# How are the distances to the most distant $\gamma$-ray bursts measured?

This question came to my mind when I read on NASA's website that an explosion (gamma ray burst, GRB 080913) took place 12.8 billion light years away from us. How do they measure such large distances?

• Possible duplicates: physics.stackexchange.com/q/24927/2451 and links therein. Jul 18, 2014 at 18:57
• possible duplicate of How do you measure distance to stars within the galaxy? Jul 18, 2014 at 20:37
• Voting to leave open: certainly not a duplicate of "distances to stars within the galaxy" as the techniques required for this much larger distance are completely different. As to the other possible dup, this measurement uses a technique not mentioned in the answers to that other more general question. Will edit this question a bit to make the distinction more clear. Jul 18, 2014 at 22:01
• – user10851
Jul 18, 2014 at 22:41
• @Kyle but what about the one I linked? Also, if the OP didn't just want GRB's, there's physics.stackexchange.com/questions/70400/… for radio.
– user10851
Jul 18, 2014 at 22:42

Did you mean 12.8 billion light years away?

If so, in this case the distance was estimated by measuring a rough spectrum for the GRB. The NASA article I've linked says:

In certain colors, the brightness of a distant object shows a characteristic drop caused by intervening gas clouds. The farther away the object is, the longer the wavelength where this fade-out begins.

This technique is used where it's hard to get a detailed spectrum and calculate the red shift.

If you're asking a more general question then this is covered in the question How is distance measured to far away stars and galaxies? and your question would be a duplicate.

• So, in essence, this is a rather fuzzy redshift calculation? Jul 18, 2014 at 13:18
• @M.Herzkamp: yes, exactly. Jul 18, 2014 at 14:00
• @JohnRennie: yes sir i meant 12.8 billion light years. sorry for omitting billion there. Jul 18, 2014 at 18:00
• I think that characteristic drop would be the beginning of the "Lyman-$\alpha$ forest". So really this is a lower limit on the distance (which, at 12.8 Gly, still tells you it's pretty darn far away). Jul 18, 2014 at 18:24

For relatively close objects, the distance can be measured through measurements of parallax (http://en.wikipedia.org/wiki/Parallax ).

• And also using Variable stars in a star cluster. Jul 18, 2014 at 18:09
• Note this was a reasonable answer to the question as stated at the time of the answer (I almost downvoted until I noticed). Jul 18, 2014 at 22:13