Question

How does the frequency of the light from a star that is moving towards earth appear to us?

Answer 1

Hint: If a star is approaching you, its Fraunhofer lines will shift toward the blue end of the spectrum ("rainbow"). When a star moves away from us, its Fraunhofer lines shift to the red end of the spectrum.

Complete answer:
A siren or a car speeding by sounds higher in pitch as it gets closer to you and lower as it moves away. The Doppler effect occurs when waves, in this case, sound waves, change in frequency and wavelength as the source moves closer to you (higher frequency, shorter wavelength) or further away from you (lower frequency, longer wavelength) (lower frequency, longer wavelength).
There is no discernible difference in sound; the car is not making a different noise. It simply sounds different because of the car's movement about you. When a star moves towards Earth, it appears to emit light with a shorter wavelength than a stationary source of light. Blueshift refers to the fact that shorter wavelengths correspond to a shift towards the blue end of the spectrum. The light from a star moving away from us, on the other hand, appears to shift towards longer wavelengths. Because this is near the red end of the spectrum, astronomers refer to it as redshift.
The degree of shift can also tell astronomers how fast the object is moving about us. A faster-moving object has a greater wavelength shift. Using various methods to determine the distance between galaxies, Edwin Hubble (and those who followed him) discovered that their velocity was always proportional to their distance. The ratio of the two became known as the 'Hubble constant,' and it represents the rate of expansion of the universe.

Note:
Ultrasonic Doppler Velocimetry is used to determine the real-time completion velocity profile of any liquid that contains suspended particles such as dust, emulsions, and gas bubbles. The flow may be pulsating, laminar, turbulent, oscillating, or stationary.