How do you calculate the age of galaxies?

Answer

The questions are, rather, Anderson: How do we know that a galaxy will form in a million years, how do we determine how long ago it is that its light reached us?

The second question is the easiest to answer. How much time it took for that light to reach us, we can estimate from the distance of the galaxy, because we know the speed at which that light comes to us. We can estimate the distance from Hubble’s law, which describes the expansion of the universe: the further a galaxy is, the greater the speed at which the distance between that galaxy and us increases. We get that speed from the redshift, the speed of expansion that we see in that galaxy. Redshift means that typical signals seen here while stationary at specific wavelengths have been systematically shifted to longer (i.e. redder) wavelengths for distant galaxies, a bit like the auditory signal from a distant source sounds lower. And Hubble’s law shows that there is a clear relationship between the degree of redshift and the distance: the greater the distance, the more redshift. That law is derived from sources of which we can determine the distance well, and is very consistent; for example, for a galaxy of which we do not know the distance directly, we can estimate it accurately.

By looking further and further, we see galaxies that are increasingly younger (relative to the age of the cosmos) than ours. So we can determine how long the light they emit was emitted after the creation of the cosmos, so after the big bang. That does not tell us directly how old they are, for that we need to know when they were born. We cannot say that for an individual galaxy. But by looking at many galaxies at different distances, we can hope to develop a global model of evolution. Very deep observations have shown that we do indeed see cosmic evolution, that the furthest – and therefore youngest – galaxies look different from those that have already had a longer evolution. But it also means that galaxies formed very early in the history of the universe.

The picture that emerges is one of galaxies suddenly appearing out of nowhere in a short space of time (1 million years). It started several hundred million after the Big Bang, but getting from the systems of that time to the systems of today took a long time. An evolution in which individual systems have merged into each other. Our system also contains several populations of stars, each of which has its own history. And who are the ‘original population’ and who are the ‘migrants’ is not very significant. What we can conclude from the observations of the oldest stars in our system today is that they formed not so long after the Big Bang. So it makes sense to say that the birth record of the galaxies lesson typically dates from about a billion years after the Big Bang.