Expert: Philip Stahl - 2/14/2007

Question
How many different types of activity are occuing in the sun? How long does it
take the energy from the core of the sun to reach the surface? Please explain
the journey! Thank you.

Answer
Hello,

There are probably hundreds of different physical, chemical processes occuring in the Sun - at different time and space scales. So many as to require an entire textbook to document them, and surely beyond the scope of a complete response in a forum like this.

These processes underscore a dozen or more forms of basic solar activity, including (but not limited to):

- solar differential rotation (the so called 'alpha-omega' dynamo believed resposible for the sunspot cycle.)

- sunspot growth and evolution

- coronal mass ejections and solar wind

- solar flare occurrence associated with magnetic instability

- all manner of wave instabilities, carrying huge amounts of energy - say from one solar active region to another via Moreton waves, for example.

- Eruption and magnetic twisting of solar prominences.

You can read up in more detail about some of the preceding at the following sites:

solar filaments & prominences:

http://www.spaceweather.com/glossary/filaments.html

solar wind:

http://en.wikipedia.org/wiki/Solar_wind

Solar flares:

http://en.wikipedia.org/wiki/Solar_flare

plus solar flare theory, relationship to corona, coronal mass ejections etc.:

http://hesperia.gsfc.nasa.gov/sftheory/

Sunspots:

http://en.wikipedia.org/wiki/Sunspot

http://www.windows.ucar.edu/tour/link=/sun/atmosphere/sunspots.html

Much of the complexity of the Sun, and the reason that solar processes are difficult to grasp, is because they can only be understood through the methods and theories of *Plasma Physics*. You can read much more about this branch of physics at:

http://en.wikipedia.org/wiki/Plasma_(physics)


The energy from the core of the Sun originates via nuclear reactions there, from which short wavelength photons escape. These photons then must diffuse outwards, and this process takes - on average -one million years.

That is, one million years for a given photon from the core to finally reach the Sun's photosphere. (From there, of course, it takes only 8 1/3 minutes to reach Earth)

The journey of the photons is basically one of repeated absorption and re-emission by (mainly hydrogen) atoms in their path. The original (gamma ray) photons from the solar core nuclear reactions make their way outwards and cover perhaps 1-2 mm before being absorbed by hydrogen atoms in their path. These atoms may then be excited and re-emit the absorbed photons, which then are usually re-directed in some random orientation. (Usually not toward the solar surface).

The photons again traverse maybe a few mm (or if lucky even a cm) before being re-absorbed again, then re-emitted in a new direction and so on. This process continues until the photon(s) complete the full distance of the solar radius - about 432,000 miles. Obviously, if a given photon is only covering a millimeter or even centimeter at a time it will not break any speed records reaching the solar surface or photosphere. Hence, the unusually long time to escape.

Now, people often ask - if this is so- how come we don't observe a black or dim Sun, or no Sun at all. They err in this question by assuming that *all photons* released from the core are *at the same stage of their journey*! In fact, there are hundreds of billions of different stages of photon exit, and we may be certain that we will never encounter any sudden "switch off" because the photon stream is continuous.

Hope you find this info useful!