The Evolution of Stars

Regardless of our age, everybody will be curious to know how the stars twinkle and how they are born. Before talking about the evolution and life cycle of stars, first, we shall talk about ‘what are stars’?

A star could be any object, but to be a star, it should satisfy one condition. It has to fuse hydrogen to helium in its core. In other words, nuclear fusion should take place in its core. To start nuclear fusion, the object should have more than 0.075 solar masses. Solar mass is a unit for measuring the mass of stars. 1 solar mass is the mass of the sun.

Formation of stars

Stars form from dust and gas clouds called nebulae. These clouds contain mainly hydrogen and helium gases. These clouds are so massive that they have their own gravitational force. Due to their gravitational force, the cloud compresses and becomes denser and denser until it has enough material to start nuclear fusion. This densely compressed cloud is called a protostar. When the protostar attains enough gravitational force to start nuclear fusion, it is called a star.

Different stars have different life cycles. The life cycles of stars are categorised depending on its mass. There are a few categories for the life cycles of stars.

• O.012 solar masses to 0.75 solar masses. (Brown Dwarfs)
• 0.075 solar masses to 0.8 solar masses. (Red Dwarfs)
• 0.8 solar masses to 8 solar masses. (Sun-like stars)
• more than 8 solar masses. (Neutron Stars and Black Holes)

Brown Dwarfs

Brown dwarfs are objects with a mass from 0.012 to 0.075 solar masses. That means that it does not have nuclear fusion in its core and hence these objects do not fall in the category of stars. These objects form from nebulae which are not massive enough and it does not have enough density and material for nuclear fusion. So dense objects without nuclear fusion form and these are called brown dwarfs or failed stars. These stars are very hot during their formation. But since they do not produce any energy, they slowly cool down. These objects are only the size of the planet Jupiter, but they are 75 to 80 times denser than Jupiter. These objects cool down and form a black dwarf. Black dwarfs are objects that do not emit any light. They are just dense black spheres of gas and dust. If protons decay, then these brown dwarfs will evaporate in many trillion years. Black dwarfs are theoretical because no brown dwarfs have died yet. This is the life cycle of brown dwarf stars. Scientists have discovered brown dwarf stars orbiting red dwarf stars also.

Red Dwarfs

Stars with a mass of 0.075 solar masses to 0.8 solar masses are called Red dwarfs. These stars form from small nebulae. The nebulae in which red dwarfs form is a bit larger than the nebulae in which brown dwarfs form. These stars form just like our sun. Red Dwarfs also have nuclear fusion in its core. But they have less fuel. So they are much cooler than our sun. The average surface temperature of the sun is around 5700 Kelvin. But the average surface temperature of a red dwarf is around 3000 Kelvin. These red dwarf stars are also one of the longest living stars we have ever discovered.

Then you would be thinking red dwarfs have less fuel than our sun, so how can they be the longest living stars that we have ever discovered? The reason is the convection zone. The convection zone is one of the layers inside a star. What this layer does is, it mixes the hydrogen and helium together. By mixing the hydrogen and helium, the hydrogen in the outer layers of the star can go to the core. Thus, it can use up all its fuel before it dies. This is what makes red dwarfs the longest living stars that we have ever discovered. The sun also has a convection zone, but this layer is very small. But in red dwarfs, this layer is overly thick. Red dwarf stars can live a trillion years. When it uses up all of its fuel, it slowly cools down and forms a black dwarf just like a brown dwarf. Scientists have discovered a brown dwarf called Gliese 229 b orbiting around a red dwarf called Gliese 229. The age of the universe is only 13.8 billion years and the lifespan of a red dwarf is a trillion years and hence no red dwarfs have died till date.

Sun like stars

Stars with a mass of 0.8 solar mass to 8 solar masses are called sun-like stars. The sun has a mass of 1 solar mass and is formed from a medium-sized nebula. Our sun also has nuclear fusion at its core and hence we get heat and light from the sun. The life cycle of our sun is much more interesting. The average lifespan of our sun is 10 billion years. Bigger the star, the shorter its lifespan and the sun had already spent half of its life. After an average of 3 billion years, our sun will begin its red giant stage. During this stage, our sun’s outer layers expand and its temperature will decrease. It expands so much that mercury, venus and earth will be inside the surface of the sun. That means our earth will be no more. After approximately 2 billion years, the sun will run out of fuel, and it sheds the expanded outer layers into space. This creates a nebula. New stars form from this nebula and this cycle goes on again and again. But the sun has only shedded its outer layers into space and the core still remains in the centre of the nebula. This is called a white dwarf. The current temperature of the sun’s core is 15 million degrees celsius. But during the sun’s red giant stage, it’s temperature decreases. The average surface temperature of a white dwarf is 100,000° celsius. A white dwarf does not have nuclear fusion so it does not produce any energy. A white dwarf is so dense that one cubic centimetre of it would weigh 1 ton or 1000 kilograms. In the core of white dwarfs, protons and electrons combine together to form a neutron. This white dwarf would only be the size of the earth, but would have ½ mass of its apparent star. They slowly cool down to form black dwarfs.

Neutron Stars and Black holes

Stars with more than 8 solar masses are different. They have a very short lifespan. After a few billion years, they have a red or a blue supergiant stage (depending on the colour of the star). This stage is the same as the red giant stage. But when the star runs out of fuel, there is no force pushing the star outward. So, the gravity of the star makes the entire star collapse on its own. This causes a huge explosion. This explosion is called a supernova. This explosion is so huge that it can outshine an entire galaxy. It also creates gamma-ray bursts that can wipe out all the life on earth from 700 light-years away. During this explosion, the entire mass of the star gets converted into light and heat. The rest of the mass of the star either gets turned into a neutron star or otherwise it gets turned into a black hole. It depends on its mass.

Neutron Stars

Neutron stars are not stars. They are one of the densest objects ever discovered. They do not have any nuclear fusion going on in its core. It creates energy by combining protons and electrons to make neutrons. They have a very strong gravitational force. They bend the light which means that you can see the front and backside at the same time. They are very much like black holes. During the formation of neutron stars, they have a very strong magnetic field. These types of neutron stars are called magnetars. Magnetars have a magnetic field that is a trillion times stronger than that of the earth. They rotate 100 times per second. Like black holes, neutron stars also suck up all the matter that is available. When a neutron star absorbs matter, it creates an accretion disk. During this time, it creates a light jet. Light jets are high energy stream of particles travelling at more than 25 % of the speed of light. These kinds of neutron stars are called pulsars.

Black Holes

Black holes are the densest objects ever discovered. It is denser than neutron stars. It grows larger and larger when it absorbs matter. Black holes have a zone called the event horizon. If any object falls inside the event horizon, the escape velocity of the black hole would be more than the speed of light. But nothing can travel faster than the speed of light. Nobody knows what would be the structure of a black hole. Quasars are one of the brightest objects that we have discovered. Quasars are black holes. Then you would be wondering how can black holes be the brightest objects we have ever discovered if we cannot even see it? The reason is that, inside the event horizon, the escape velocity is the speed of light. But when a black hole absorbs high energy-matter, it creates a disk around the black hole. This disk is called the accretion disk. It is located outside the event horizon of the black hole. The matter of the other object goes around this disk. As it gets closer and closer to the black hole, the density and temperature of the matter increases. When the matter gets very close to the black hole, it starts emitting X-ray from both poles of the black hole. This is so bright that it outshines the brightness of an entire galaxy. When this black hole emits a light jet, it is called a quasar.

“Take a stroll when the darkness of the night reaches at its peak coz when the first ray of light falls, you find a stronger version of yourself”

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