Download Identifying Old Stars

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,
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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?