Lois & Clark Forums Forums General Discussion Off Topic Oh my, we're doomed!

I have something to add. (Don't I always?) Well, I'd like to say a few more things about what a white dwarf is.

A white dwarf is the hot but cooling core of a star that was as massive as the Sun, or a bit more massive. A white dwarf has blown off its outer layers, revealing its "naked core". During a fleeting stage of a Sun-like star's life, it will be a so-called "planetary nebula". The term was coined by astronomers who looked at planetary nebulae through early telescopes and thought that these nebulae looked like planet Uranus in the telescope. Unlike stars they had an obvious "disk" and were slightly greenish in color. Here are some pictures of planetary nebulae:



Abell 39.



Soap Bubble Nebula.



Ring Nebula.



Cat's Eye Nebula.

The "disks" you can see here are the cast-off atmospheres of former Sun-like stars. When a star like the Sun reaches the end of its life, its core will grow hotter, which will cause the outer atmosphere of the stars to expand. Later the core will start "hiccuping", which will blow the outer atmosphere even further away from the core. At the same time the newly exposed very hot core will make the cast-off atmosphere glow. The green color of many planetary nebulae is caused by the irradiation of oxygen ions in the cast-off atmosphere by the very hot core at the center. Take a look at the first planetary nebula posted here, Abell 39. You can see the green color of the cast-off atmosphere, but also the striking blue color of the exposed glowing hot core at the center of it. Strangely enough, the blue color means that the core is very hot.

All these formerly Sun-like stars have reached the end of their "nuclear fusion"-line. Nuclear fusion has ceased in their cores, which now consist of a mixture of helium, carbon and oxygen. They can't go on synthesizing heavier elements, because they aren't massive enough to make their cores hot enough, but they also won't collapse in on themselves like the dead cores of the really massive stars, and the reason for this is again that they aren't massive enough. If I remember correctly, astronomers talk about the "Pauli principle" (beware that I haven't even googled it), which says that when electrons get packed so closely that they basically "touch", they will resist further packing extremely strongly. In the core of a white dwarf star, the electrons are packed that closely. The gravity of a white dwarf star is not enough to overcome the outward pressure of the electrons inside it, so the white dwarf will not collapse under its own weight.


Our Sun will become a white dwarf one day, but it will have no chance whatsoever to become a white dwarf supernova. The first reason is that it is not massive enough. A white dwarf is just the dead core of a star, but in order to become a supernova the core needs to weigh 1.4 times as much as the Sun. Our Sun weighs, unsurprisingly, exactly one solar mass , but when it has cast off its outer atmosphere to become a white dwarf it will weigh less. Also a white dwarf has to have a closely orbiting stellar companion in order to ever go supernova, because it has to suck matter from the other star in order to gain enough mass to explode.

Here you can see the relative sizes of a normal star like the Sun, compared with the size of a red giant (a Sun-like star which has ceased nuclear fusion in its core and puffed up its atmosphere) and a white dwarf (a former red giant which has blown off its atmosphere). Note that a white dwarf is about the same size as the Earth, but it contains about as much mass as the Sun.



Ann