Here we will assume that there is no absorption. The mass of helium is 4.0026 and the mass of carbon is 12.0000 atomic mass units. Compare this to the energy produced in fusing one gram of H into He. The luminosity of the star while this goes on is 100 times the solar luminosity. 14 sec which is approximately 10 million years.
Density is mass over volume, or is proportional to mass over radius cubed.
Calculate the density of a black hole of one solar mass.