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Swinburne Online Education Exploring Stars and the Milky Way
Module
: 7:
Module
Evolving onto the Main
Sequence
Activity 2:
From Joining
© Swinburne University of Technology
the Party
Summary
In this Activity we will examine how a star evolves from
a cloud of cool gas into a hydrogen-burning star.
To begin the Activity, however, we will take a quick look at
how astronomers observe star-forming regions.
We will then follow a star’s evolution on the H-R diagram.
How to observe star formation?
In the Activity Starbirth we saw that star formation involves the
collapse of a cool molecular cloud.
In the final stages of this collapse, the cloud’s core becomes
sufficiently hot and the gas is under enough pressure for fusion
reactions to begin: this is a young stellar object (YSO), a baby
star.
Obviously, astronomers are very interested in the stages of the
cloud collapse that occur before the star is born. The
protostellar cloud is not very luminous, however, since it is not
generating energy by fusion reactions. So our first question is
this: what kind of observations can astronomers make to
observe the early stages of the star formation process?
Celestial Microwaves
In some cases, microwaves are emitted by protostellar
clouds in an intense, narrow, single-frequency beam.
How in space can this happen? Is there an extraterrestrial
out there with a nebula-sized malfunctioning oven or
remote control?
10-14 10-12 10-10 10-8 10-6 10-4 10-2
Wavelength of photon (metres)
1 102 104 106 108 1010
Radio
Microwave
Infra-red
Visible light
Ultra-violet
X-rays
Gamma
rays
2x10-12 10-14 10-16 10-18 10-18 10-20 10-22 10-24 10-26 10-28 10-30 10-32 10-34
Energy of photon (J)
Lasers, Masers
It’s not my fault!
It’s these idiots
beside me
Humans are able to make
intense, narrow single-frequency
beams of visible light, called
lasers.This stands for Light
Amplification by Stimulated
Emission of Radiation.
The microwave version is called
a maser, and it is probably not a
sign of extraterrestrial
intelligence. It happens naturally.
Hey, why the microwaves
all of a sudden?
molecular
cloud
Click here to learn more
about MASERs
Masers detected in our Galaxy typically extend over
only a few thousands of an arcsec.
Sources of maser emission are:
• newly-forming stars and
• old stars losing mass.
In each case it seems that the maser emission comes
from extremely dense clouds or disks of material
containing the right molecules. These clouds surround
sources of excitation (such as bright young stars,
shockwave energy from exploding stars, or radiation from
centre of an active galaxy) which provide the radiation to
trigger the maser emission.
Real-life masers
The photo shows part of the Orion
Nebula, where since 1963 hydroxyl
(OH) masers have been detected in the
hot, thick gas surrounding the bright
central area of new stars.
The location of the first known maser is
marked by the yellow dot.
We’ll see more observations of star
forming regions and young stars
throughout this Activity. For now, let’s take
up the story of protostellar formation using
the H-R diagram...
Deep in a cloud
Question: Where do you
think such a molecular
cloud would be on this
H-R diagram? Near A?
B? C?
Answer: None of the above.
Luminosity (compared to the Sun)
We left our study of the birth
of stars (in the Activity
Starbirth) at the time when
the star was just becoming
visible through the remains
of the molecular cloud.
A
Sun
1
10-2
C
10-4
B
28,000
6,000
3,500
Surface temperature (K)
The contracting protostar is initially very cold
and faint, deep within a molecular cloud.
Because it is cold, it is way over to the right of
the H-R diagram.
Luminosity
Because it is faint, it is near the bottom of the
luminosity scale.
young
protostar
Temperature
Luminosity
Gravity, gravity
The cloud then contracts under its own gravity, but
becomes red-hot in the process and begins to glow.
Question: Can you work out what this change should look
like on the H-R diagram?
young
protostar
Temperature
Answer: the protostar moves up and to the left as
it becomes brighter and hotter.
Luminosity
hotter
brighter
young
protostar
Temperature
Smaller and …?
The protostar in its cocoon of
gas and dust continues to
collapse under gravity. It
becomes smaller, and
therefore hotter.
Question: Will the luminosity of
the protostar increase, or
decrease? Luminosity depends
on both the size of an object and
its temperature...
Hotter
= brighter?
Smaller
= dimmer?
Luminosity
Answer: It turns out that although the protostar does get
hotter, its size is significantly reduced.
So the protostar actually becomes a bit dimmer during
this contraction.
Hotter
and dimmer
young
protostar
Temperature
T-Tauri stars
How do we know all this?
The evidence is the existence of
T-Tauri stars: stars of 0.2 to 2
solar masses embedded in small
dark molecular clouds.
These stars have not yet quite
reached the main sequence, but
they are getting there.
At least, we suspect so: these
small stars evolve so slowly that
we won’t be around long enough
to actually see them arrive!
This is the cluster RCW38 in the constellation
Vela. Images at visual wavelengths show only
a murky molecular cloud, but this photograph,
taken by sensing infra-red radiation, reveals a
multitude of young stars and protostars.
Luminosity
The contraction continues, with an increase in the core
temperature of the protostar.
Suddenly one day the density and temperature of hydrogen
nuclei in the core are enough to overcome their electrostatic
repulsion, and ...
Hydrogen
fusion!
young
protostar
Temperature
ZAMS
The protostar is no longer a protostar, but now is a real hydrogenburning star. It has joined the ZAMS: that is, the Zero Age Main
Sequence.
As for the time all this took:
it varies according to how large
the star is. What do you think?
Would nine months be enough?
“Nine months”?
More like
googoo
MILLIONS
ofbaba
years!
Joining the ZAMS
It took our own Sun 30,000,000 years to reach the main sequence!
Mass of star
Luminosity
Here you can see
tracks of how
different-sized stars
reach the main
sequence, according
to how their mass
compares to that of
our Sun (Ms).
The time estimates
are in millions of
years.
15 MS
0.16
0.7
8
5 MS
2 MS
1 MS
0.5 MS
30
Time to reach
main sequence
100
Temperature
Larger goes faster
This is because the
first thing that
happens is the
contraction of a
cloud due to gravity.
The more massive
the cloud, the
stronger the
gravitational force
and the quicker the
cloud will contract.
Millions of years
Huge, massive stars form very quickly, while little stars take a
long time to form.
100
90
80
70
60
50
40
30
20
10
0
snore
Dum de dum...
Zip!
0.5
1
2
5
Mass (in solar masses)
15
Membership is exclusive
Luminosity
No way, Tiny Tot!
A star isn’t allowed into the
main sequence club if its
mass is too high or too low:
these stars have another fate
in store. Let’s start by
considering stars with
insufficient mass to reach the
main sequence.
mutter grumble
Temperature
Electrostatic loathing
Yeuchhh! Your
Wild
Stop
horses
mucking
couldn’t
about.
charge is positive!
drag
Get in
methere
in there
and ...
fuse!
As you learned earlier, the core of a
protostar is a seething mass of incredibly hot
hydrogen in an ionised state. In other words,
it consists of bare protons.
These protons have the same charge. So
they loathe each other, and the closer they
get the worse it gets.
Fusion zone
However, for fusion to start the protons have
to be very close indeed.
So unless there is sufficient mass in the
protostar for gravity to overcome this
repulsion, fusion can never begin.
Eeeeek!
Gravity
could …
So there
is yours!
but
just
ain’t enough
From gravity to P&T to fusion
Yeuchhh!
We’ll fill in a bit of detail here because
it’s not quite that simple.
You start off with a large cool cloud
(which we’ll call a protostellar cloud),
and gravity collapses it into a smaller,
hotter cloud.
In a larger, cooler cloud, the protons are
moving more slowly and tend to be further
apart. That is, the gas is at a low
temperature and a low pressure.
The protons have enough time, and space,
to avoid each other.
Eeeeek!
Not quite there
In protostars with about
0.08 M, the core
never reaches a high
enough temperature to
trigger fusion.
The only energy source of
these stars is gravitational
contraction of the outer
stellar layers. They are
called brown dwarf stars.
Gliese 623a
Gliese 623b
a brown dwarf
Little Gliese B
This little brown dwarf star is about the same size as
Jupiter but has about 20 times the mass. It orbits Gliese
A at 40 AU.
Our Sun
Jupiter
5.2 AU
Gliese B
Not to scale
Gliese A
40 AU
40 times heavier
than Jupiter
Not so hot
Hello
there
YOW!
We can tell Gliese B isn’t a normal
star because it has methane
absorption lines in its spectrum.
Methane (CH4) is pretty stable on Earth, but
not inside a star, even a cool red dwarf. At
temperatures higher than 2 500 Kelvin, all
the methane molecules would be destroyed.
So Gliese B can’t be star; it must be just
a warm ball of gas.
Rare birds?
When we examine the skies we don’t find many brown dwarfs.
While it is true that their low luminosity means they are hard to
detect, it is unclear whether brown dwarfs are really
uncommon or whether our observations are just not powerful
enough.
As you will see much later
in the Unit - in the Activity
Dark Matter - an accurate
count of brown dwarfs is
extremely important for
resolving the mystery of
missing mass in the
Universe.
A Palomar and an HST image of another
brown dwarf candidate, Gliese 229B.
Monster stars
What about stars that are
too massive to ever reach
the main sequence?
Aaaargh!
Too bright!
Luminosity
Let’s leave the protostars
that are too light to join the
main sequence and look
at the other possibility.
heavy sigh...
Temperature
Pressure can win
Stars with very high mass have extremely
high temperatures & pressures in their
cores.
When the mass is greater than
about 100 times that of our Sun,
the core temperature is so great
that the radiation pressure due to
fusion reactions can be stronger
than the gravitational forces.
pressure
gravity
Pardon me!
When that happens, the star
will belch out a plume of hot
gas, like a solar prominence
only much, much larger and
more violent.
Ooops
GO TO YOUR ROOM!
Untidy stars
Stars with these extremely high
masses are therefore not stable.
They can spread huge gusts of
hot matter through surrounding
space. Because the matter is so
hot, it glows and we can see it.
You wouldn’t want to live near
one!
CLEAN UP THAT MESS!
What mess?
Blast it!
A good example is the star
Eta Carinae in the Keyhole
Nebula. Buried in this vast
cloud of hot gas and dust
is a star of about 100 solar
masses.
About 100 years ago, pressure won for a short time and
the star expelled several huge gusts of gas and dust.
These explosions are likely to keep on happening, but we
can’t predict when.
A closer view
Here is another picture
of Eta Carinae. It was
taken by the Hubble
Space Telescope, using
higher resolution which
only imaged the very
centre of the nebula,
around the star itself.
It shows two clear lobes of expelled gas, a lot of dark
dust and some mysterious streaks and lumps.
Quick overview
Let’s finish this Activity with a review of what you have learned:
Luminosity
If it is too massive
it becomes unstable
If it has enough mass
fusion begins and
it reaches the ZAMS
… but also
smaller
It becomes hotter
and brighter
The large, cool, dark
molecular cloud
begins to contract
If it’s not massive
enough it won’t
be a star at all
Temperature
Summary
This Activity has shown you how a star evolves from a
molecular cloud until it joins the Zero Age Main
Sequence.
You have also learned what happens when a molecular
cloud has far too much mass, or not enough mass, to
end up as a star on the Main Sequence.
In the next Activity we will follow a main-sequence star
through its youth and middle age.
Image Credits
MSSSO © M. Bessell (used with permission)
Large Magellanic Clouds,
Eta Carinae
AAO © D. Malin (used with permission)
Star trails in the Southern Cross
Gliese 623B
http://antwrp.gsfc.nasa.gov/apod/image/9911/gl623_hst_big.jpg
Gliese 229B
http://antwrp.gsfc.nasa.gov/apod/image/gl229b_hst.gif
Eta Carinae
http://antwrp.gsfc.nasa.gov/apod/image/etacarinae_hst2.gif
Hit the Esc key (escape)
to return to the Module 7 Home Page
Stimulated emission
When atoms or molecules get
into excited states, they don’t
always decay back to the ground
state quickly.
Hey, you in the
metastable state!
Wake up!
Aren’t you supposed
to be emitting a
photon like me?
For some
transitions, they can wait
quite a while in what is
called a “metastable state”
to decay naturally, or until
another photon (with the
same energy as the one
they should be emitting)
reminds them of what they
should be doing.
Oh, yeah!
Thanks for
reminding me!
This is called stimulated emission.
The maser process
As far as we understand it (or think we do), there are a
few steps in this process and it differs according to the
particular molecule you are “stimulating”. In molecular
clouds, such molecules include silicon oxide (SiO),
hydroxyl (OH) and water molecules (H2O) .
If a molecular cloud releases maser radiation due to
water molecules, it is called a water (H2O) maser, and
that’s the example we’ll study.
2. Add a newly-forming
nice and hot OB star
(or group of stars)
in a cocoon of dust
4. Let the water be
irradiated with some of
the infrared photons
1. Take a molecular cloud
containing water molecules
3. Let the dust
absorb most of
the gamma rays
from the hot young
stars and reradiate
infrared radiation
The rest of the recipe
The water molecule has one of these
“sleepy” metastable energy levels.
When the molecular cloud is bathed in
infrared photons from the nearby dustenshrouded young stars, an unusually
large number of water molecules can
end up in the metastable energy levels.
It does this by absorbing an infrared
photon to move up to an excited state,
then releases a photon and drops to the
metastable state, where it would
normally stay for an extended period of
time.
5. The water molecule is excited
by an infrared photon into a
higher energy level
6. The molecule decays
to a meta-stable state
excited state
meta-stable
state
infrared
ground state
The way that the infrared
photons aid in populating the
meta-stable state in the water
molecules is called pumping.
Usually most molecules would
be in the ground state, with
only a few in the metastable
excited state.
However in these circumstances
an unusually large proportion of
the population of molecules is in
the metastable state - what we
call a “population inversion”.
5. The water molecule is excited
by an infrared photon into a
higher energy level
6. The molecule decays
to a meta-stable state
excited state
meta-stable
state
infrared
ground state
The thermal distribution of photons
coming from the dust surrounding
the young stars will peak in the
infrared, but will include higher and
lower energy photons, including some
in the microwave region.
Molecules in the metastable state
can be stimulated to drop to the
ground state early, emitting a
microwave photon, if a photon of
exactly the right energy comes
past and reminds the molecule to
emit an identical photon.
5. The water molecule is excited
by an infrared photon into a
higher energy level
6. The molecule decays
to a meta-stable state
7. … until a “reminder”
microwave comes past
Get moving! Release
that microwave!
excited state
meta-stable
state
microwave
ground state
This is essentially the same
process of stimulated
emission that makes lasers
work, except there it is visible
light photons
that are emitted, not
microwave photons.
5. The water molecule is excited
by an infrared photon into a
higher energy level
6. The molecule decays
to a meta-stable state
7. … until a “reminder”
microwave comes past
excited state
meta-stable
state
microwave
ground state
Copy-cats
So instead of the one incoming photon,
you now have two, which trigger
another two, and so on, all through the
cloud. This can amplify the relevant
molecular emission line by
factors of up to 10 billion (1010).
These molecules are wimps,
with no initiative. All of the
outgoing microwave photons
have the same wavelength
(or frequency), and are headed
in the same direction as the
original trigger microwave.
Repeated
all through
cloud
Heaps of
microwaves
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