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Q & A: Seeing far-away stars

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Most recent answer: 07/30/2016
Q:
Tonight I saw a picture we recently took of the M8 Lagoon Nebula which is situated approximately 5,000 light years from our planet. A stunning sight. But I am struggling to comprehend the picture. If it has taken the light from this nebula 5000 years to reach us, and every second this light has been bombarded and deflected from every direction from every other source of light from our universe, how can we really be seeing the actual sight 5000 years later ? Humbly accept I am not scientifically minded and would be grateful for an explanation.
- Steve (age 59)
UK
A:

Hello Steve

Except for regions in and around galaxies, space is pretty much empty.  The average number of protons in one cubic meter is only about one.   In addition the scattering cross section is rather small so a photon can travel for millions of light years without being affected.   Light by light scattering is negligible.   A problem arises if a photon tries to pass through a galaxy.  If it runs edge-on through a spiral disk galaxy  it will likely bump into a "dust" particle and be absorbed or deflected.    If the photon hits the galaxy perpendicularly there is a good chance it will pass through unscathed.   Blue photons have a higher scattering cross section than red ones so the light spectrum that makes it through will be shifted toward the red.   This effect is similar to the sun being reddened at sundown.  

LeeH

 


(published on 07/30/2016)

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