How old is an average star?

Answer 1

An interesting question and one that is difficult to answer simply, but lets try.

The universe itself is about 13.7 billion years old (the time of the big bang) and

we think the the first stars began to form about 400 million years after this event (and they have been forming and dying continuously ever since).

However, these first stars were massive 30-300 times as massive as our Sun (and made of almost pure hydrogen and a bit of helium) and they lived fast and died young, seeding the universe with the products of their fusion processes (in other words all the elements heavier than lithium). There are none of these first stars left and as we look at their replacements they incorporate more and more heavier elements (produced by the earlier stars) with time - a concept known as "Metalicity".

On the basis of Metalicity, stellar populations are categorized as I, II, and III, with each group having decreasing metal content and increasing age. New stars with high metal content are population I and are forming now while population II stars can be seen in globular clusters while the oldest stars are population III (non of which are now observable as they have all died). Thus the oldest stars currently visible are from the II population.

OK, the above has explained how stars are born and die but the picture is more complicated because after population III, it was possible to form stars of much smaller mass than the first stars indeed, now it is possible to form a star with just 0.075 of the mass of the Sun. The length of time a star lives (how long it can hold itself up against gravity with the nuclear fusion energy going on in its core) depends on its mass, the heavier the star the shorter its life. Really big stars live only a few million years - a star 15 times as massive as the Sun will live 10 million years, while our Sun will live for 10 billion years (it is about 5 billion years old now). However a small, red dwarf star will have a life expectancy of trillions of years (No red dwarf star has eved died so far because the universe itself is not old enough for this to have happened).

Thus red dwarf stars are the most common stars to be found in the universe today and they will be around for the longest period.

This meas that as the universe ages, the age of the "average" star has changed. For population III star epoch, the average age was only a few million years. For the population II times the average age at the time extended to a few 100-1000 million years while in our current population I epoc the average age has exteneded to 10 - 100 billion years and in future when the only stars left will be red dwarf stars (because the rate of new star formation is decreasing as the universe ages), the average age of stars will be trillions of years.

Answer 2

2 million years old is young for a star.