In our last article we read about the Red Giant stars. Only those stars end up as Red Giants that:
- Are at least 1/3rd the mass of our own Sun or,
- Have same mass as our Sun or,
- Are up to 8 times as big as our Sun.
With these figures in mind, 97% of all stars that are present in our Milky Way galaxy will end up as Red Giants and that includes even our Sun! The question is, ‘what happens after the Red Giant state?’ We covered this partially in our article on Red Giant stars but now, let us learn that in details. Just a heads up, after the Red Giant state comes the White Dwarf stage. So, let us visit the skies once again and learn 20 interesting White Dwarf star facts. You ready?
Interesting White Dwarf Star Facts: 1-10
1. After a Red Giant gets rid of outer layers as planetary nebula, what remains is the central core that is rich in carbon and oxygen.
2. Because the size of the remnant core is not big enough, the core fails to produce temperature required for fusing carbon atoms. The required temperature is extremely high at 1 billion Kelvin.
3. Though the temperature fails to reach that high, the core is still hot at around 100,000 Kelvin and just hangs out there in the vast universe and goes through gradual cooling.
4. While the core sits there and cools gradually, it keeps emitting faint white light along with low-energy, soft X-rays. This explains the name White Dwarf.
5. The cooling process takes place over next hundreds of billions of years (it is better we call this time frame eternity). After all the heat trapped in the remnant core is released in space, the core stops emitting light and becomes absolutely dark. This is when it is known as black dwarf.
6. Though we call it a White Dwarf, it is far beyond being a dwarf that we are usually accustomed to think of. A White Dwarf star is as big as Earth in terms of size.
7. In terms of mass, a White Dwarf has the same mass as our Sun. So, the total mass of Sun compressed and put in an object the size of Earth. This makes White Dwarfs extremely dense objects.
8. A White Dwarf can have more mass than that of our Sun but the maximum it can attain is 1.4 Solar Masses (that is 1.4 times the mass of our Sun). This is called Chandrasekhar Limit. Well, we are not going to go into those technical explanations. Just know that the mechanism that prevents White Dwarfs from collapsing on themselves because of their own gravity cannot support a mass bigger than 1.4 Solar Mass (M☉). This mechanism is called electron degeneracy pressure.
9. Again, it is not necessary that a White Dwarf will have at least the same mass as our Sun. It can have lower mass. The lowest possible mass is 0.17 M☉. Most of the White Dwarfs have mass within the range of 0.5 M☉ and 0.7 M☉.
10. Just to put in perspective, if we manage to scoop out a teaspoonful of matter from a White Dwarf, it will weigh as much as 5.5 tons – almost equivalent to the weight of an elephant! Got it?
Interesting White Dwarf Star Facts: 11-20
11. Logically Black Dwarfs are formed from White Dwarfs. However, it is not practically possible to find a Black Dwarf. Why? Simple! Scientists say that the age of the whole Universe as we know today is 13.8 billion years. A White Dwarf can take hundreds of billions of years to cool down and become a Black Dwarf. Since our universe itself is not old, practically no Black Dwarfs have been formed till date.
Till now we learned the basic ABCs of White Dwarfs. Let us learn something more. Let us learn about the variations of White Dwarfs.
12. A White Dwarf can be formed from a star that initially had less than half the mass of our Sun. Such a White Dwarf is formed after the hydrogen is burnt out and the core is left with helium. However, temperature is not high enough to start fusing helium into carbon and oxygen. So, nothing more will happen. The hot helium core will sit there, hanging out and cooling down with mainly helium-4 nuclei making up the remnant core.
13. The second variation of White Dwarfs come from main sequence stars that initially had the mass of anywhere between 0.5 M☉ to 8 M☉. After the initial hydrogen is burned out, there is enough mass and temperature to trigger fusion of helium into carbon and oxygen. However, when helium is burned out, there is not enough temperature to trigger fusion of carbon into neon. Thus, the core becomes carbon-rich, wrapped in a thin layer of burning helium, which in turn is covered with an outer layer of burning hydrogen. The basic ABCs that we learned so far till point #11 were about this second variation of White Dwarf.
14. The third variation of White Dwarfs come from main sequence stars that initially had the mass of anywhere between 8 M☉ and 10 M☉. A White Dwarf formed from such a star will have enough temperature to start fusing carbon into neo and then again, will be left with enough temperature to start fusing neon into iron. The iron thus produced will move towards the very center of the White Dwarf.
15. Over billions of years iron is produced through fusion of neon. This iron at center will not have enough temperature to fuse into heavier element. Thus, the non-fusing core simply keeps growing. Eventually, it grows so big that the outward electron degeneracy pressure that prevents the White Dwarf from collapsing on itself will fail to counter the inward gravitational pull of the iron core.
16. This is when the core will collapse and then explode as a supernova known as core-collapse supernova. What will be left behind after the explosion is either a black hole or a Neutron Star or far more compact star that is way more exotic than any form known to us!
17. While the White Dwarfs have a core temperature of about 100,000 Kelvin, the temperature at the surface is way less at about 10,000 Kelvin. This low temperature may provide a habitable zone at a distance of anywhere between 0.005 AU and 0.02 AU. AU stands for Astronomical Unit. 1 AU is the distance between our Sun and Earth. Some propose that if such a habitable zone exists, life can be sustained there for at least 3 billion years.
18. According to the hypothesis, Earth-like planets may form or migrate inward to that distance and create a habitable zone. Some scientists however say that such closeness may lead to extreme tidal forces, which in turn can cause greenhouse effects strong enough to make the planet inhabitable.
19. It is possible that a White Dwarf may be formed in a binary star system. In such a case, if the White Dwarf is pulling in (accreting) matter from the companion star, the mass as well as density of the White Dwarf will increase and trigger a fusion reaction at the core. The reaction will start with an explosion and eventually, the White Dwarf will be reduced to the state of a Neutron Star.
20. Another possibility states that in a close binary star model, both stars can be White Dwarfs and both can radiate energy. The energy thus radiated takes the form of gravitational waves which will pull both the White Dwarfs closer and closer and eventually, the two will merge into a single star.