This diagram showing the pre-main sequence track might be helpful:
https://www.ucolick.org/~bolte/AY4_00/w ... ation.html
The proto-star in the diagram starts out with a radius of a hundred solar. It is in the process of shrinking, and it is really bright, much brighter than it will be when it first reaches the main sequence (blue line). But at this stage the pre-main sequence star is also very cool.
When the proto-star has shrunk to a radius of ten solar, it is still considerably brighter than it will be when it reaches the main sequence. But it is less bright and less cool than when its radius was a hundred solar.
What happens when the star reaches the main sequence is that the star stops shrinking, and the heat production that came from the continuous shrinkage stops. Instead, the star starts fusing hydrogen to helium right in its very core. Over time, the star will grow brighter even while it stays on the main sequence, probably because more and more of its central region gets involved in hydrogen fusion - the hydrogen-fusing part of its core grows larger, or so I believe.
Of course, when the star has depleted its core hydrogen, the core starts shrinking again, which generates a lot of heat. Meanwhile, the star has now yet another heat source, namely a hydrogen-fusing shell around the shrinking and therefore heat-producing core. This double heat production makes the star's outer layers expand mightily, turning the star into a red giant.
The outer layers of the red giant are eventually lost, and the hot shrunken core is revealed: A tiny white dwarf.
The white dwarf can't shrink any more, due to electron degeneracy pressure. The electron degeneracy pressure stops any two electrons from occupying the same "space". (There is a quantum term for this, but in layman's terms that is what electron degeneracy pressure means.) The electron degeneracy pressure will keep the white dwarf stable as long as its mass does not exceed 1.4 solar masses. Above this mass, the electron degeneracy pressure is overwhelmed.
There is one thing that could make a white dwarf grow in mass to the point that the electron degeneracy is overwhelmed, and that is the presence of a very nearby companion star that could start dumping gas onto the white dwarf:
https://w.astro.berkeley.edu/~basri/ast ... lec16.html
In the illustration, the donor star is a main sequence star. That is rarely the case. Instead the donor star is typically a swollen red giant.
Often the process of a companion dumping matter onto a white dwarf just leads to explosions on the surface of the white dwarf, nova explosions, which leave the white dwarf itself unharmed:
But occasionally, if matter is dumped onto the white dwarf too fast, and the mass of the white dwarf itself is close to 1.4 solar masses, this could lead to mass increase that overwhelms the electron degeneracy pressure and makes the white dwarf shrink precipitously. When the white dwarf shrinks so violently its temperature spikes. And because the white dwarf is really made of "prime fuel", helium, oxygen and carbon, then 1.4 solar masses of fuel suddenly explodes in a raging "deflagration" that leaves nothing behind.
Or so I understand it.
We don't have to worry about Sirius triggering a supernova explosion in its white dwarf companion, in the unlikely event that humans are still here when Sirius turns into a red giant. Sirius and its white dwarf companion are too far apart for any mass transfer to happen between them.
Ann
Young Stars, Dark Nebulae
