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The two statements that explain why light from stars provides evidence that the universe is expanding are:
1. Light waves from stars spread out as the stars move away from Earth. This is known as redshift. As stars move away from Earth due to the expansion of the universe, the light waves they emit get stretched out, causing their wavelengths to become longer. This elongation of wavelengths shifts the light towards the red end of the spectrum, indicating that the universe is expanding.
2. The frequency of a star's light waves decreases due to the Doppler effect. The Doppler effect causes the frequency of light waves to decrease as the source (in this case, a star) moves away from the observer (Earth). This change in frequency results in a shift towards the red end of the spectrum, confirming that the universe is expanding.
These two phenomena, redshift and the Doppler effect, are crucial pieces of evidence that support the theory of the expanding universe. By observing these changes in the light from distant stars, scientists have been able to infer that the universe is indeed expanding.
