Life Cycles of Typical Stars
The stages of evolution of a star largely depend on its initial mass, the mass which it started with. Based on this stars are divided into 3 categories : the low mass stars, the medium mass stars and the heavy ones.
Low mass stars have a mass of about less than 0.8 times the mass of the Sun. Since they are less massive, the gravitational pressure at their cores is low, making their fusion really slow. So such stars can undergo fusion for a really long time, which is estimated at around 10 trillion years. These stars are commonly referred to as red dwarf stars and have a low surface temperature, due to low fusion.
Medium massed stars, like our Sun, have a mass from 0.8 to 10 times the mass of the Sun. These stars have a higher fusion rate as compared to low mass stars. When these stars run out of hydrogen to fuse, they look for the next best thing to fuse, Helium. Helium fusion occurs in the following steps
Calculating the energy released by mass defect, we find that this reaction produces at least three times the energy produced in a hydrogen fusion reaction. This huge increase in fusion energy wins against gravity and forces the star to expand into what is called a red giant. It stays as a red giant for some time when it continues helium fusion. When it runs of helium fuel too, gravity overpowers radiation pressure, and the star shrinks. It sheds its outer layers and becomes what is called a white dwarf.
White dwarfs are very dense because they are composed of mostly carbon and fusion ultimately stops. The star is now dead, and now the star is balanced by its own gravity and electron degeneracy. The white dwarf slowly radiates its heat energy into the vacuum of space,and becomes a cold black dwarf.
In the third case, where the star is more than 10 times the mass of the Sun, the star has a much more interesting and rather short life, spanning only few tens of million years. This is due to the high fusion rate, caused due to high gravity. The star depletes its hydrogen and helium faster, and goes on even to fuse heavier elements like carbon, nitrogen, oxygen and much more.
When the core reaches iron, fusion is no longer feasible because more heavier elements are less stable than iron. So gravity takes over, and increases the temperature of the core. This causes a sudden surge in radiation pressure and forces the outer layers to expand suddenly, causing a supernova - one of the grandest explosions in the Universe.
The life of the star after this again depends on the mass of the left over material. In some cases the star might completely annihilate itself leaving just clouds of gas in its place. If the remnant has a mass less than 3 Solar masses, it forms a neutron star.
A neutron star is formed when the remnant core ( which is composed of heavy metals ), collapses under gravity so much, that the electron degeneracy pressure fails and electrons combine with protons to form neutrons ( hence the name ). Now the star is balanced between gravity and neutron degeneracy pressure, and the star is extremely dense. Neutron stars too radiate their heat out into the Universe and become colder over time.
If the mass of the remnant exceeds 3 Solar masses, even neutron degeneracy pressure is not strong enough to fight gravity and the star collapses to a singularity ( a point with theoretically infinite pressure and zero volume ). This is what we call a black hole. Black holes have extremely high gravitational fields, and swallow mass around them to increase their mass and size. But they also release matter outside in relativistic jets called Hawking radiation. This eventually depletes the black hole and it just disappears.
( An artist’s rendering of a black hole with its Hawking radiation )
This answer is a gist of the life cycle of typical stars. There are many more situations where some conditions maybe different leading to an alternate life cycle.
Of course, the Universe is an interesting place and each star is unique.
Sources :
Comments
Post a Comment