien’s displacement law: “A black body radiation curve for different temperature will peak at different wavelength, that they are inversely proportional to each other.”
• Stellar spectra:
  • O Type:
    • Color: Intense Blue
    • Temperature: 30000K to 60000K
  • B Type:
    • Color: Bluish White
    • Temperature: 10000k to 30000K
  • A Type:
    • Color: White
    • Temperature: 7500K to 10000K
  • F Type:
    • Color: Yellowish White
    • Temperature: 6000K to 7500K
  • G Type:
    • Color: Yellow
    • Temperature: 5000 to 6000K
  • K Type:
    • Color: Yellowish Orange
    • Temperature: 3500K to 5000K
  • M Type:
    • Color: Red
    • Temperature: < 3500K

    Note: The stellar size and mass increases from ‘M’ type to ‘O’ type.
    This stellar spectra is based on the Wien’s displacement law.

• The solar mass is represented as M☉ which is the mass of Sun approximately equal to 1.99×10³⁰kg.
• E=mc², Einstein’s mass energy equivalence relationship: 1kg of mass if converted to energy becomes 9×10¹⁶J.
• Triple alpha process: set of nuclear fusion reactions by which helium (₂He⁴) are transformed into carbon.
• Neutron degeneracy pressure: Degeneracy pressure is a pressure exerted by dense material consisting of fermions (neutrons, protons, electrons etc.).
• Nucleosynthesis: process of creating new atomic nuclei from pre-existing nucleons.

Hertzprung-Russell diagram:

This diagram is a plot of luminosity (absolute magnitude) against the colour (temperature). The ordinary hydrogen-burning dwarf stars like the Sun are found in a band running from top-left to bottom-right called the Main Sequence. Giant stars form their own clump on the upper-right side of the diagram. Above them lie the much rarer bright giants and Supergiants. At the lower-left is the band of white dwarfs – these are the dead cores of old stars which have no internal energy source and over billions of years slowly cool down towards the bottom-right of the diagram.

Classification of Stars

Stars are classified into five main luminosity classes. These are the five classes:

• Supergiants : Very massive and luminous stars near the end of their lives. They are sub classified as Ia or Ib, with Ia representing the brightest of these stars. These stars are very rare – 1 in a million stars is a supergiant.

• Bright Giants: Stars which have luminosity between the giant and supergiant stars

• Normal Giants: These are mainly low-mass stars at the end of their lives that have swelled to become a giant star. This category also includes some high mass stars evolving on their way to supergiant status.

• Subgiants: Stars which have begun evolving to giant or supergiant status

• Dwarfs: All normal hydrogen-burning stars. Stars spend most of their lives in this category before evolving up the scale. Class O and B stars in this category are actually very bright and luminous and generally brighter than most Giant stars.

This article was originally posted at cosmologybyhashim.blogspot.com : http://cosmologybyhashim.blogspot.com/2020/03/stellar-evolution-life-of-star.htmlhttps://cosmologybyhashim.blogspot.com/2020/07/stellar-evolution.html