Fate of the stars

What happens when the stars exhaust their fuels?

To summarize, when a normal star exhausts its nuclear fuel, it contracts under its own gravity. Sometimes it becomes a white dwarf (can be understood as mass of a sun with the size of earth, thus they are very dense). The more massive the white dwarf is the smaller it is in size. However no white dwarf is able to weigh more than 1.4 times the mass of sun.This is known as the Chandrashekhar limit.

So what would happen to more massive stars? Their centers collapse into a singularity and gives rise to a black hole. The singularity ruptures space-time fabric and leads to very high gravitational field. The stronger field makes light near the surface bend inwards more. Eventually, when the star has shrunk to a certain critical radius the gravitational field at the surface becomes so strong that the light paths are bent inwards to the point that light can no longer escape.

Let’s look more deeply into the subject for stars in different categories of mass:

1. Low mass stars

After a low mass star exhausts the supply of hydrogen in its core, there is no longer

anything to support the core against gravity. Hydrogen burning continues in a shell around the core and the star evolves into a red giant.

Meanwhile, the core of the star collapses under gravity's pull until it reaches a high enough density to start burning helium to carbon. The helium burning phase will last about 100 million years, until the helium is exhausted in the core and the star becomes a red super giant. . During this brief phase of its existence, which lasts only a few tens of thousands of years, the star will lose mass in a powerful wind. Eventually, the star will lose all of the mass in its envelope and leave behind a hot core of carbon embedded in a nebula of expelled gas. Radiation from this hot core will ionize the nebula, producing a striking "planetary nebula", much like the nebulae seen around the remnants of other stars. The carbon core will eventually cool and become a white dwarf, the dense dim remnant of a once bright star.

2. Massive Stars

In mid-sized stars, fusion stops once the core is Carbon and Oxygen. However, in giants, the temperatures rise to high levels, kicking off an explosive chain of fusion reactions. Oxygen fuses to Neon, Neon and Carbon fuses to Silicon, Silicon to Nickel and Iron.Following the fusion of Iron, the core collapses, leading to a supernova.supernovae are the most violent explosions in the known universe. The aftermath of supernova leaves us with a massive cloud of matter - a nebula - and a stellar remnant. This can go in two broad directions.

2A The less massive stars become neutron stars ( can be understood as mass of earth packed in size of a pea). Fast rotating stars emit beam of light and are called pulsars .

2B The more massive stars convert into black holes.

After calculations it was found that the minimum mass required for a star to become a black hole is 3 times the mass of our sun. Thus the massive stars with mass less than 3 solar masses become neutron stars and stars with mass more than 3 solar masses become black holes. L Thus our sun would not convert into a black hole when its fuel gets exhausted.