The Hubble Space Telescope is in the news every now and then. How can it photograph galaxies that are millions of light years from Earth?

We can understand that light comes from galaxy and Hubble "catches" it. But does nothing come between the galaxy and Hubble that absorbs or scatters the light over a distance of say 30 million light years? How? Or is there any other concept?

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    $\begingroup$ maybe there is really "nothing" in between? $\endgroup$
    – Shing
    Commented Aug 18, 2015 at 11:15
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    $\begingroup$ possible duplicate of How can we see objects that are so far away? $\endgroup$
    – Kyle Kanos
    Commented Aug 18, 2015 at 13:14
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    $\begingroup$ Let me quote the introduction of the Hitchhiker's Guide to the Galaxy: "Space is big. Really Big, You just won't believe how vastly hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space. Listen..." $\endgroup$
    – Jim
    Commented Aug 18, 2015 at 14:31
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    $\begingroup$ This question is clearly asking about extinction and is not a duplicate to that labelled as a duplicate. The answer to the question about the IGM density is of course related, but also not a duplicate. $\endgroup$
    – ProfRob
    Commented Aug 18, 2015 at 15:03
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    $\begingroup$ @RobJeffries very true. That's why I made mine a comment and upvoted yours. I really just wanted a reason to use the HHGTTG quote $\endgroup$
    – Jim
    Commented Aug 18, 2015 at 15:31

2 Answers 2


The typical extinction for a line of sight out of our Galaxy (but avoiding the Galactic plane) is of order a few tenths of a magnitude at visible wavelengths (it is a factor of 10 less in the infrared and factors of a few more in the UV).

This means that the typical attenutation of a signal arriving at the HST from outside the Galaxy is around say $10^{-0.2/2.5} = 0.83$ i.e. 83% of the light gets here.

Of course if you observe in a direction such that the light path travels in the Galactic plane, then the the extinction is much higher and does stop you seeing extragalactic objects (or even objects on the other side of our Galaxy).

In the intergalactic medium the attenuation is much lower, because the density of material (atoms, molecules etc.) that might absorb or scatter the light is also much lower -- typically $10^{-7}-10^{-6}$ H atoms per cc, compared with perhaps $10$ H atoms per cc in the Galactic interstellar medium (in the Galactic plane).

This in itself is not an explanation for why light isn't absorbed/scattered, since light travelling into the Galaxy might encounter a column density of H atoms of $10^{19}$ to $10^{20}$ cm$^{-2}$, but even at the very low IGM densities, similar columns could be reached over distances of tens or hundreds of millions of light years.

I think the principle reason is that the dust that is mostly responsible for blocking visible light is even less common with respect to gas in the intergalactic medium than it is in the interstellar medium. The dust is produced by stars and supernovae within galaxies, and although some of it does escape into the intergalactic medium, there is very little. Observations of type Ia supernovae at different redshifts suggests that the extinction due to intergalactic dust is less than around 0.1 mag for photons travelling from redshift of 0.5 (about 5 billion light years away) and that the dust-to-gas ratio is only a hundredth of the Galactic value (Inoue & Kamaya 2003).

  • $\begingroup$ If light is absorbed, you don't know what happened to it. The light you see is the light you get. In other sectors of physics, this light doesn't exist. One can just speculate, even if it is probably the good answer. $\endgroup$
    – user46925
    Commented Aug 23, 2015 at 10:20
  • $\begingroup$ "If light is absorbed, you don't know what happened to it". Of course you do. It is either scattered or heats up the material that absorbs it. Either way it has a clear wavelength dependence imprinted on the unabsorbed light, which means that it can be measured and is certainly not speculative. @igael $\endgroup$
    – ProfRob
    Commented Aug 23, 2015 at 10:41
  • $\begingroup$ you are just talking about light you get, directly or after transformation. If you assume many and many hypothesis, then you're right. Else it's just speculations. @robJeffries $\endgroup$
    – user46925
    Commented Aug 23, 2015 at 11:04
  • $\begingroup$ Clearly not an astronomer. $\endgroup$
    – ProfRob
    Commented Dec 27, 2017 at 0:16

Because the light left them long ago. We see distant objects not as they are now but as they were when the light left. So we can study the progress of evolution of stars and galaxies by looking at them at different distances and therefore different times!


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