The H_α Spectral Line of Hydrogen Has a Wavelength of 656.3

question 28

Multiple Choice

The H_α spectral line of hydrogen has a wavelength of 656.3 nm as measured in the laboratory. The H_α spectral line is observed in light from a distant star at a redshifted wavelength of 789.0 nm. If Hubble's law is valid for this star, estimate how far is it from Earth if the Hubble constant is 22 km/s per million light-years. (c = 3.00 × 10⁸ m/s, 1 ly = 9.461 × 10¹⁵ m, 1 y = 3.156 × 10⁷ s)

• A) 2.5 × 10⁹ ly
• B) 9.1 × 10⁸ ly
• C) 2.7 × 10⁸ ly
• D) 9.1 × 10⁷ ly
• E) 2.7 × 10⁷ ly

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