Practice Mid-term 1. Hydrogen emits a spectral line with a wavelength of 21 cm. We receive radiation from some gas moving at a certain speed that appears to have a wavelength of 21.01 cm. A spacecraft moving towards the gas at the same speed as the gas would receive radiation with a wavelength of precisely 21 cm. The gas is moving away from us.
All photons emitted by hydrogen have the same energy.
The red line has photons of the highest energy. The red line has photons of the lowest energy. The green line has photons of the highest energy. The hydrogen is in the ground state. Two stars appear from Earth to be equally bright, i.e., they have the same apparent magnitude.