Identifying Old Stars - PowerPoint Presentation

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?