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What is Star? Everything You Need to Know

by Sankalan Baidya
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What is Star

On a clear cloudless night we look up at the sky and see hundreds of thousands of twinkling specks of light that appear to be ornaments on the dark body of the celestial gloom – the all prevailing darkness that seems to be eager to engulf everything! Those tiny specks of lights are stars. But, what is star? How is a star formed? Why can we see stars only during night? Why does a star twinkle? These are some of the commonest question that most of us have asked or ask to our parents and teachers. In this article, we are going to take a somewhat detailed look into the answers of these questions and many other questions. Ready? Let’s get started…

What is Star? The Definition

A star is a giant ball of gas. The gas is so hot that it glows. The gas ball is primarily composed of two elements – Hydrogen and Helium. The question here is, that “if a star is made of gas, why doesn’t the gas disperse?” That’s are really good question. Here is the answer you seek: that ball of gas is so huge that the gas atoms are held together by their own gravity. This is the reason why the ball of gas doesn’t disperse.

Now, here comes another important question: If gravity holds the ball of gas, isn’t it possible that because of the gravity at the center, the star will collapse on itself?

Um… yes! That’s precisely what should happen and that is what really happens. You see, inside the ball, the gravity is intense and the gas atoms actually fall into the center and causes a tremendous rise in temperature. It is this high temperature that triggers a type of nuclear reaction called ‘Fusion Reaction’. In this type of reaction, elemental atoms fuse together forming new elements, basically heavy elements.

When this fusion takes place, immense amount of energy is released. This energy exerts an outward pressure from the center and acts as a counter-balancing force against the inward gravitational pull. This keeps the star as it is and it doesn’t collapse on itself because of gravity.

What is Star and How are Stars Formed? Method of Star Formation!

This is an obvious question that follows after the question ‘what is star?’. Now, answer to this question is really complex but we will try to provide a simple explanation as far as possible.

You know what? Stars follow the same cycle of birth and death. However, the stages between birth and death are a bit different for stars compared to that of humans. Stars follow the following cycle:

  1. Gas and dust cloud (Raw ingredients)
  2. Protostar (Birth)
  3. Main Sequence (Adulthood)
  4. Death

Let us take a look at each stage separate and understand how star is formed and what happens to the star eventually. This section is going to be long. So, grab a coffee, tea or snack, sit back, relax and read! We will keep it simple. Promise!

Stage 1: Gas and Dust Cloud: Nebula

There is gas and dust that are scattered throughout the universe and are present in almost every galaxy. These gas and dust just stay there, doing nothing.

However, these gases and dust can be gravitationally disturbed by an external event such as a comet passing through or a supernova explosion somewhere in the vicinity that releases shock waves or something else.

This sudden gravitational stir up will make these gases and dust to collapse on each other because of their own gravitation. This gravitational collapse will make these gases and dust particles to clump together, forming humongous or ginormous clouds known as Nabulae (singular Nebula).

Fun Fact: Did you know Nebula is a Latin word which means ‘Cloud’?

A single nebula can stretch for hundreds and thousands of light years. Now, these Nebulae are called stellar nurseries. That is, stars are formed inside these ginormous clouds.

Stage 2: Protostar (Birth of a Star)

Inside a nebula there can be turbulences, because of which, knots of masses can be created. That is, clumps of large amounts of gases and dust can be formed. These knots or clumps, when they have sufficient mass, will start collapsing on each other because of their own gravitational attraction. As this collapse continues, the material right at the center starts heating up gradually.

This hot core is called Protostar. It sits at the very center of a collapsing cloud and some day, it will become a star This protostar will continue to grow for some time as more and more cloud collapses on the core because of the gravitational pull. As a result, the temperature of the core will also keep on increasing.

Stage 3: Main Sequence Star

At one point, the protostar will attain a critical temperature when the hydrogen atoms in the gas will start fusing together forming helium atoms. This fusing is called fusion reaction. When the fusion reaction starts, enormous amounts of energy gets released. At this stage, that is when the fusion reaction starts, the collapse of gas and dust continues until the point where the energy released by fusion reaction is just equal to the gravitational pull at the core. This state is called hydrostatic equilibrium state and the protostar becomes what is known as Main Sequence star.

What really happens in the hydrostatic equilibrium stage?

This is when the core of the star is exerting an inward gravitational pull but at the same time, the energy released by the fusion reaction at the center is pushing outward. The inward gravitational pull and the outward push balance out each other and star comes to maintain a spherical shape. These stars are called Main Sequence Stars. It is also called the mature phase or adulthood of a star.

Did you know?

  • It may take millions of years for a star to reach adulthood from the starting of the collapse. Our sun took 50 million years to reach adulthood!
  • Most of the stars that we see in the universe are Main Sequence Stars.
  • Main Sequence Stars will stay in adulthood for a very long time up to billions of years. For example, our Sun will stay a Main Sequence star for up to 10 billion years in total. It has been a Main Sequence star for last 4.5 billion years and will continue to stay so for another 5.5 billion years.
  • A star remains a Main Sequence star as long as there is fuel for nuclear fusion reaction. This means, as long as there are hydrogen atoms for fusing together into helium atoms, the adulthood will continue. When the star runs out of fuel, the Main Sequence star enters its last phase or death phase.
  • A star usually spends 90% of its total life in the Main Sequence stage.
  • How long the Main Sequence stage will last depends on size of the star and how hot the star is.

Stage 4: Death of a Star

This is where things get really interesting. There is a general rule of thumb that the stars follow. The rule states that larger a star, the shorter its life span is. So, most star will stay in Main Sequence for billions of years while others that are really big will last for only a few million years. Either way, all stars eventually die. What happens at death is really fascinating.

The death of a star is marked by the phase where all the hydrogen present in the core of the star is burned out to form helium. With no more hydrogen is left in the core, the nuclear fusion reaction will stop. Once this nuclear fusion stops, the star will not have enough energy to support itself. The hydrostatic equilibrium will simply break. This is when the core of the star will star collapsing on itself.

As the core starts collapsing on itself, the temperature of the core will gradually increase and it will become hotter and hotter. At the same time, outside the core, the star may still have hydrogen in the shell. This means that fusion reaction will continue to take place in the shell.

The energy released by the fusion reaction in the shell will cause the shell the expand. Simultaneously, the outer layer will also be pushed outward by the increasingly hot core. As the shell keeps expanding, it keeps on cooling and eventually the star becomes what is known as red giant.

If the dying star was really massive, its collapsing core would be big enough to trigger other nuclear fusion reactions that are far more exotic. This means that helium in the collapsing core will fuse together and will form other heavier elements including iron.

Unfortunately, these exotic nuclear fusion reactions are not really stable. They are temporary and sometimes the core burns furiously while at other times, the fire just dies down. This instability and variation eventually makes the whole star pulsate. The pulsating star will then throw off its expanded outer layer, forming a cocoon of dust and gas around the core. From here on, it is the size of the core that will determine the ultimate fate. Let’s see what happens next!

White Dwarfs

An average star is basically a star of the mass of our Sun. Yes, our Sun is an average star. However, any star with the mass of up to 1.4 times the mass of our Sun is considered as an average star. For these average stars, nothing much happens. Once the outer layers are completely ejected by the star because of its pulsations, the interior core gets exposed. This core is really hot and it is known as White Dwarf. A White Dwarf will be around the same size as our home planet – Earth. However, this White Dwarf will have far more mass than Earth. Astronomers were once puzzled by this. They asked: “if a White Dwarf has nearly the mass of a star, why doesn’t it collapse on itself?”. The answer to this question is pretty interesting.

It turns out that inside the White Dwarf, there are fast-moving electrons that exert an outward pressure and prevent the White Dwarf from collapsing on itself. Here are some interesting facts on White Dwarfs:

  • The bigger the Main Sequence star, the more massive its core will be and hence, denser the White Dwarf will be.
  • The smaller the diameter of a White Dwarf, the bigger is its mass!
  • Only average stars will become White Dwarfs. This means that any star of equal mass of our Sun or up to 1.4 times the mass of our Sun will become a White Dwarf.
  • If a star has mass bigger that 1.4 times the mass of our Sun, it will not form a White Dwarf because the outward pressure created by the fast-moving electrons in the core will not be able to counterbalance the gravitational collapse. A different fate will await such stars!
  • White Dwarfs are really common in our known universe.

White Dwarfs to Novae

Nova is the Latin word for ‘New’. Nova is singular and Novae is plural. Coming to the point, it may happen that a White Dwarf may be a part of a binary star system or a multiple star system. In such an event, it may happen that the White Dwarf may be close enough to its companion star(s). The proximity (closeness) may allow the White Dwarf to use its gravity to attract matter from outer layer the companion star. It will mostly attract hydrogen. This will result in the formation of outer layer for the White Dwarf itself.

If the White Dwarf manages to drag in enough matter, fusion reaction may kick in and the White Dwarf will suddenly brighten up a lot. This is when the White Dwarf is called a Nova but again, the fusion reaction on the surface layer will make it expand and eventually the surface will be expelled. Once the surface layer is expelled by explosion, the newfound glow of the White Dwarf will then subside within a few days. The White Dwarf will then restart the cycle all over again and form Nova once again.

Something else may also happen! If the White Dwarf is really big and formed out of stars with around 1.4 times the mass of our Sun, the White Dwarf can drag in enough material that it will collapse because of its own gravitational pull and eventually explode and become a Supernova.

Supernovae

This is what we like to call cosmic firework – something that we puny humans can never match even with the most powerful atomic bombs we have created. Supernova (plural: Supernovae) happens to Main Sequence Stars that have at 8 times the mass of our Sun or more than that. A Supernova is way different from a Nova. How?

In a Nova, only and only the outer surface explodes but in a Supernova, the core also explodes along with the outer surface.

What really happens is that in such big Main Sequence Stars, a number of really exotic nuclear reactions take place in the core and eventually iron is formed. Production of iron means that the star has produced all the energy it possibly can. It cannot produce any further energy.

Here one may argue that another round of fusion reaction can convert iron into heavier elements and produce energy. But that’s not going to happen because for any other nuclear reaction to convert iron into heavier metals, energy is not released, it is consumed. So, further nuclear fusion reaction is not really possible.

At this stage, since there is no energy to counter-balance gravity, the iron core collapses on itself. It only takes a few seconds for a core that is roughly 5,000 miles across to shrink down to a few dozen miles across. This rapid collapse eventually increases the temperature to at least 100 billion degrees or even more than that.

Initially, when the core starts collapsing, the stars outer surface also collapses along with it but as the core rapidly shrinks, the sudden burst of energy within a few seconds makes the core as well as the outer layer rebound back outward but only this time, it happens with a spectacular explosion. The explosion is extremely violent and releases so much energy that we just cannot imagine it. This explosion is called Supernova explosion and when the explosion occurs, it can for days and weeks, outshine the entire galaxy in which the star ended up as Supernova.

Neutron Star

In a Supernova, if the core is as big as anywhere between 1.4 times to 3 times solar mass, the core will keep collapsing until a point where the protons and electrons in the core will fuse together forming neutrons. This will give rise to Neutron Star. These Neutron Stars are super dense. Extremely high amount of mass is packed in them, giving them extreme gravitational force even on the surface.

If such Neutron Stars are formed in binary star systems or in multiple star systems they will collect mass by stripping off gas from nearby stars. The powerful magnetic fields of the Neutron Star will then accelerate all atoms close to its poles. This acceleration will lead to very powerful beams of radiations. These beams will shoot out in space like searchlights as the Neutron Star keeps rotating.

Now, if the Neutron Star has such an orientation is space that the beams its produce are periodically pointed towards our Earth, we can see those beams in periodic pulses whenever the magnetic poles of the Neutron Star crosses our line of sight. In such a case, a Neutron Star is known as a Pulsar.

Black Hole:

This is pretty interesting. In a Supernova, if the core has mass bigger than 3 solar masses, the core will collapse completely and lead to the creation of what is known as Black Hole. This Black Hole will be intensely dense and all the matter will get packed into an infinitesimally small dense point called Singularity. The gravity in the Black Hole will be so intense that nothing can escape from its immediately proximity. When we say ‘nothing can escape’, we also mean light. Since light cannot escape its gravity, we cannot see a Black Hole. All the instruments we have created so far is designed for detecting photons (light is made of photons).

So, how do we see a Black Hole. There is an indirect method. When Black Hole pulls in matter, a spiraling disk is created around it, which heats up to enormous temperatures and in the process, releases Gamma rays and X-rays. We can detect these rays and this allows us to detect Black Holes.

What happens to material released by Novae and Supernovae?

The material that is released from Novae or Supernovae eventually blend into the surrounding gas and dust present between stars. Heavy elements and various other chemical compounds are produced after a star dies as Nova or Supernova. They all get recycled and are again used for creation of new stars and planets and other celestial objects! Remember, matter is never lost!

Now that we know what is star and we know how a star is born and what happens when a star dies, it may sound that we have covered everything we need to. Alas, we are far from being done. There is much more that we need to learn when we try to answer the simple question – What is Star?

For instance, we need to learn about the Types of Stars. We need to answer the question – Why Stars Twinkle? We need to learn about Classification of Stars and more! We will continue this in our next article.

Sources: 1, 2, 3, 4

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