Understanding the evolution of stars can help us to find out how our universe formed. Read everything about the birth, life, and evolution of stars in this article.
Stars form when cool, relatively dense clouds (molecular clouds) of interstellar gas and dust shrink upon themselves as a result of gravitational collapse. In a spiral galaxy, such as the Milky Way, star formation is usually triggered when gas clouds are compressed by shock waves from nearby supernovae.
The Birthplace of Stars
Stars are born in regions of unstable clouds of dust and gas which are scattered throughout the outer spiral arms of our Milky Way. Hundreds of newborn stars are nursed in the Orion Nebula, which is just visible to the naked eye as a fuzzy speck of light in the constellation of Orion.
However, the lit up regions of the nebula only represent a small portion as much of the nebula is actually full of dense molecular clouds which absorb visible light and can only be probed with radio waves.
To understand the evolution of stars and where they come from, it is also important to know the formation of stars.
The Formation of Stars
Once the clouds start to collapse, they break down into massive lumps, and as these continue to collapse, gravitational compression causes the lumps of cloud to warm up as gravitational potential energy is converted into heat. It is these lumps that will eventually form a single star, two stars or even a star with its own planetary system in the evolution of stars.
As the pressure and temperature increases in these lumps, a sphere made up of superhot gas called a proto-star (a potential star) is formed. This proto-star will continue to collapse until its core approaches close to 10 million kelvins as that is the required temperature to undergo nuclear fusion.
The entire formation process for a star like our sun might take about 50 million years. Once a star has begun the conversion of hydrogen into helium, the remainder of its life will be determined exclusively by its mass.
This nuclear reaction releases heat and produces an outward pressure which supports and holds up the star against further gravitational collapse for as long as there is enough nuclear fuel to burn.
The Composition of Stars
All the stars that are born at this moment will start out with roughly the same mix of raw materials; hydrogen, which accounts for just under 75 percent, helium at just over 25 percent and a small amount of heavy elements such as zinc. Stars from earlier generations contained slightly more hydrogen and less helium with very few, if any, heavy elements.
It is fortunate for us that our sun was formed some 4.6 billion years ago as opposed to 10 billion years ago as the lack of heavy elements in early stars would have made it difficult for life on any of the planets to form.
The End of an Era in the Evolution of Stars
After exhausting all readily available hydrogen in its core, nuclear fusion comes to a halt and the core begins to contract due to gravity. Stars with masses between 0.5 and 8 solar masses (1 solar mass is the mass of the sun) expand to become red giants.
These massive stars have a radius several hundred times the size of our sun and will ultimately expel its outer shell. The expelled material is called a planetary nebula as they resemble giant planets when observed through an optical telescope.
This ring of expelled material will remain visible for about 10 thousand years before gradually dispersing into interstellar space. The star is now dead as it has no fuel. All that is left is a white dwarf star.
A Violent Death ends the Evolution of Stars
The evolution of stars with masses greater than 8 solar masses isn’t as pleasant but much more dramatic. As a result of their sheer size, these stars burn through their hydrogen supply at a faster rate but at the cost of a considerably shortened lifespan. Once their hydrogen to helium conversion phase is over, they expand to become red supergiants.
These are the largest stars in existence in terms of radii. Once all their energy is spent, the red supergiant collapses and explodes as a supernova, shining briefly with the light a billion suns. Most of this material is expelled into space. All that remains is a spherical body of incredible density which will either collapse into a neutron star or possibly, a black hole.