Last week I was doing about cepheid variables with my SL class and we were talking about why they have a changing brightness. The simple explanation is that they are expanding and contracting, if the temperature of the star is constant then this would cause the luminosity and hence the brightness to change. Unfortunately it's not that simple.

Its all to do with the outer layers of the star.

• The core of the star is producing heat which is absorbed by the outer layer causing the outer layer to get hot.
• As the outer layer gets hot it expands and cools.
• The outer layer now absorbs less radiation. This means that more radiation is escaping which results in an increase in brightness.
• As radiation escapes the gas cools. The cooler gas now contracts due to the force of gravity. This causes it to get hot so it absorbs more radiation and the cycle repeats.

The animation shows this although the graph is a bit misleading, I'm not sure why the blue lump moves around. It would make more sense for the curve to be static with a point moving along it. I'm also not sure why the core is changing size.

Why don't all stars do this?

This is because not all stars have an outer layer that absorbs more radiation when it is hot. One way this can happen is if the layer consists of helium. Hot helium is ionised which absorbs light better that cold helium which isn't.

So why does ionised Helium absorb radiation better than non ionised Helium?

Hmm. I would expect it to be the other way around since without electrons to be excited how can the Helium absorb radiation. The answer is that ionised Helium or Plasmas don't absorb radiation in the way that atoms do. A Plasma is a gas made of nuclei and electrons moving rather quickly in random motion. When EM radiation passes through a plasma the charges become excited by the varying Electric field thereby absorbing energy. Normal gaseous Helium on the other hand can only absorb the specific wavelengths that excite atomic electrons.

Here is an alternative explanation

Courtney Seligman's website

• The outer layers of a star are made of gas.
• If a gas gets hot it expands.
• The outer layers of gas in a stable star stay the same size because the temperature is constant. The radiation entering the layers from the hot core = the radiation leaving
• If a layer becomes too dense it absorbs more radiation leading to an increase in temperature. The gas expands which reduces the density until equilibrium is again established.
• In a Cepheid the dense layers are made of ionised He which gets hot and expands however as it expands the electrons recombine with the ions resulting in a release of energy. This stops the temperature falling so the gas continues to expand until all the He is recombined, then the temperature falls.
• At this point the star is very large so is emitting a lot of radiation (this is the bright part of the cycle). The gas now cools and starts to contract. As this happens the temperature of the He rises again and the atoms again become ionised - the temperature of the gas staying constant allowing the contraction to continue.