Astronomy Practice Exam

What happens to the spectral lines of a galaxy that is moving rapidly away from the observer?

1. They shift to shorter wavelengths

2. They remain unchanged

3. They shift to longer wavelengths

4. They disappear completely

The correct answer is: They shift to longer wavelengths

When a galaxy is moving rapidly away from an observer, the phenomenon observed is known as the redshift. This occurs because the light emitted from the galaxy is stretched as it travels through space. As the source moves away, the wavelengths of the light increase, shifting them toward the red end of the spectrum, which corresponds to longer wavelengths. The redshift can be quantitatively described using the Doppler effect, which applies to all types of waves, including electromagnetic waves like light. When the source of the light moves away from the observer, the frequency of the light decreases, leading to an increase in wavelength. This shift to longer wavelengths is a direct consequence of the galaxy's motion relative to the observer. Thus, the correct answer reveals that the spectral lines of a galaxy moving away appear shifted to longer wavelengths, demonstrating the fundamental concept of redshift in cosmology and astrophysics.