2011

When a distant galaxy formed its stars

Round: Theory
Points: 10
Level: Silver
Taught in: Section 13.5

Adapted from IOAA 2011 (Katowice), theory problem 11. We have restated it in our own words and kept the original data. The solution was last checked on 21 October 2026.

The spectrum of a galaxy at redshift $z = 6.03$ shows that its stars are 560 to 600 million years old. At what redshift did the galaxy form its stars? Take the present age of the Universe as $t_{0} = 13.7 \times 10^{9}$ years, and assume a flat universe with no cosmological constant, in which the scale factor grows as $t^{2 / 3}$ .

I solved it without the solution

AnswerThe stars formed between $z = 17.3$ and $z = 20.8$ .

This problem also appears among the olympiad problems of Section 13.5, The age and history of the Universe, which teaches the ideas it uses. Equation and figure numbers refer to the course.