When an emission line of helium (or any spectral line) from a galaxy is shifted toward the red end of the spectrum, it indicates a phenomenon known as "redshift." Redshift occurs when the wavelength of light is stretched, resulting in longer wavelengths that appear redder than they would if the source of the light were at rest relative to the observer.
In the context of galaxies, redshift is primarily due to two main factors:
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Doppler Effect: If the galaxy is moving away from us, the light it emits is stretched due to the relative motion, akin to the way sound waves stretch as an ambulance moves away. This results in the spectral lines shifting to longer (redder) wavelengths.
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Cosmological Redshift: This occurs due to the expansion of the universe. As the universe expands, the space between galaxies stretches, and so does the light traveling through that space. This results in the observed light being redshifted.
In summary, a redshifted emission line of helium from a galaxy indicates that the galaxy is moving away from us. This is often used in cosmology to infer the velocity of galaxies and to understand the dynamics of the universe, such as supporting the idea of an expanding universe. The amount of redshift can be quantified using the redshift parameter \( z \), which is a measure of how much the wavelength has increased compared to the original wavelength.
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