Facts and observations on the Chandrasekhar border

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Facts and observations on the Chandrasekhar border

Some of these stars will die as slowly cooling stellar embers known as white dwarfs, while others will just pass through this stage. They will then explode in tremendous cosmic blasts known as supernovas, forming neutron stars or possibly black holes.

Before reaching a white dwarf state, stars first lose mass by shedding their outer layers. This means that the 1.4 solar masses usually represents the stellar core that is left behind.

If it is in a binary system, however, a stellar core doesn’t need to begin with enough mass to exceed the Chandrasekhar limi. For white dwarfs with a binary partner, there is another way they can exceed this mass limit.

According to Swinburne University, the beginning mass for stars that remain white dwarfs is 8 solar masses, though other predictions suggest a star has to be ten times the mass of the sun to leave a core with enough mass to exceed the Chandrasekhar limit.

Story Highlights

  • Stars are fighting against their own gravity, which will eventually lead to severe and radical changes that signal the end of their main sequence lifetimes.

  • The Chandrasekhar value for a white dwarf star is generally considered to be 1.4 solar masses , according to The SAO Encyclopedia of Astronomy —  that is 1.4 times the mass of the sun. First predicted by Subrahmanyan Chandrasekhar in 1931, the Chandrasekhar limit mass has so far corresponded well with observations as we are yet to find a white dwarf with a mass above 1.4 solar masses.

If a white dwarf at the edge of the Chandrasekhar limit is accreting mass from its partner — referred to as a donor star — then this can push it beyond the Chandrasekhar limit. This results in further thermonuclear burning, usually the fusion of carbon and oxygen, and pushes the white dwarf towards a supernova explosion.

These circumstances lead to a very specific type of supernova called a Type Ia supernova different from supernovas caused by core collapse.

WILL THE SUN EXPLODE?
In around 4.5 billion years the sun will run out of hydrogen in its core meaning it can no longer sustain nuclear fusion. This will signal the end of the outward pressure that stops its core from collapsing under gravity. As the core collapses, the outer layers of the sun will puff out in a series of outbursts beginning a short-lived red giant phase for our star. In the core helium created by the fusion of hydrogen will begin to fuse into carbon.

The shed outer layers will spread out to the orbit of Mars, consuming the inner planets including Earth, eventually becoming a planetary nebula that surrounds a scorching hot, albeit gradually cooling stellar core known as a white dwarf. This is how our sun and other low to medium mass stars will remain for trillions of years, meaning the sun will not explode.

This isn’t the end for all stars, however. Some have enough mass to push past this white dwarf phase and initiate further nuclear fusion, a supernova, and the transformation into an exotic stellar remanent. The dividing line between these fates is the Chandrasekhar limit.

This is the principle from quantum physics that prevents two electrons from occupying the same quantum state and essentially prevents them from cramming too close together, providing the pressure to support the white dwarf against its own gravity. But even this limit can be exceeded. BEYOND THE CHANDRASEKHAR LIMIT
In stellar cores with a mass greater than 1.4 times that of the sun, carbon burning can be initiated creating neon, according to The SAO Encyclopedia of Astronomy. This leads to further stages of core contraction and the burning of successively heavier elements until the heaviest element that can be synthesized in stars ,  iron,  fills the core.

A white dwarf with a mass of 1.4 solar masses or less can’t initiate carbon burning but continues to contract until this is halted by electron degeneracy pressure. WHAT PROTECTS A CHANDRASEKHAR MASS STAR AGAINST FURTHER COLLAPSE?
With all the hydrogen of a stellar core exhausted at the end of the main sequence the white dwarf that remains consists mainly of carbon  — created by the fusion of helium in the red giant stage.