Asked by Danielle, Melbourne
The stars in our Universe are so far away that it can take years for their light to reach us here on Earth.
We get light years as a measure of distance from this: one light year is the distance light travels through space in one Earth year. So, if a star is born one light year away, we’ll only be able to see its birth a year later.
This is why new telescopes, on the ground and in space, are always straining to see further into the Universe: the further away you look, the further back in time you can see, giving tantalising hints as to what the early Universe looked like.
Back to Danielle’s question: the answer, then, is that you can see any star of any age: it just depends where you are.
We can see 10-year-old stars when the time’s right, but the light takes so long to travel to us that when we see a 10-year-old star, the star itself could now be 20 years old. To see a star before it reaches the ripe old age of 10, it has to be less than 10 light years away.
As a comparison, our Sun is just 0.000015 light years away, but after that the nearest star to us – Proxima Centauri – is 4.2 light years away, so when we look at it through a telescope, what we’re actually seeing is what Proxima Centauri looked like 4.2 years ago.
There are 12 stars (including our Sun) that are less than 10 light years away from us, so we would have been able to see them before they reached the age of 10.
[Originally posted over on Ask a Scientist | References: http://www.astro.wisc.edu/~dolan/constellations/extra/nearest.html]