Background
While I was in graduate school, I put together some cartoon-like comparisons of multiple stars to show the order of magnitude differences in radii.
At the time, VY Canis Majoris was the largest known star by radius (it appears from my graphic that it was then thought to be ~1950 $R_{\odot}$, where it is now thought to have a radius of $1420 \pm 120 \ R_{\odot}$). I see now that UY Scuti has taken that title with a radius of $1708 \pm 192 \ R_{\odot}$. I recall that at the time the mass of VY Canis Majoris was not well known (as suggested by my cartoon image) but now I see that it is reported to be $17 \pm 8$ $M_{\odot}$. Even more interesting is that UY Scuti has an even smaller mass of ~7-10 $M_{\odot}$.
As a comparison, one of the more massive stars in our catalogues is Eta Carinae, which is a binary system where the primary, $\eta$ Car A, has $r \sim 60-800 \ R_{\odot}$ and $M \sim 100-200 \ M_{\odot}$.
A quick survey of Wikipedia shows me that there are over a dozen stars with $r \geq 1000 \ R_{\odot}$ and over a dozen different stars with $M \geq 100 \ M_{\odot}$.
Questions
- What causes a star like UY Scuti to have such a large "radius" but so little mass while the much more massive $\eta$ Car A is less than half the size?
- Is it their respective ages?
- Is it their composition (i.e., fuel source)?