What is the density of krypton gas at stp

The blackbody curve for a star name zeta is shown below. what is the peak wavelength for this star ?

How is energy related to the change of state represented by the model? atoms gain energy as a solid changes to a liquid. atoms gain energy as a solid changes to a gas. atoms lose energy as a solid changes to a liquid. atoms lose energy as a solid changes to a gas.

Which change to the system wood cause the freely-moving piston to lower?