The Life of a Star

To our ancestors, stars were eternal objects which shine in the heaven above. However, research throughout the previous century has shown us that this is not the case. Stars are dynamic and ever changing. The life of a star doesn’t imply that stars are alive!! It is just a metaphorical way of representing the changes a star goes through, as long as it exists.

Life cycle of a star

Birth of a star

75% of the matter in the Universe is hydrogen and 23% is helium; these are the amounts left over from the Big Bang. These elements exist in large stable clouds of cold molecular gas and dust called nebulae. At some point a gravitational disturbance, from a nearby star or due to clumping of matter causes the process of star formation.

As the gas collects together, it heats up. Conservation of momentum from the movement of all the particles in the cloud causes the whole cloud to begin spinning. Most of the mass collects in the center, but the rapid rotation of the cloud causes it to flatten out into a protoplanetary disk. It’s out of this disk that planets will eventually form, but that’s another story.

The center of the disc is where our interest lies as that is the place where a star will be born.Here, the high concentration gases begin to heat up from the gravitational collapse of all the hydrogen and helium, and over the course of about 100,000 years, it gets hotter and hotter becoming a protostar. A protostar is a dense and hot body that is a precursor to the birth of stars. Although not a full fledged star, it does have many properties of one. Matter continues to collapse for a 100 million years until temperatures and pressures at its core can become sufficient enough for nuclear fusion to begin. From this point on, the object is a main sequence star.

Main Sequence Phase

Hereon the star lives through the longest stage of its life: the main sequence stage. The main feature of this stage is the nuclear fusion that occurs at its core. This is the process that all-stars go through as they fuse atoms of hydrogen into atoms of helium. This reaction gives off more heat than it requires, and so the core of the star releases a tremendous amount of energy. This is pretty much why the Sun shines and all life on Earth is possible in the first place.

To understand this process, let us take a detour into the process of fusion to see what is exactly going on.

The process of fusion that occurs in stars is also known as the proton-proton chain. The core of a star is really hot and I am talking about an order of a 100 million Kelvin hot!! At such high temperatures, protons are whizzing around at tremendous speeds. Thus collisions are a common thing. An attentive reader may question, how do protons collide if they have the same charge as like charges repel? Well, that’s because of the speed they are travelling. When two protons come insanely close to another, one of them changes into a neutron, which has no charge. The exact reason for this change requires us to delve into quantum mechanics which is not the main focus of this post.

The protons and the neutrons fuse together to form deuterium or a tritium nuclei. Deuterium and tritium are just isotopes of hydrogen, containing a proton and 2-3 neutrons. The last step is to fuse the deuterium and tritium nuclei to form helium. As the energy of helium is less than the protons that formed it, the extra energy is released and this is the energy that is radiated out.

The energy produced in fusion is basically what supports the star. All of the energy radiating out of the core pushes the star outward, while the gravitational force crushes it inward. This delicate balance between the forces of fusion and gravity, prevents the star from being crushed in on itself due to its own weight or ripped apart by nuclear fusion. A star at this stage of life is held in balance.

Red Giant Phase

However, nothing last till eternity, not even the Universe. Over the course of its life, a star is converts all the hydrogen into helium at its core. When a star is exhausted of all its hydrogen, the nuclear reactions stop. Without the internal force that supported the star from crumbling due to its own weight, the star begins to contract inward due to gravity.

The time required to reach this stage varies from star to star. The least massive stars, like red dwarfs with half the mass of the Sun, can use their fuel for hundreds of billions and even trillions of years as their rate of fusion is comparatively slower. Larger stars, like our Sun will typically live in the main sequence phase for 10-15 billion years. The largest stars have the shortest lives, and can last a few billion, and even just a few million years as they burn and use their fuel rapidly, thus quickly being depleted of hydrogen to fuse.

As the star begins to crash inwards due to its immense gravity, the hydrogen at its outer layers begin to heat up to the point to restart fusion. (Remember, that the hydrogen in the core is depleted, not the hydrogen at the surface layers). This process causes the star to brighten up again by a factor of 1,000-10,000 and the outer layers of the star begin to expand outward, increasing the size of the star to, many times its original. In an instant, the star springs back to life and begins fusion in its outer layers. Our own Sun is expected to do this in the next 4 billion years, engulfing the Earth in this process. The star is now a Red Giant and will be so for the next few million years.

Death of a Star 

From here on the fate of each star varies. For stars lesser than 8 solar masses, after the hydrogen in their surface is also depleted, it is pretty much the end. The temperatures at the core will never rise high enough to support further fusion of helium to carbon and heavier elements. Such stars will slowly eject their outer layers into space, and then contract down, eventually becoming a white dwarf. The star is now dead and what is left behind is called a stellar remnant.

The story for bigger and more massive stars is quite different. The temperature and pressure at the core of the star will eventually reach the point that helium can be fused into carbon. Stars much more massive than our Sun continue on in this process, creating heavier and heavier atoms. Carbon will be fused to oxygen, oxygen to neon and so on. However, death strikes when the bar hits iron! Due to its extra stability, no star can reach temperatures high enough to fuse this element and fusion stops abruptly. With no fusion to support the star, gravity wins and crushes the star inwards.

The core collapses first leaving the outer layers to rush in on the core and rebound back into space creating a spectacular explosion called a supernova. The exploding layers rub against each other creating temperatures so high that elements heavier than iron are formed. The elements that the star fused throughout its life are then spread out into the Universe to form planets, other stars and maybe even life. Thus the building blocks that make up everything we see around us was once cooked up in a star. We are star-dust!!

Now, at the site of the supernova explosion lies a beautiful planetary nebula that may sow the seeds for the future stars and the cycle begins again.

Stellar Remnants

Stellar remnants are what is left of a star after it dies. The stellar remnant, in case of stars like our Sun is a white dwarf. These objects start out very hot, although there are no fusion reactions taking place inside it any more. The reason for this has to do with electron degeneracy pressure which is beyond the topic of this post. Over the course of the next hundreds of billions of years, the white dwarf eventually cools down to the background temperature of the Universe. Once it has done so it is called a Black Dwarf.

However, black dwarfs are hypothetical as the time required for a white dwarf to turn black is longer than the age of the Universe. This means no black dwarfs are thought to exist presently in our Universe.

On the other hand, the stellar remnants formed by the bigger and more massive stars are the real bad boys. Enter neutron stars and black holes. To begin with, neutron stars are balls made entirely of neutrons. The possess exceptionally high gravitational and magnetic fields. They are so dense that a spoon of neutron star matter will weigh as much as a mountain!! Although they are only 20 km across, their magnetic field can suck all the haemoglobin from your blood from 1000 km away. Some neutron stars called Pulsars spin at an incredibly fast rate. I am talking about 500 times per second!! Now that is fast!!

And finally, the baddest one of them all: a black hole. Black holes are objects with a gravitational field so high that not even light can escape them. A black hole contains mainly of an event horizon and a singularity. The event horizon is the boundary beyond which nothing, not even light can escape. Once you across this veil, there is no escape. The singularity is an infinitely small, infinitely dense point at the centre of a black hole. All those infinities have troubled scientists for a long time and till date no one has been able to figure out the exact dynamics of one.

Thus we have it, a life of a star. To summarise, take a look at the image given below.

 

Although they may seem as just pretty points of light in the sky, stars live an ever so dynamic life and are much more complex than they seem. The continuous birth and death of the billions of stars in the Universe seeds the environment with the basic building elements. So the next time you look up at the night sky, remember… the stars you see have a played a part to make it possible for you to look and marvel at them.