When most main sequence stars exhaust their hydrogen supply, they become giants. Yet on the diagram, there are not many stars between the main sequence and the giants. What would explain this fact?

kepler used the data from tycho blahe (his mentor) to construct the laws of planetary motion.

3. dark matter

empty space could also be a simple vacuum

you'd use the equation that establishes a relationship between  energy  and  wavelength, which looks like this

e=h⋅cλ, where

λ  - the wavelength of the photon; c  - the speed of light, usually given as  3⋅108m/s; h  -  planck's constant, equal to  6.626⋅10−34j s

plug the value you were given for the energy of the photon in the above equation to get the wavelength

e=h⋅cλ⇒λ=h⋅ce

λ=6.626⋅10−34js⋅3⋅108ms−1.257⋅10−24j=1.581⋅10−1m