Identifying Old Stars

Here’s What I Claimed As sun-like stars run out of Hydrogen in the core, they swell up into red giants. Question: do you believe me? You should say: give me evidence!

First, An Apology 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. Sorry about that.

Our Objective We’d like to identify stars which are nearing the ends of their lives, to test our theories of how they will evolve (i.e. change) as they use up their fuel. We’d like to identify stars which are nearing the ends of their lives, to test our theories of how they will evolve (i.e. change) as they use up their fuel. For instance, we claim that sun-like stars will turn into red giants. If so, then maybe all the red giants are very old (say, ten billion years old). How will we recognize ‘older’ stars?

People: Aging Made Manifest We see gradual, continuous change.

Stars Don’t Do That! Stars last for very long times, but they change very little externally as that happens. So a “middle-aged” star looks very much like it did when newly-formed. The outer layers tell us essentially nothing about the interior structure. How can we hope to progress?

One Fruitless Approach There is no point in monitoring individual stars, even for many centuries, to see how they change.

Stars are Too Long-Lived! Look at these representative main- sequence lifetimes Even a huge short-lived star (100 solar masses) lasts a few million years, and will not change a bit on human timescales.

Sometimes Things Happen Fast! At some stages, like a supernova explosion, things happen very quickly! [But that’s related to stellar death, not the slow steady change of stellar aging and maturity.]

So: The Fundamental Question How will we recognize a star that’s been around for a long time? What outer face will an old star present to the world?

By My Earlier Arguments.. We might expect stars like the sun to shrink, becoming very compact, hot objects, converting He into other elements in their cores. Do we see such stars in the sky? Is that perhaps what the white dwarfs are? Or should we be looking for something else?

Red Giants I already told you the answer: the outer parts will expand even though the core contracts. But how did we learn that? Surely it’s not just because “the equations said so…!”

This is a Challenge It’s not by watching! We have never yet observed a single star turn into a red giant. And it is essentially impossible to determine the precise age of a single star in empty space!

So?? How do we work out how stars evolve (i.e. change in their appearance and structure as they age)?

Over to ET Suppose ET was given a wide-ranging glimpse of life on Earth -- but a static one, like a set of still photographs. How much would ET learn about human birth, life, growth, and death?

A Thought Experiment Let us suppose that ET hypothesized that human males grow older (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.

What Can ET Do? ET has no direct way of determining the ages of individuals! How does ET test this proposition? [Let’s brainstorm this a bit…]

Answer: Study Fraternal Twins!

Examples Examples

Problem (Partly) Solved You may not know their individual ages, but within each pair you know that they are the same age! So you can test to see if the male is flabbier and greyer than the female, and see whether any such difference depends on anything else that may be related to age.

So: Forget Individuals! Extend This Reasoning to Families ET could look at family settings to see couples, parents, siblings, etc in different stages of development relative to one another. This would lead to a general understanding about human aging, etc.

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.

An Important Reminder

‘Metabolic Rates’ Unlike living creatures: the more massive stars live short, showy lives, while their tiny brethren last a long time Can we use this to advantage?

An Older ‘Family’ of Stars In an older stellar family, the most massive stars will have used up their fuel, and will have changed in some way!! But the low-mass stars will still be unchanged, and still on the main sequence! A perfect place to test theories of stellar evolution!

The Analogy Completed Let us identify a collection of stars (a star cluster) that are of identical (but perhaps unknown) age, and of varied masses. Now look for differences between the high- mass and low-mass stars, in a way that hints at evolutionary behaviour.

The Pleiades [these stars are all the same age!]

A “Globular Cluster” [these all have the same [different!] age]

Nature Gives Us This: the Star Clusters! You can’t watch a single cluster evolve (or even the individual stars within it) but you can still intercompare stars within clusters, and eventually identify clusters of different ages. This provides snapshots of stellar evolution.