When excited electrons decay to ground state, hydrogen emits along distinctive line near the visible red. An astronomer observes what appear to be hydrogen emission lines from a distant star but they are in the infrared spectral band. She concludes ________.
◦ the star must be moving very fast away from Earth
◦ the star's hydrogen emission must be being absorbed by water in the atmosphere, which then re-emits in the infrared
◦ her instrument needs to be recalibrated
◦ the star must be moving very fast toward Earth