Download After the ZAMS - Lincoln-Sudbury Regional High School

Swinburne Online Education Exploring Stars and the Milky Way
Module
: 8:
Module
Life on the Main Sequence
Activity 2:
From After
© Swinburne University of Technology
the ZAMS
Summary
In this Activity you will learn about
• the life of stars after they reach the ZAMS,
• how the time they spend there depends on their mass,
• whether stars gain or lose mass and why, and
• open clusters and how they have taught us so much
about the life stories of stars.
Our own Sun
Luminosity
Our Sun will spend about 80% of its entire life on the
main sequence.
10 billion years …
What will happen then?
that’s a nice long
middle age ...
You’ll find out in the next
Module!
While on the main
sequence, though, the Sun
ZAMS
will very slowly and quietly
(Zero Age Main Sequence)
keep on turning hydrogen
into helium.
Temperature
Middle Age of a 1-solar-mass star
Our Sun’s part of
the main sequence
Luminosity
Helium is of course more
compact than the four protons
from which it was made.
So during this phase the core
shrinks, which increases its
temperature.
The radiated heat swells the
outer layers: the star grows
slowly larger and its surface
becomes cooler.
Sun moves
a little on
the H-R
ZAMS
Temperature
Watch out!
Many people get the wrong idea about stars and H-R
diagrams, because of the term main sequence.
The sequence is actually a
sequence of mass, e.g. more
massive stars appear on the
upperleft side and low mass
stars on the lower right.
It’s got nothing to do with the
history of any single star.
I reckon stars
move like this...
Luminosity
They think that stars move
up the main sequence, but
that isn’t so.
WRONG!!
Temperature
You’ll shortly see a rough map of
how a star might move on the H-R
diagram.
Its actual development will depend
on its mass, its surroundings and its
history.
During the animation, keep an eye
on the chart below as well.
Luminosity
What really happens
The important thing for you to
remember is that stars don’t move up
and down the main sequence. If
anything, they move across it!
protostar
main sequence star
Temperature
old star
Gaining mass?
Er ...
Do stars gain or lose mass while they are on the main
sequence?
You might think that because they are so incredibly
massive that they would trap any gas or dust in the
vicinity.
Come and have
some turkish delight ...
Losing mass
A nice idea, but it forgets the stellar winds.
Stars are constantly losing a bit of mass because of the
matter-to-energy conversion in their cores.
However the stellar winds are a much more serious
source of mass loss.
These winds commence as soon as a
molecular cloud starts to heat up.
But the effect really becomes important
when fusion starts in the core.
A big problem
AAAARGHHHHH!
This means that anything near a star will be blown away, or eroded.
If it moves anywhere, it will be most likely away from the star, not
towards it.
But far greater than this is the loss of
material from the star’s outer layers: it
is just swept into space by the
pressure of radiation from the core.
A large star can lose up to 60% of its
mass in stellar wind.
All of the really light elements in its
outer atmosphere can be blown away.
You mean I’m
leaking?
Low mass stars
The less mass a star has, the longer it takes to evolve onto the main
sequence and the longer it will spend there.
sigh
protostar
main sequence star
old star
The Universe simply isn’t old enough for many very low mass stars
to have made it onto the main sequence, let alone off it!
This slowness is however of great benefit to us on Earth, with a Sun
which has evolved not terribly fast.
The Big Ones
Very massive stars evolve very quickly, in
stellar terms, and so we can see a lot of
these at different ages and stages.
Remember too about look-back time: the
further away a star is, the longer its light
takes to reach us.
So our cameras may be receiving light (and
therefore images) which left the star
thousands of years ago. A 40 solar mass
O5 star in the nearby galaxy Messier 31 will
have evolved along the Main Sequence in
1/2 the time it takes light to travel to us!
For these reasons, we have a much better
idea of the entire life story of the larger stars
than we do of the smaller ones.
This is Betelgeuse, as seen
recently by the HST.
As Betelgeuse is a red
supergiant, it will probably
explode fairly soon.
But as it is 520 light years
away, we won’t know for
520 years!
Features of a Big One
Live fast,
die young ...
A massive star is characterised during its time on the main
sequence by a few clear markers.
Its time on the main
sequence is very short
The star turns hydrogen
into helium more
through the CNO cycle
than the p-p cycle
There is likely to be carbon
fusion and nucleosythesis of
heavier elements in the core
The stellar wind can be
so very fierce that it
strips the outer layers of
their lighter elements.
There are also profound
differences in the star’s old
age (but they’ll be covered
in a later Module).
A measure of age
While you can guess the age of a human by looking at their skin, hair,
body shape and so on, with a star one of the best leads that you have
is the proportion of hydrogen to helium and other elements.
I’m a B-class,
but I haven’t got
much helium in
my core yet
You’re just a baby!
I’ve been on the main
sequence so long
my core’s just
full of it ...
A bit about Open Clusters
You’ll be studying globular clusters in a later Module, but
we are now going to have a look at open clusters as they
can tell us so much about life on the main sequence.
A cluster is open if
its stars are pretty
thinly scattered:
usually 100 to
1000 stars in a
space of a few tens
of light years.
This is the Jewel
Box cluster.
N330
Here is N330,
a young cluster
in the Lesser
Magellanic
Clouds.
Lovely, isn’t it?
Let’s look at it
a bit more
closely.
Blue = lots
of UV
(very hot)
Yellow-white = A-type
supergiants
Colour
Orange = red
supergiants
To tell the truth,
the colours in
this photo aren’t
realistic.
They have been
enhanced to
indicate each
star’s strongest
radiation.
Red = H
(Balmer series)
Pink = blue stars
with lots of H
All in the family
Here’s a family like many
human families: you can
see the resemblances.
Looking at the parents
allows you to predict
what might happen as
the children grow up.
It’s pretty likely that as
they get older both kids
will get darker hair.
It is probably less likely that
the boy will wear lipstick
and the girl will grow a moustache,
but you never know ....
Stellar families
The stars in open clusters are usually much like our
own Sun: younger stars that are relatively rich in
heavier elements being recycled from older, exploded
stars.
The larger the star, the faster it will evolve. So a photo
of an open cluster is like a family photograph. It will let
us predict to some extent where a star is headed by
looking at its faster-developing relatives.
Astronomers have learned a great deal about stars in
general by studying such clusters, but we have to make
a few assumptions.
1. Ageist assumptions
We have to assume that all the
stars in the cluster are of about
the same age.
If we didn’t do that, we’d be unable
to say as definitely as we do that
larger stars evolve faster.
Having seen clusters forming in
molecular clouds, though, supports
the idea that stars in a cluster were
“born” at about the same time.
This is the Lagoon Nebula, an
active region in which clusters of
stars and protostars abound.
2. Compositionist assumptions
We have to assume that all the stars in the cluster are
made of much the same kind of stuff: that they
formed from the same molecular cloud, and it was wellmixed.
other
If we didn’t make this assumption,
we’d be unable to predict what will
happen to one star in the cluster
by looking at others in the same
cluster that are further along in
their development.
He
26%
2%
Composition
of our Sun
H
72%
four
3. Groupist
three
assumptions two
hup
Third, we have to assume that all the stars
fourin the cluster
are staying with the group and moving roughly in the
three
same direction.
The stars within the group may move around a bit, but
the group as a whole heads in a common direction.two
hup
four
Within the group
three
two
hup
Click to see that animation again and watch
four just one of
the stars: you’ll notice that the stars within this particular
three
group are circling each other.
If we record the proper motion of the stars over a period
of
two
time, we can work out both the direction that the cluster is
heading, and the motion of the stars within the cluster.
hup
What clusters tell us
If you make those three assumptions, you
can really get down to business.
It is like having an entire family photo album
so that you can study and predict the
changes in a standard member over time.
A sort of time-line
In this family, the hair apparently gets darker as middle-age approaches,
then goes gray, then white.
At about the same time, the top lip disappears, sags appear under the
eyes and the cheeks start to drop.
All through life the nose, ears and chin get bigger (but that’s true of all
humans).
6 months
7 years
28 years
51 years
74 years
Stretching the truth
The trouble with these nifty analogies is that sooner or later they fall
apart. In a human family, people are born at different times...
but (usually) age at about the same rate.
Born 1925
Born 1948
Life expectancy
about 80 years
Life expectancy
about 80 years
Born 1971
Life expectancy
about 80 years
Born 1992
Life expectancy
about 80 years
Born 1999
Life expectancy
about 80 years
A cluster, on the other hand …
In a stellar family, a cluster, the stars do the reverse.
They are born at the same time ...
Time on main sequence
(millions of years)
Born 1642
Born 1642
2000,000
Born 1642
15,000
Born 1642
Born 1642
3
500
3,000
… but they age at very different rates:
the bigger, the faster.
(We chose 1642 for this example
because that’s when
Isaac Newton was born)
Star family time-lines
Astronomers treat photos of clusters as if they were family photos, and
use them to look for changes in particular types of stars with time.
However, human family members have their own peculiarities (like
wearing lipstick or moustaches), and so do the stars in a cluster.
They are of different
masses (and therefore at
different stages), for a start.
They are subject to different
stellar winds from
neighbouring stars, different
gravitational pulls, and
different accidents (such as
running into a nebula).
But we can still see a lot of
patterns.
The Pleiades
Spotty bit
= Pleiades
luminosity
This is the Pleiades cluster:
a well-known, bright and
beautiful group of stars in
the constellation Taurus.
If you plot an H-R diagram
for the Pleiades cluster,
you can see that different
stars are at different
stages of their evolution
… because they are of
different mass.
Red line
= ZAMS
temperature
The Pleiades H-R
At the bottom, there are the small, newly-formed stars and protostars:
cool and not very luminous, and very slow to reach the ZAMS.
great huge
bloated stars
that have
already left
the ZAMS
Once a star joins the ZAMS, it
usually becomes a bit brighter and
cooler, so the middle-sized stars drift
up and right of the ZAMS.
The very large stars, with the highest
temperature and luminosity, are so
mature that they are leaving, or have
already left, the ZAMS.
(The next Module looks at those.)
PLEASE NOTE: all of the stars in
this diagram are the same age!
middle-sized
stars
small stars
protostars
Summary
This Activity has shown you how stars of different masses
continue to evolve very slowly after joining the Zero Age
Main Sequence.
The evolution of the star will be controlled mostly by its
mass.
Open clusters give us a sort of “family album” that lets us
see stars of the same age and type (but different mass) at
different stages of their evolution.
Image Credits
MSSSO: Michael Bessell © (used with permission)
The Jewel Box cluster (NGC 4755), containing Kappa Crucis
N330
Lagoon Nebula
RCW 38:
http://antwrp.gsfc.nasa.gov/apod/image/9812/RCW38_vlt_big.jpg
AAO: Pleiades cluster © David Malin (used with permission)
http://www.aao.gov.au/local/www/dfm/image/uks018.jpg
Now return to the Module home page, and read
more about stars on the main sequence in the
Textbook Readings.
Hit the Esc key (escape)
to return to the Module 8 Home Page