The age of the sun and the earth are known, but I've never heard anyone specify the age of the galaxy - the Milky Way?
If it is known, how do they know?
what a splendid series of podcasts you guys are producing,
Tamela - We do know the age of the sun and the Earth. It’s about 4.5 billion years old. The way we start looking at how old our galaxy is, is by looking at the contents of it, so looking at the age of the stars that it contains. If we look at the stars in our Milky Way, we find that some of them are actually about 13 billion years old. So, really dating back to the very, very early beginning of the universe. Possibly, there's even an older generation of stars which have already died out, but we can look at the abundance of heavier elements within our galaxy. These are only formed in supernova explosions – so, the death of stars or through cosmic ray interactions. This gives us an idea, sort of like carbon dating– how old our galaxy must be. Some estimates put about 13.2 billion years old. So, that gives you a rough answer, but if you're talking about the actual shape of the spiral galaxy itself, that's probably much younger. So maybe 10 billion years or so, for it to actually have built up into the structure that we recognise today.
Chris - Still pretty old though, isn’t it, when you think that the universe as a whole is only 13.8 or so billions years old.
Tamela - It’s extremely old, yeah.
Chris - So Milky Way is quite an old thing.
Richard - Well, that's why I was wondering. Does that mean that pretty much all the galaxies came fully formed very quickly after the big bang?
Tamela - Quick in the timescale of the whole universe I suppose, but they did build up from – initially, they were just small perturbations, the local areas in the universe that were denser then others, they started to form stars. The stars gravitationally bound together into globular clusters and stuff. We believe that there was this hierarchy building up into the galaxies that we recognise are spirals and elliptical galaxies.