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Q & A: Seeing the invisible?

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Most recent answer: 10/22/2007
Q:
How can we see a nonluminous object?
- Sarah Michael (age 12)
London
A:
Nice question. There are several ways. The simplest is that the non-luminous object can get in the way of light coming from other things. For example, we can easily see that the moon is round during a solar eclipse, when it blocks light from the sun.

There are other ways too. Non-luminous objects have effects on luminous objects that make the presence of the non-luminous ones obvious. For example, non-luminous black holes often have luminous stars whizzing around them. You can tell from how fast the stars circulate how much gravity the black hole has. For example, the black hole at the center of our galaxy has as much mass as about three million stars like our Sun.

Mike W.

Lee H
okay

(published on 10/22/2007)

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