Medium Mass Stars
This includes stars with an initial mass from about 40% of a solar mass up to about two to four solar masses. It is hard to be precise about the maximum initial size as it depends on how much mass is lost over the life of the star. The star, having burned all the hydrogen in its core, collapses under the force of gravity, the internal temperature again rising with increasing pressure. Hydrogen burning now moves to a shell around the core exerting more inward pressure on the core, and causing the outer shell to expand and cool, as the star starts on the path to becoming a red giant.
With no fusion going on in the core, gravity and the increased temperature from the hydrogen fusion around it, causes the core to compress. The temperature rises to around 130 Million degrees K. This is the point where it can start to convert helium into carbon via Beryllium-8. In stars of up to about 2¼ solar masses, the contraction compresses the helium in the central part of the core until it becomes degenerate. The degeneracy pressure now stops further contraction in this central area while the rest of the core continues to increase in temperature. The degeneracy pressure overcomes the thermal pressure that is trying to expand the central core, so it does not increase in volume, and the pressure does not go down. Thus, when helium burning starts, for the first few seconds it is highly explosive; this is the "core helium flash". This causes the temperature to rise so high that thermal pressure dominates and eliminates the degeneracy in the helium, allowing the core to expand, cool down, and stabilize helium burning. Stars with about 2¼ to 4 solar masses heat up enough to start helium burning without the central core becoming degenerate, avoiding the helium flash.
The star now settles down to a period of stable helium burning producing carbon. Some Carbon-14 atoms combine with a helium atom to produce oxygen-16. This whole process is much less efficient than burning hydrogen to helium, and lasts only about 10% of the time main sequence burning lasted. In the case of the Sun, for example, helium burning will last about one billion years compared to ten billion years of hydrogen burning.
Once the Helium is used up, the hot carbon core cools and starts to contract. The temperature rises but, while it is very high, it is not high enough for further fusion reactions to start. The energy generated by collisions between free electrons stops the contraction in the core due to electron degeneration pressure. The outer layers drift away to form a planetary nebula around the star. The star has lost so much material that it now has a mass of up to about 1.4 solar masses. It contracts until it is approximately the size of the Earth; very hot with a density of about one metric ton per cubic centimeter. It is now a white dwarf.
Although it is very hot, the star has a very small surface area from which to radiate heat. Over time, the white dwarf slowly radiates all itsí heat and becomes a black dwarf. No one knows how long this will take, but estimates vary between 25 billion and one trillion years. At less than 14 billion years, the Universe is not old enough for any white dwarfs to have become black dwarfs yet, although many have become very cool and dark red.
This is the fate of our sun. In about 5 billion years, it will start to become a red giant and will expand so far, it will probably engulf Mercury and probably Venus too, even though the planets will have moved their orbits outwards as the Sun lost mass. When finally it stops nuclear processes, after perhaps another billion years or so, its' planetary nebula could reach beyond the current orbit of Jupiter. Whether the Earth is still around at this stage is conjectural. The pictures below illustrate a number of typical planetary nebula, similar to that which the sun may well form during the process of becoming a white dwarf.
Astronomy & Cosmology -
Stars - Life & Death of Stars
The Ring Nebula
The Helix Nebula
The Cat's Eye Nebula
The Necklace Nebula