14 DAY TRIAL // Almost all small and medium-size stars will end up as white dwarfs, after nearly all the hydrogen in their cores has been fused into helium. Near the end of its nuclear burning stage, such a star goes through a red giant phase and then expels most of its outer material (creating a planetary nebula) until only the hot (T > 100,000 K) core remains, which then settles down to become a young white dwarf which shines from residual heat.
There are two types of red giants, one contains lots of carbon and the other a lot of oxygen. When the carbon-rich stars die, large clouds of carbon particles such as soot and graphite are produced. Soot and graphite are pitch-black, so when the rays from the dying star hit the soot particles, they stop the light and are pushed out into space, where they are seen as gigantic dust clouds.
This is a 20-year-old theory which fits both the observations and the model calculations.
But for the other type of red giant stars, which are oxygen-rich, it has been a mystery how their material is blown out into space. The oxygen-rich stars produce large quantities of water and silicates, like quartz or grains of sand. Water and silicates are transparent, which means that when rays from the star hit the particles, they go straight through-just like when the sun rays go through glass. The star's radiation, therefore, can't push on the particles and be the motor which drives the dust clouds out into space.
Some astrophysicists like Anja C. Andersen and Susanne H?fner, from the Niels Bohr Institute, in Denmark, are not convinced by this classical explanation, and have pondered the problem for many years and weighed possible solutions.
First, they considered whether there could be iron in the silicates, so that it wasn't transparent. But then calculations showed that with iron, the dust grains would have melted and were therefore unstable.
Next, they thought that perhaps many molecules had been produced, which could block out the light and thereby create a wind which blew the star's outermost layer into space. But the model calculations showed that the molecules could not block enough light to drive the process alone.
When the star becomes a red giant, it pulses in its death-throes, and perhaps this could "spark" the process into motion and force the star's material out. But this theory didn't rally with real observations of these stars.
And then it hit them: "What if the oxygen-rich stars also make some carbon that works with the transparent dust, and altogether can give a push to the process by forcing the star's outer layer into space?"
Could it be so simple, and in reality, it's the same process that is happening in both types of stars?
It seems the theory also fits with the observations obtained from dying red giants. The carbon may be the explanation for how both types of stars blow their material into space, where they are ultimately reused in the cosmic cycle in new stars and planets.