Expert: Philip Stahl - 7/9/2004

Question
  Iwould just like to know of the interactions going on inside the sun and other stars.  I read a small bit on convection currents working adiabotically in the atmospheres of stars but didn't fully understand it.  Also what causes solar flares, prominences, and other solar phenomenon?  Thanks for your time!

Answer
Hello.

Basically, you can think of the Sun as a typical "garden variety" star. It is a gigantic, gravity-bound nuclear reactor. The highest temperatures are in its central core (imagine it constructed like a giant 'onion' with layer upon layer) which also has the highest density. Here is where you will find protons fusing together as well as other particles, to yield helium plus a great deal of energy.

The average temperatures in the core are about 14 million Celsius. This is hot enough to enable nuclear (fusion) reactions - which are responsible for giving off the radiant energy in the star.

For example, one such reaction is:

H1 + H1 + H1 + H1 ->  He 4  + energy

That is, four protons fusing (nuclear fusion) to give helium, plus an amount of energy given off that is equal in amount to:

E = (total mass of 4 H1 - total mass of He 4) c^2

where the above is one form of the famous Einstein mass-energy equation, E = mc^2. (Note that the total mass of the reactants on the left side always exceeds the total mass of the formed nuclei on the right. The mass difference is what provides the energy equivalent for the reaction)

As one leaves the core one comes to a large 'radiative' zone - so called before the high energy photons leaving the core zig-zag out through this region toward the cooler layers above. The first such major zone above the radiative, is the "convective".

The temperatures vary from the millions of degrees - near the core boundary, to thousands of degrees at the bottom of this 'convective zone'.

Within the latter, energy is transported by convection to the surface or photosphere, which is at a temperature of approximately 5500 C.  This means a given volume of heated gas (actually plasma) expands, and as it does so rises to near the photosphere-surface. There it delivers its heat, whereupon the volume cools, and descends back toward the hot core. There the cycle resumes, as it is re-heated and once more expands. (Recall that as a given gas volume expands from heating, it becomes less dense- hence tends to rise).

Above this (convective) region is the chromosphere, where temperatures are somewhat cooler - then the higher altitude corona where they suddenly increase to 2 million degrees. There is as yet no consistent explanation for this temperature inversion.

At the normal stable photosphere one usually finds "sunspots".  These are darker (than the surrounding photosphere)as a direct result of the gases within the sunspot being trapped in a "magnetic bottle". This "bottle" insulates the trapped plasma from higher temperatures and the temperature difference (up to 1500 C) leaves them darker to an Earth observer.

Note also that ordinarily the sunspots appear very dark at the center (umbra) with a lighter 'ring' (penumbra) outside. This is a direct result of the most intense magnetic field being toward the center of the magnetic "bottle" or tube.

For some reason, very large sunspots with "complex" magnetic structures(that is, multiple magnetic polarities in the same sunspot)can become unstable and given rise to extremely violent flares. We believe that what happens can be somewhat likened to twisting a magnetic tube over and over - like a large rubber band. Eventually, so much (potential magnetic) energy is stored inside by twisting, that it erupts. This is what we call a "solar flare".

Its effects can be dramatic, since for powerful flares the particles don't stay in the Sun (or near it's photosphere) but travel in space toward Earth.


One major flare recorded in 1989 caused a major outage in the Ottawa (Canada) power grid. These eruptions also have the potential to disrupt aircraft navigation, and very strong particle emissions from them have been documented as destroying the electronics on satellites. (I recall this occurred during cycle 22, but cannot recall the exact name of the satellite).

Prominences often represent the large scale eruption of high altitude magnetic "tubes" (coronal arches) on the solar surface- with the plasma exploding into space. In many cases, prominences erupt as offshoots of flares - in other cases they are difficult to distinguish from flares. Generally, however, it is agreed that prominences go on for a longer time than flares and the changes in their physical -magnetic conditions are nowhere near as dramatic or abrupt. In other words, they often tend to highlight (via intense brightening) their associated magnetic structures (e.g. arches) rather than obliterate them entirely - as is common with major flares.