Star – Space Age

Stars in The Night Sky- Stars Explained

I am sure that there has been a time in your life when you looked up at the stars in the night sky and thought to yourself. Hmm, I wonder what makes them shine so brightly? There must be some secret behind it. Well, it’s no secret; it’s science! For centuries, stars have been used widely in poetry, essay, children’s rhymes, and lullabies. We have always gazed up at the night sky and tried to identify the constellations. Oftentimes, we are able to identify some prominent stars which are closer to Earth as they usually shine brighter. Let us now delve deeper into the cosmos and understand how these wonderful twinklers get their shine.

How are Stars Born, and what are they made of?

Stars are massive gas giants, not the ones like Jupiter but the ones which are way more massive than Jupiter. Did you know that it requires the mass of 70 Jupiter-sized planets to form the smallest star? 70 Jupiter-sized planets are the smallest amount of material that is enough to trigger nuclear fusion to qualify as a star. This type of star is known as a red dwarf. To understand the principle of star formation, we first need to understand the key concepts of nuclear fusion [1], nebula, and the influence of gases like Hydrogen, Helium, and oxygen.

A nebula is a large accumulation of gas and dust in the fabric of space. Gravity influences this nebula, and the gas and dust begin to shrink and divide into small balls which look like swirling clumps. When each clump becomes ball-shaped, they continue to shrink the material, which begins getting hotter. Once the temperature reaches 10 million degrees centigrade (1 crore), an explosion would occur, thereby resulting in a nuclear fusion. When this fission starts consequently, and at a large scale, a new star is born. There are other ways how a star can be born; one such way is through a supernova explosion (which will be covered in this article), where the death of a massive star gives rise to a new high-density star known as a neutron star.

The Life of a Star

Learning the life of a star would be quite interesting as it delves into various aspects which could broaden the perspective that you have about our universe. It will also help you see our Sun in a new light and understand how its life was and will come to be in the distant future. The life span of a medium-sized star such as our Sun is around 5 billion (500 crores) years. Larger stars would have shorter lifespans; this is because their energy would burn out quicker. Similarly, stars that are smaller than our Sun tend to last longer. Have you ever wondered why stars don’t seem to change at all? A star would look the same to you now just as how you had observed it when you were five years old. This is because your lifespan would be like a second to that of the lifespan of a star.

The Birth of a Star

Not all stars follow the same path; their path would depend on the mass of the star or how much gas was collected and collapsed to form that star. This gas would serve as fuel for the star. In a nebula, when nuclei of an atom collide with enough energy, there would be a large electromagnetic repulsion between them. The strong nuclear force would take over, and the nuclei would fuse, with a small fraction of their mass converting into energy. Only when nuclei collide and fuse into the core of a star will they release enough energy to counteract the gravity crushing inward. The matter that forms a star determines the amount of fuel. With a variety of other factors, the lifetime of a star can be determined.

As we now know that any star begins from a large cloud of gas and dust; this material needs to be at least a few lightyears across. During the earliest era of star formation, this material would exclusively be Hydrogen and Helium. This was what existed in the brief 17 minutes after the big bang nucleosynthesis [2]. Hydrogen and Helium collect due to gravity and push inwards as it contracts. In this process, immense heat is generated, resulting in a nuclear fusion. This entire process is not immediate as it occurs over a few million years. A yellow or red main sequence star will be born in the process of the fusion reaction, and the glow will be the result of this reaction happening in a nearly endless cycle.

The Red Giant Phase

As long as there is hydrogen to fuel the nuclear fusion reaction, the star will continue to shine over a few billion years, depending on the mass of the star. Once the hydrogen fuel begins to deplete, the core of the star will slowly begin shrinking. When the core starts to shrink, it will become much hotter, resulting in the hydrogen fuel burning faster. The fast-burning fuel will result in extra energy released by the reaction, which radiates outwards, resulting in the outer layers of the star being pushed away from the core. This process would result in the star’s outer layers expanding, making it appear like a red giant. When the outer layer cools, the star becomes red. The star will continue to expand until most of the residual fuel is burnt. A star like our Sun will reach this stage of the life cycle around 5 billion years from now. At present, our Sun has already lived 50% of its life. A star will typically stay a red giant for around a billion years.

White Dwarf- The Death of a Star

Yes, even stars die out eventually, but they would have lived a glorious life of 10 billion years (1000 crore) before they die out (This is for a medium-sized star such as our Sun). After all the reserve hydrogen fuel in the star depletes, the core begins to get even smaller and even hotter. This results in a helium flash phase. Wherein the star becomes so hot that it begins fusing heavier helium nuclei into larger nuclei through a process known as triple alpha. This means that the star has a whole new fuel other than the one that it burns for billions of years. This results in the star starting to pulsate as it runs to the final energy fuel. At this point, the star starts to contract and becomes smaller, hotter, and appears blue as most of the Helium has fused into larger nuclei. The core then becomes mostly carbon and oxygen, with a shell of Helium and hydrogen around it.

At this phase, the star has very little material left to burn, and the core will collapse. This results in the star expanding to a red giant once again until its last bursts of energy eject the outer shell. Once the outer layer of the star dissipates, only the core, which is roughly the size of our Earth, will remain. This core will gradually cool due to a lack of fuel to burn as it is not hot enough to fuse carbon or oxygen nuclei. This results in the core contracting further until the star appears to be white, earning the name “white dwarf.” This white dwarf phase would last for 10s or 100 billion years until eventually further cooling and becoming a black dwarf. At present, there are around 8 white dwarf stars discovered among hundred-star systems that are closest to our Sun. There are no known black dwarfs in our universe, as it would take trillions of years for a white dwarf to reach that stage, and our universe is just 13.8 billion years old.

Planetary Nebula- The Circle of Life

Now that we have covered the life cycle of a star let us understand a few things that influence the birth and death of stars. During the red giant phase of a dying star, the shell that ejects from the core is known as a planetary nebula. The name could be misleading as it didn’t appear from a planet; it was because there was confusion during its discovery. A planetary nebula comprises large gas and dust particles, which are nothing but the remnants of a dead star. This gas and dust will become available to join another nebula in the vast universe and form yet another star.

What Happens to Larger Stars?

A star that is over 15 times the mass of our Sun is known as a high-mass star. These stars have much shorter lifespans ranging between 100 million years to even 10 million years. As mentioned before, the larger the star, the shorter its lifespan. High-mass stars usually die out with a bang, which is known as a supernova. Larger stars are formed by larger gas clouds contributing to more mass, which implies more gravity. In these starts, the force pushing inward will be much stronger, resulting in the star being much hotter. This results in faster fusion, generating a great outer pressure to counteract the inward pull of gravity. This type of star is known as the main sequence star, which appears big bright, and blue.

During the last stage of the main sequence star, the fuel runs out, and the core contracts and heats. This results in the star becoming a giant star, as in the case of low and medium-sized stars. However, when the core of a high-mass star begins to compress, it becomes much hotter than a star the size of our Sun. When the core compresses, it forms not only Helium, Carbon, and Oxygen but also Neon and Silicon. These layers are separate and pushed down on the super-hot core, which turns the core into iron. Each layer performs a particular type of fusion until no fuel remains.

A Supernova Explosion Occurs

The core of the star comprises iron nuclei which are so stable that further fusion would release no more energy. When this occurs, gravity wins the fight as there is no longer enough fusion reaction to counteract the force of gravity. All of the outer layers bounce off the core in a single second, resulting in a large massive explosion ejecting all the heavy nuclei and the remaining fuel into outer space. This explosion is what is known as a supernova. Here is a better way to understand it, if you compress an object like a small iron ball or marble using a hydraulic press, the marble ball would eventually explode due to the pressure leaving dust and debris. This is the force of compression at a small level, but the compression that triggers a supernova explosion is much higher.

A supernova explosion is known to be the most violent and energetic phenomenon in the universe. The explosion would be brighter than the entire galaxy that they belong to. It is so bright that its glow would be visible through the naked eye even if you are hundreds of lightyears[3] away from the explosion.

The explosion of a supernova does not leave behind a white dwarf star, unlike low-density stars. If the core of the star before the explosion was between 1.4 or 3 solar masses, the core would not be able to support itself against gravity and will collapse. This collapse will be so powerful that all the electrons will squeeze into protons such that they combine to form neutrons. The shockwave from this event is the supernova. The remnants would be a ball of neutrons bunched up together, which make a large atomic nucleus the size of 20 km in diameter. This resulting core would be so dense that a teaspoon from it would weigh a whopping 10 million (1 crore) tonnes. This core is what is known as a neutron star.

If the core of the resulting neutron star from the supernova is above 3 solar masses, the outward pressure of neutrons pressing against each other (neutron degeneracy pressure) will result in neutrons being crushed together to a single point of infinite density. This single point of density is what is known as a black hole.

Conclusion

Now that we have learned the lifecycle of a star and the birth and death of stars, I will be covering the types of stars in my upcoming articles. Scientists are still on the road to researching different types of stars and are on the verge of discovering new principles that would influence future events in our galaxy. We are still in the infancy stage, and there is a lot more to understand about stars. Now you can gaze at the night sky, knowing how these wonderful twinklers came to life and how they would die. It’s all a circle of life; when one star dies, its remnants (planetary nebula) will influence the birth of another beautiful star.

Glossary

[1] Nuclear fusion– Nuclear fusion occurs when two large nuclei in an atom merge together to form a larger, heavier nucleus. In this process, a lot of energy is released as the overall mass of the resulting nucleus is less than that of the two original nuclei. This leftover mass translates into energy.

[2] Nucleosynthesis- The creation of atomic nuclei at the center of atoms which comprise protons and neutrons. This process occurred within the first few minutes of the big bang.

[3] Lightyear- Distance traveled by light in one year

Neutron Stars- The Most Powerful Stars In Our Universe

What would happen if a star with over 10 to 25 times the mass of our Sun goes supernova? There are two probabilities due to the resulting supernova explosion; one would lead to forming a black hole, and the other leading to the formation of Neutron stars. With the most powerful gravitational field and magnetic force, Neutron stars are spinning balls of collapsed stars that illuminate the night sky. Astronomers have discovered nearly 2000 Neutron stars in the milky way and theorize that there could be over 1 million in our neighbouring galaxies.

How is a Neutron Star Formed?

In our universe, new stars are formed from the remnants of collapsed stars. The same is true in the case of a Neutron star. Before understanding how Neutron stars were formed, we need to know a little about stars and the cause of a gravitational collapse. Stars comprise millions and billions of hot plasma that is being pushed into the core due to gravity that nuclei fuse. Hydrogen fuses into Helium, thereby releasing energy that pushes against gravity and tries to escape. Stars are quite stable as long as this balance exists. However, over a few billion years, this Helium will get depleted and result in the star growing into a red giant. Medium-sized stars like our Sun will burn Helium into Carbon and Oxygen during the end of their life, swelling into a red giant. These medium-sized stars will turn into white dwarfs.

Gravitational Collapse Resulting in a Supernova

However, for stars that are 10 to 25 times the mass of our Sun, the internal reaction would be far different once the Helium gets exhausted. The balance of pressure and radiation collapses, and gravity will squeeze the star tighter, and the core would burn hotter and faster, resulting in the star swelling hundreds of times. At this stage, heavier elements will begin to fuse; carbon burns to neon in a few centuries. Neon burns to Oxygen in a few months, Oxygen burns to Silicon in months, and Silicon burns to Iron. This Iron ball is nuclear ash with no energy to give and therefore cannot be fused. Without the outward pressure from fusion, the core gets crushed due to the enormous weight around it.

Due to the collapsing weight of the star, the electrons and protons fuse into neutrons and further gets squeezed together. This is known as a gradual gravitational collapse. Here, an iron ball the size of the Earth gets crushed into a small ball the size of a city. This will result in the whole star imploding with gravity, pulling the outer layers of the star at 25% the speed of light. This implosion bounces off the iron core, producing a massive shockwave with the remnants of the star spewed into space. This is known as a supernova explosion, which is so bright that it could outshine galaxies. After the explosion, what remains is a Neutron star with the mass of over a million Earths but compressed to an object which is nearly 25 km wide.

The Nature of a Neutron Star

A Neutron Star’s gravity is the second strongest in the universe, first to black holes. If a Neutron star gets denser, it could even become a black hole. Even light gets bent around it, so you can only see the front and parts of the back. They are massively hot as they burn at a million-degree Celsius. Just like planets, Neutron stars also comprise an atmosphere, crust, and core. The crust is very hard as the outermost layers comprise iron leftover from the supernova explosion. On the crust, there are enormous nuclei with millions of protons and neutrons shaped like spaghetti. Physicists call this nuclear pasta which is known to be one of the densest and strongest materials in the universe. Nobody knows what the core of a neutron star might comprise due to its dense nature. Physicists theorize that protons and neutrons might dissolve into an ocean of quarks known as the Quark-Gluon plasma.

Celestial Ballerinas

Have you ever seen a ballerina spinning by pulling her arms in? When Neutron stars collapse, they begin spinning very fast, several times per second. PSR J1748-2446ad is one of the fastest spinning neutron stars in our universe, spinning at 716 times a second, which is nearly 25% the speed of light.

The spin of Neutron stars creates radio pulses that can be detected. These are known as radio pulsars, which are the best-known type of neutron stars. These fast-spinning celestial ballerinas are known as magnetars until they calm down. Magnetars are 1000 times stronger than regular neutron stars, with a magnetic field that is 100 million times stronger than the most powerful man-made magnets.

The Collision of Two Neutron Stars Forms a Black Hole

The best types of Neutron stars are friends with other neutron stars. While radiating energy like gravitational waves and ripples in space-time, two Neutron stars would collide as their orbits decay. Their collision would result in both stars getting destroyed in a killonova explosion, forming a black hole. The remnants of the explosion and debris of a killonova explosion will mix back into the galaxy. Some of them end up in a cloud that gravity pulls together, leading to the formation of stars and planets. This process would repeat as a cycle.

Even our solar system is the product of the remains of collapsed Neutron stars. In fact, all the elements in our technological world are built out of the elements Neutron stars made billions of years ago.

Betelgeuse: The Amazing Red Giant

Betelgeuse is the only star that glows red in the night sky. The average lifespan of a star is about 13.5 billion years (1,300 crore). Our sun is around 5.3 billion years old, halfway through its life. Betelgeuse on the other hand is a giant star, much bigger than that of our sun. It has exhausted almost all of its fuel and is nearing the end of its life.

Size Comparison With Our Sun

Our sun is a medium sized star that will turn into a red giant in another 5 billion years ( 500 crore). At that time, as a red giant, it will encompass the orbits of Mercury and Venus. Our earth will be uninhabitable as it will be stripped off its atmosphere and left burnt. Hey don’t worry, this won’t happen even in your great great grandson’s lifetime. The sun will remain a red giant for a million years. In this phase, it will burn its reserve fuel. After a million years, the outer layer of our star will fade away as a gaseous shell, thereby exposing its core. This event is known as a planetary nebula. The exposed core will appear as a glowing white dot known as a white dwarf.

However, Betelgeuse is a massive star which is more than 15 times the mass of our sun. Which means that the mass of the core will collapse and the star will explode into a supernova instead of forming a planetary nebula.

So When Will it Go Supernova?

Betelgeuse is currently a red giant that could go supernova any time. Astronomers are studying the star closely, however, they are not able to determine when it will explode. When it does, the people of earth will experience a breathtaking view of the explosion. The sky will light up and for a brief period it will seem like there are two suns present. The light emitted by the explosion will be so bright that it will engulf a part of the sky.

You may wonder whether we might get destroyed by the explosion. You need not give it a thought as the star is 642 light years away.

Where is it?

On a clear night sky, Betelgeuse will appear as a red star. In fact, it is the only star that appears as a red dot. You can spot it easily as it is a part of the Orion constellation. Refer to the image below.