What Is a Black Hole?

   What Is a Black Hole?  A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying. Because no...

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Does light have mass?

Does light have mass?   The short answer is "no", but it is a qualified "no" because there are odd ways of interpreting the question which could justify the answer "yes". Light is composed of photons, so we could ask if the photon has mass.  The answer is...

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Why is the sky blue?

Why is the sky blue? A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light.  When we look towards the sun at sunset, we see red and orange colours because the blue light has been scattered...

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Dark Energy, Dark Matter

  Dark Energy, Dark Matter In the early 1990's, one thing was fairly certain about the expansion of the Universe. It might have enough energy density to stop its expansion and recollapse, it might have so little energy density that it would never stop expanding,...

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