One day, while researching on blue dwarfs in Wikipedia, I stumbled across something that I cannot understand:

Rather than expanding, however, red dwarfs with less than 0.25 solar masses are predicted to increase their radiative rate by increasing their surface temperatures and becoming "bluer". This is because the surface layers of red dwarfs do not become significantly more opaque with increasing temperature.

But I could not find more data on this topic.

However, this also means that the Sun and massive stars expand into red giants and supergiants, as the star becomes "opaque" with increasing temperature and cannot radiate heat fast enough to cool itself down. So it swells up in size to increase the surface area to radiate excess heat away.

However, red dwarfs are many, many times more dense than the Sun. This should mean that a red dwarf would be incredibly opaque, so it blocks heat and light from the core. This should mean that a red dwarf should theoretically swell up to radiate heat away. But, a red dwarf does none of these. It simply gets hotter.

Why do red dwarfs remain "transparent" to internal heat, whereas the Sun and other stars remain "opaque" despite them being many times lighter than red dwarfs?

  • 2
    $\begingroup$ Note that the photospheric radius of a star is where it stops being opaque, by definition. The text you quote does not say that the Sun expands to become a giant because it becomes opaque. All stars are opaque. $\endgroup$
    – ProfRob
    Commented Nov 5, 2022 at 8:05
  • $\begingroup$ Can you provide a link to the article you're quoting from? Contextless quotes like that are harder to work with. $\endgroup$ Commented Nov 5, 2022 at 10:28
  • $\begingroup$ Question needs rewriting to omit all the supposition and assumptions that you are making. The bold face question cannot be answered because it isn't true. $\endgroup$
    – ProfRob
    Commented Nov 5, 2022 at 10:34
  • $\begingroup$ @PeterErwin The quote appears to be from the wikipedia article on blue dwarfs, and the OP has edited the link into the question. $\endgroup$ Commented Nov 5, 2022 at 12:52
  • 1
    $\begingroup$ Re However, red dwarfs are many, many times more dense than the Sun. That isn't true, at least not in the core where fusion takes place. The cores of red dwarfs have a much lower temperature and a much lower density than do larger stars. $\endgroup$ Commented Nov 5, 2022 at 12:55


You must log in to answer this question.

Browse other questions tagged .