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Life Cycle of a Star

Our star is pretty insignificant. It is not very big, and it is only one of a huge amount of stars in the universe. It is about halfway through its life, in a stage known as main sequence. In a few billion years, our Sun will die, ending all life on Earth. Our Sun provides the gravitational pull that keeps the planets and other objects orbiting around it, and provides a source of energy which supports all life on Earth.

The length of a star’s lifetime depends on its mass. If the star has a lot of matter and therefore a high mass, its lifetime will be shorter. This might seem a bit counter-intuitive, because one might wonder if more nuclear fuel would mean the star would be able to shine for a longer time. Smaller stars are actually more efficient with the fuel they have; however, larger stars use their nuclear fuel at a much faster rate. The mass of a star depends on how much matter there was in the cloud, known as a nebula, that created the star.

Stars of a similar mass to that of our Sun all have a similar life cycle. They start as a nebula. A nebula is a cloud of dust and gas which can range in size. To make a star the size of our Sun, you would need a nebula several hundred times the size of our solar system. This cloud, which contains the building blocks of the star, collapses due to gravity. As the cloud shrinks in size, its temperature increases, as the particles that make up the cloud collide with each other. When this collapsed cloud reaches a certain temperature and pressure, nuclear fusion can occur. At this stage, the ball of gas is known as a protostar. Nuclear fusion is a nuclear reaction where two light nuclei combine together, forming a heavier nucleus and energy. It is this energy which is radiated from the start. The amount of energy produced in these reactions can be calculated from E=mc2. “E” is the amount of energy, “m” is the change in mass, and “c” is the speed of light in meters per second.

When the outward pressure from the nuclear fusion is balanced with the gravitational force pulling the star together, we can describe the star as stable. Stars that are stable like our Sun are said to be in the main sequence stage of the star’s lifetime. There comes a point where the star runs out of its hydrogen fuel, and this is when the end of the star’s life begins. Stars run out of their fuel after millions or billions of years, depending on their size. When the star runs out of its fuel, the nuclear reactions in its core can’t continue. This means the outward pressure decreases, allowing the force due to gravity to start collapse in the core. The outer layers expand and cool slightly. This cooling changes the color of the star to a red color. At this stage, the star is known as a red giant. This will be the fate of our star in a few billion years. Our Sun will swell up and expand to a few hundred times that of its original size. When this happens, all life on Earth will die.

The outer layers of the star then drift off, leaving a hot, dense core. These can produce a very beautiful phenomena known as a planetary nebula. The hot core of a planetary nebula is known as a white dwarf. A white dwarf is a dead star that still shines due to the residual heat. They are very dense, with one teaspoon of a white dwarf having a mass of several tons. Over time, this dead star will cool and dim. This dead star which has cooled and no longer emits light is known as a black dwarf.

Stars that are much bigger than our star follow a different cycle throughout their lifetimes. While smaller stars, like our Sun, are formed by a collapsing nebula, larger stars’ nebulae contain a lot more matter. They also go through a main sequence stage but have a blue hue due to the higher temperatures associated with them. When it comes to the end of the larger stars’ lives, they do it in a much more dramatic way. Massive stars can have cores that are hot and dense enough to provide an environment where nuclear fusion can occur for additional elements. Like stars of a similar mass to our Sun, massive stars also grow when they start to run out of nuclear fuel.

This ends in a large explosion known as a supernova. Supernovae are some of the brightest objects in the sky. Elements heavier than iron are thought to be formed in a supernova. The dead stars are now known as neutron stars, and they are extremely dense. If a star is very large and has enough mass, then a black hole could form at the end of the massive star’s life. A black hole is an area of space where the gravity is so strong that even light cannot escape.

Nebula A nebula is a cloud of dust and gas that collapses under its own weight. As the cloud collapses, it gets warmer. When it reaches a certain temperature, nuclear fusion starts.

Massive Main Sequence Star At this stage, the pressure from the nuclear fusion reactions are balanced by the force of gravity. The star will spend millions or billions of years in this stage, depending on its size.

Red Supergiant When the nuclear fuel runs out, the star swells in size. As the star grows, the outer layers cool, giving the star a red color.

Supernova The star’s core collapses, causing a violent explosion, and throwing the outer layers of the star into space.

Neutron Star or Black Hole What is left after the explosion is a very dense core known as a neutron star. If the star is extremely big, a very dense neutron star known as a black hole could form. A black hole is an area of space where the gravity is so strong that even light can’t escape.