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Why are objects in space visible if they do not emit their own light?

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When the wavelength of a spectral line emitted from an object increases which end of the visible light spectrum does not move toward and what is the objects motion relative to earth?

Increasing wavelength is an indication of a Doppler shift caused by an object moving away from the viewer. Longer wavelengths (of the visible spectrum) are redder, shorter wav

How do scientist know that black holes exist if these objects don't emit visible light?

Black holes do not, themselves, emit light. But the area around them does. Not light as we think of it, but highly energetic X-rays caused by the mass AROUND the black hole as

What must an object do to be seen if it does not emit light of its own?

It must either reflect light, as in the case of the Moon under ordinary circumstances, or block light, as in the case of the Moon during a solar eclipse. Of course in everyd