Reply To: X-ray cooling?

[jamesr]

slane wrote:

Somebody got cosmology wrong again.
Here is a quote from the infamous internet information source:
“The core is the only location in the Sun that produces an appreciable amount of heat via fusion: the rest of the star is heated by energy that is transferred outward from the core”
http://en.wikipedia.org/wiki/Sun

I certainly know it very well.
Astronomers think that the sun can maintain stable nuclear fusion reactions in its inner core just by a centripetal force (gravity), it is a good idea, but it cannot be verified by experiments, because nobody can go in inside of the sun. I must emphasis that physics is an experimental science.
Fusion scientists think that they can achieve stable nuclear fusion reactions just by electromagnetic forces, but they have not succeeded.
We do not know that heats can produce forces directly.
Sunspots kind stable nuclear fusion reactors are governed by a centripetal force and electromagnetic forces.

Fusion in stars is incredibly inefficient & slow, hence why they last so long. The temperature in the core of only 10million degrees is only just enough to get the hydrogen to fuse. Further out the temperature drops rapidly till at the surface it is only 5400C. Sunspots are dark because they are upto 1000 degrees cooler. There is no way any fusion can occur at these cold temperatures and low density.

In larger stars where the temperature & pressure is higher you can get other reactions such as the CNO cycle which make large stars burn much faster, hotter & brighter. Although there are certainly still large gaps in the knowledge of the internal workings of stars. The observations of millions of stars and their spectra, backed up by fusion probability cross section measurements on earth give us pretty good confidence that the fusion process in the core is the right model.

To get fusion on earth at any appriciable rate you need to satisfy the Lawson criteria. That is you need the product of temperature, particle density and confinement time to be above a critical value. Magnetic fields are useful in obtaining the confinement but if you don’t have the temperature to go with it nothing will happen.

For D-T reactions, which are by far the easiest, you need around 100million degrees (10 times hotter than the core of the sun) and for pB11 we will need an order of magnitude higher again. There are no shortcuts, or cunning ways to make it easier. To obtain breakeven (ie Lawson criteria) if you reduce one factor like temperature you need to make up for it with the others, eg by increasing the density.

For those that want to get into the hard core physics, these are good introduction: Nuclear Fusion Reactions and Plasma Physics. They may look a little scary but just skip over the equations if you just want an overview.