This is actually a homework question, but its probably better suited for this forum. Gas accreting onto burned-out stars like white dwarfs, neutron stars, and black holes can often be seen in the X-ray part of the EM spectrum, at energies of about 10 keV. What temperature does this suggest that the accreted gas is heated to? I can't make sense of my class notes, and the textbook does not cover this well. I can't find a formula that relates wavelength to energy to give temperature. Formulas using Watts is the closest I can come, but Watts is Joules / s, and there is no time in this problem. What did I scribble down in my notes that I can't read??