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Earth's atmosphere comprises different layers that include gases, particles, and moisture. When light from a star passes through the atmosphere before reaching a telescope on Earth, it interacts with these elements. These interactions can lead to absorption or scattering of specific wavelengths, changing the observed spectrum. This effect is called atmospheric extinction.

The Hubble Space Telescope is located in low Earth orbit, approximately 340 miles (547 km) above the Earth's surface. At this altitude, it is placed well above most of the Earth's atmosphere. This means that the light from stars does not have to pass through the layers of the atmosphere before being observed, resulting in a more accurate and undisturbed spectrum.

Atmospheric extinction causes some wavelengths of the star's spectrum to be absorbed or scattered by the Earth's atmosphere, altering the observed spectrum as a result. This occurs at specific wavelengths depending on atmospheric composition and can result in the loss of information or the introduction of false signals. Thus, the observed spectrum from a telescope on Earth would differ from the star's true spectrum.

Another factor that causes a difference in the observed spectra is the Doppler effect. The Earth and the Hubble Space Telescope are both moving relative to the stars, albeit at different velocities. This relative motion causes the wavelength of the light from a star to change as the distance between the observer (the telescope) and the star changes. The Doppler shift causes spectral lines to shift either towards longer wavelengths (redshift) or shorter wavelengths (blueshift), depending on the relative motion between the observer and the source of light. In conclusion, the main reasons why the spectrum of light from a star observed by the Hubble Space Telescope is different from the star's spectrum observed by a telescope on Earth are the position of Hubble above Earth's atmosphere, avoiding atmospheric extinction, and the Doppler effect due to relative motion.