Structure of a Star
The Core
Recall that a star is constantly under pressure due to gravity compressing it. This means that at the centre of the star there is a great pressure since there is more stuff compressing the core. As we all know, more pressure means that things get hotter. Thus, it is at the core, where the temperature of the star is the highest.
Now, we all know that nuclear fusion needs a high temperature to occur. After all, why would scientists want cold fusion if it happened at room temperature? Given the core has the highest temperature in the entire star, it is logical that it is the location where nuclear fusion occurs.
Now that we know how energy in the star is made. How is it produced? Let us find out.
Note
The subsequent analysis of the radiative zone and convective zone is done with respect to a sun-like star. Depending on the mass of the star, the radiative zone can be the inside or outside layer of the star.
The Radiative Zone
On its journey out, energy in the form of photons first enters the radiative zone. As the name suggests, radiation is the main form of energy transfer here – though not quite the form that you would expect!
Keeping in mind that the stars are very dense, photons don’t travel straight through. Rather, they travel in a zigzag manner. This zigzag motion is caused by the photons "bouncing" around within this dense layer. Note that this layer of plasma is so hot that it is fully ionized (all electrons are stripped from atoms to form nuclei), so the constituent particles interact strongly with light. Overall, due to the sheer density of the radiative zone, a photon takes a few million years to reach the edge of the radiative zone from the core. This is in spite of the fact that photons travel at the speed of light. This bouncing also gradually causes the photon to lose energy over time, and its wavelength will increase, causing it to fall from the gamma range to the visible-UV range in the electromagnetic spectrum.
Conditions for Energy Transfer via Radiation
Energy transfer by radiation occurs when there is a low temperature gradient and low enough opacity to allow energy transfer by radiation. It also requires a high temperature as the hydrogen atoms need to be ionised for the star to be transparent to ultraviolet radiation.
The Convective Zone
At this point, temperatures are sufficiently low for electrons and atomic nuclei to recombine and form atoms. These atoms will then absorb the photons and heat up. Thus, the plasma at the bottom of the convective zone is hotter and less dense. As a result, it rises while the cooler plasma (which is denser) sinks.
This setups up convection currents which are visible to the naked eye (see the picture of convection cells on the sun's surface). When the plasma reaches the surface of the star, its energy is released as heat.
Conditions for Energy Transfer via Convection
Energy transfer by convection occurs when there is a large temperature gradient a given parcel of gas within the star will continue to rise if it rises slightly via an adiabatic process.
Different Mass Stars
Different mass stars have different orders of layers (see above). Click on the tabs below to see why.
Stars with masses less than 0.5 solar masses do not have a radiative zone and only have a convective zone. This is because the temperature in these low mass stars is low enough such that the hydrogen is largely neutral and thus opaque to UV photons. Thus, energy transfer is dominated by convection.
Stars with masses between 0.5 and 1.5 solar masses have a radiative zone on the inside and a convective zone on the outside. In these stars, hydrogen-to-helium fusion occurs primarily via proton–proton chains, which do not establish a steep temperature gradient. Thus, radiation dominates in the inner portion of solar mass stars. The outer portion of solar mass stars is cool enough that hydrogen is neutral and thus opaque to ultraviolet photons, so convection dominates. Therefore, solar mass stars have radiative cores with convective envelopes in the outer portion of the star.
Stars with masses above 1.5 solar masses have a convective zone on the inside and a radiative zone on the outside. In these stars, the core temperature much higher, so hydrogen-to-helium fusion occurs primarily via the CNO cycle. Due to the strong temperature sensitivity of the CNO cycle, the temperature gradient in the inner portion of the star is steep enough to make the core convective. In the outer portion of the star, the temperature gradient is shallower but the temperature is high enough that the hydrogen is nearly fully ionized, so the star remains transparent to ultraviolet radiation. Thus, massive stars have a radiative envelope.