Question

A star emitting a continuous spectrum shines through a cool interstellar cloud composed primarily of hydrogen. The cloud is falling inward towards the star (and away from Earth). Which best describes the spectrum seen by an observer on Earth?

a) blueshifted hydrogen emission lines
b) blueshifted hydrogen absorption lines
c) redshifted hydrogen emission lines
d) redshifted hydrogen absorption lines
e) a hydrogen continuum, neither redshifted nor blueshifted

Answer 1

The spectrum seen by an observer on Earth, when a star's light passes through a cool interstellar hydrogen cloud moving away from Earth, will exhibit redshifted hydrogen absorption lines.

Step-by-step explanation:

The correct answer to the question is d) redshifted hydrogen absorption lines. A star emitting a continuous spectrum that passes through a cool interstellar hydrogen cloud would result in absorption lines being observed.

Because the cloud is composed of hydrogen and cooler than the star, it will absorb specific frequencies of light that correspond to hydrogen's energy transitions. Due to the cloud moving away from Earth, these absorption lines will be redshifted because of the Doppler effect.

An observer on Earth viewing the star's light that has passed through the cloud will see a spectrum that is less intense at the frequencies corresponding to hydrogen's energy level transitions. As the cloud is falling toward the star and away from Earth, the absorption lines in the spectrum will appear shifted toward the red end of the spectrum, indicating motion away from the observer.