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View Full Version : How Do We Calculate A Star's Lifespan? (For an assignment, please help)



Science Turtle
2016-Oct-11, 03:24 AM
I have an assignment to complete for my astronomy class, and the topic I've decided on is 'How did we calculate the lifespan of different stars if we are yet to observe a star live out its full life?' I understand how we calculate a star's lifespan, factoring in mass, luminosity, the elements and amount of elements that the star's comprised of, etc. What I need now is data.
Who worked out how to calculate a star's lifespan, and how did they do it? Do we have any numerical data? Are there any recorded examples of the calculation process itself? What I'm looking for is this: This star has a radius of this and is comprised of this amount of these elements, therefore the star will live for this long before burning out.

Ken G
2016-Oct-11, 01:00 PM
There is not a short answer to your question, you have asked about a large chunk of the history of all of astronomy. A key paper on stellar nucleosynthesis was BBFH, which stands for Burbidge, Burbidge, Fowler, and Hoyle. You will want to google that paper as it was a big advance in how we understand stars theoretically. Observationally, although we cannot sit on one star and watch it for its whole life, what we can do is look at many stars at various stages of their lives. The key parameter is the stellar mass, and stars with the mass of our Sun take about the time that there have been stars in the universe to go through their full evolution. Stars more massive than the Sun evolve faster than that, and very massive stars end their lives as supernovae, which we have observed.

So the game that is played is a bit like if you were an alien visitor to Earth, and all you could do is travel all over and observe humans, and you want to answer "how do humans age?" You wouldn't sit on one baby human and observe it for a century or so, instead you would sample large numbers of people at the same time, and notice you are seeing them at all different ages. It would be easier for humans because we all age approximately the same, but for stars you can get the same effect by binning them by their mass, and assume all stars of the same mass age approximately the same. Then you will notice that higher mass stars show much more evidence of being at advanced stages of evolution, so you conclude they age faster. What helps us a lot is that stars often form in large clusters all the same age, so we see a bunch of stars of different mass that have been around for the same amount of time, and that really shows us how mass affects the rate of evolution. Also, we have binary stars, where the gravity of one star acts on the other in a way that lets us infer the mass of the stars. So this is how we bin based on mass and age, and that's the key to understanding evolution observationally.

publiusr
2016-Oct-15, 06:07 PM
In general, the larger a star is, the shorter its life span.