The Life Cycle of Stars

Stars originate from nebulae, vast clouds of gas (mainly hydrogen) and dust in space. Due to the influence of gravity, these gases and dust aggregate and form a protostar.
As the temperature and pressure in the core of the protostar increases, nuclear fusion begins. This is the process involving hydrogen atoms joining to form helium, unleashing a large amount of energy in the process. This marks the start of a star’s main sequence, which is the majority of its life.
During the main sequence, the outward pressure produced by nuclear fusion is balanced by the inward pressure of gravity. This creates stability in the star for billions of years.
When most of the hydrogen in the star’s core has been converted into helium, the star begins to die. What happens next is determined by the star’s mass.
For smaller stars, like our Sun, the helium in the core contracts and the outer layers of the stars expand, cool down, and glow red. This is the creation of a red giant. The outer layers of the red giant eventually drift off into space, forming a planetary nebula, leaving behind a hot, dense core known as a white dwarf. After billions of years, white dwarfs cool to become black dwarfs.
For larger stars, the core heats up more when the hydrogen fuel is exhausted and extreme pressure squeezes the helium atoms together to form heavier elements such as carbon or oxygen. The star bulges into a red supergiant.
When no more fusion can take place, the core collapses under gravity which causes the outer layers to explode in a supernova.
Post supernova, if the core is less than three times the size of our Sun, a neutron star is formed. If it is greater than three times that of our Sun, the core collapses completely to form a black hole.
Black holes are regions of space where the gravitational pull is so strong, not even light can escape. They can only be detected by considering the effect of their gravity on nearby objects.
Light years measure astronomical distances. One light year is the distance that light travels in one year (approximately 9.5 trillion kilometres). Star distances are often expressed in light years.
Parallax is the method scientists use to measure the distance to nearby stars. It is the apparent shift in position of a close star against the background of distant stars.
Spectroscopy, the study of the interaction between light and matter, is used to determine what stars are made of. When a star’s light is spread out into its spectrum, dark absorption lines appear, revealing the star’s chemical composition. This is often represented in a graph known as an absorption spectrum.