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 ID: 576059
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Slide1
Identifying Old Stars
Slide2
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.) Slide3
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?Slide4
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.Slide5
The ‘Next Most Obvious’ Way
I
dentify 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?Slide6
For People, Aging is Made
Manifest
Seniors develop grey hair, lines on the face, etc.Slide7
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.)Slide8
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?Slide9
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.Slide10
ET Would
Like to Test This
But ET can’t watch it gradually happen (no time!)
And suppose further t
hat he has no
direct way of determining the
actual ages
of individuals
! (No one will tell him.)
How does
ET test this strange proposition? Slide11
A Productive Approach:
Consider
Fraternal
Twins
! Slide12
Examples
Since they are
twins,
ET can be sure that
they are the
same age
.Slide13
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.Slide14
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?