This is a follow up to: Is there a theoretical maximum size limit for a star?

The answer there talks about the radiation pressure preventing a star from forming.

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What reaction is causing this radiation pressure and why does this potentially prevent a star from forming?


2 Answers 2


Radiation pressure is nothing but electromagnetic interaction.

Imagine a hydrogen atom hit by a stream of photons coming from the same direction. Although the atom as a whole is neutral, the electron and the proton are physically displaced, forming a dipole, i.e. a positive-negative charge couple. Some of the photons therefore scatter against the dipole transferring to it some momentum. So the atom start moving in the same direction as the photons. If the photons are in the ultraviolet, the electron can be exited to higher orbitals and possibly stripped from the atom. In this case the scattering is even more efficient.

Now imagine a star surrounded by a layer of hydrogen. Gravity attracts the layer towards the star. The photons emitted by the star try to push the hydrogen atoms away from it, through the electromagnetic force.

Very massive stars are very luminous and hot, which means that they emit a lot of ultraviolet photons. When the pressure transferred from the photons to the layer is larger than the gravitational attraction, then the layer begins expanding, effectively stopping growth of the star.

In the figure posted by the OP there is also dust. I don't know the details of photons-dust-gas interactions (we need a stellar atmosphere expert, I guess), but the basic principle is nonetheless the same.


To add to @Frencesco's answer, if you're talking about why temperature prevents a star from forming in the first place, the same principles apply.

For a star to form you need a cloud of gas to cool off significantly so that it can condense, much further along it can begin fusing hydrogen at its core. For it to cool off and begin collapsing it needs radiates away energy (this is partly why there are thought to be no dark matter stars). This process can take time. However, once the star has formed, ionizing radiation works to act against gravitational collapse, transferring energy/momentum back into the gas.

Summary: You first need to radiate away energy before you can create a star. For the most massive stars, the rate of energy/momentum transferred into the outer layers of the star is simply to great (see above).


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