The Universe's early days are illuminated by light. This helps us to pinpoint when the stars turned on after the Big Bang.
It is easy to determine the date of birth if you know someone's age. Astronomers now use the same method to calculate the age of some of our Universe's oldest galaxies. This has allowed them estimate the time of cosmic dawn, the period in which the first stars appeared.
Nicolas Laporte, University of Cambridge, UK and his collaborators examined six galaxies previously identified as potentially old. The team looked at the starlight from the galaxies to determine their age. They also searched for hydrogen signatures that are more prominent than stars in a galaxy's age. It was also found that some galaxies' light had traveled more than 13 billion year before reaching Earth. This means that we are now seeing them in the same time period as when they were created.
These two measurements together suggest that the Universe was 250 million to 350 million years old when these first stars were born.
The James Webb Space Telescope of NASA, due to launch in late 2012, is expected to allow the authors to view galaxies like these in their early stages of formation.