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Identifying Old Stars An Important Semantic Point In biology, individuals do not evolve, but a species can. Evolution depends on how traits are inherited from generation to generation through reproduction, and because differential survival can be affected by genetic changes. But in astronomy, we use the words “stellar evolution” to refer to how single stars change as they get older. “Stellar development” or “stellar aging” would be a better term, but we are stuck with the usage. (We will see later, by the way, that there can be important changes that accumulate as ‘generations’ of stars come and go. In particular, this leads to an enrichment in the heavy element content.) Our Contention: As sun-like stars run out of Hydrogen in the core, they swell up into red giants. Where is the observational evidence? How can we test the veracity of this statement? The ‘Obvious’ Way Let’s simply watch a star of one solar mass and see how it changes as time passes. This is clearly impossible – the pace of change is simply far too slow. The ‘Next Most Obvious’ Way Identify a number of stars that are the same mass as the sun, but focus on those that are rather old – say, 8, 9 or 10 billion years. See how their properties compare to the current sun, to determine if they are indeed starting to look like red giants as they age. This sounds better, but there is a problem: How will we recognize ‘older’ stars? For People, Aging is Made Manifest Seniors develop grey hair, lines on the face, etc. Stars Don’t Do That! Stars last for very long times, but they change very little externally as that happens. So a “late-middle-aged” star looks very much like it did when newly-formed. The outer layers tell us essentially nothing about the interior structure and the depletion of fuel resources. (The Solar System is special. We can determine the ages of meteors, the moon, and the Earth; and helioseismology also helps us determine the Sun’s age.) So?? Other than in the computer, how do we work out how stars evolve (i.e. change in their appearance and structure as they age)? What observations will help? A (Helpful?) Thought Experiment Imagine a visiting extraterrestrial (ET), here on Earth for just a day, who for some reason has developed the strange hypothesis that human males age in appearance much more dramatically than females. That is, hair gets greyer skin gets wrinkled muscles get flabby mind starts to fade much more quickly for men than for women. ET Would Like to Test This But ET can’t watch it gradually happen (no time!) And suppose further that he has no direct way of determining the actual ages of individuals! (No one will tell him.) How does ET test this strange proposition? A Productive Approach: Consider Fraternal Twins! Examples Since they are twins, ET can be sure that they are the same age. Problem (Partly) Solved ET may not know their individual ages, but within each pair, he knows that they are the same age! So he can test to see if, in general, the male is flabbier, greyer and more lined than than the female. He can then test whether any such difference depends on anything else that may be related to or indicative of age. Back to Astronomy… Suppose you could create a whole family of stars, all at the same time – some of them very massive, others quite lightweight. Within such a family, the stars would use up their fuel at different rates. They all would be the same age, but present different faces to the world in a way that depends on age. Are there such families of stars?