Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: yamo on 07/01/2015 06:01:08

Title: Is there a limit to how far we can see?
Post by: yamo on 07/01/2015 06:01:08

Is there a limit to how far from its source light from a star can be seen?  I understand that eventually universal expansion will someday exceed c but is there any other limit?

Title: Re: Is there a limit to how far we can see?
Post by: PmbPhy on 07/01/2015 07:13:05

Quote from: yamo

Is there a limit to how far from its source light from a star can be seen?  I understand that eventually universal expansion will someday exceed c but is there any other limit?

Yes. I'm sure there is. I can't imagine that we can see a star which likes biilions of light years away. Only galaxies can be seen that far away.

Title: Re: Is there a limit to how far we can see?
Post by: CliffordK on 07/01/2015 09:05:05

The perceived intensity of an omnidirectional light source decreases by the inverse of the square of the distance. 

So, say a star is perceived with X intensity at 1 LY away, then:

Distance	Intensity
1 LY	X
2 LY	(1/4)X
10 LY	(1/100)X
1,000 LY	(1/Million)X
100,000 LY (size of Milky Way)	(1/10 Billion)X
10 Billion (1010) LY (about radius of visible universe)	(1/1020)X

Anyway, a star 10 Billion light years away gets mighty dim.  In fact, I'm not sure if individual stars can be discerned by the naked eye outside of our own Milky Way, although the galaxy Andromeda can be seen.

However, one of the most distant objects ever observed was actually the very bright supernova from a single star.

Time and Distance is a little funky in our ever moving Universe, but if we accept that the Big Bang happened about 13.8 billion years ago, that also puts an upper limit on the distance of visible objects, 13.8 billion LY.