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Module :
Activity 2:
From Joining the Party
Module 7: Evolving onto the Main
Sequence
Swinburne Online Education Exploring Stars and the Milky Way
© Swinburne University of Technology
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.
In the Activity Starbirth we saw that star formation involves the collapse of a cool molecular cloud.
How to observe star formation?
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?
In some cases, microwaves are emitted by protostellar clouds in an intense, narrow, single-frequency beam.
2x10-12
10-1010-1210-14 10-8 10-6 10-4 10-2 1 102 104 106 108 1010
10-1610-14 10-18 10-18 10-20 10-22 10-24 10-26 10-28 10-30 10-32 10-34
Visible
light
Radio
Microw
ave
Infra-red
Ultra
-violet
X-ra
ys
Gam
ma
rays
Wavelength of photon (metres)
Energy of photon (J)
How in space can this happen? Is there an extraterrestrial out there with a nebula-sized malfunctioning oven or remote control?
Celestial Microwaves
Lasers, MasersHumans 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 microwavesall of a sudden?
Hey, why the microwavesall of a sudden?
It’s not my fault!It’s these idiots
beside me
It’s not my fault!It’s these idiots
beside me
molecular cloud
Click here to learn more about MASERs
Click here to learn more about MASERs
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.
Masers detected in our Galaxy typically extend over only a few thousands of an arcsec.
Real-life masersThe 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 cloudWe 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.
28,000 6,000 3,500Surface temperature (K)
28,000 6,000 3,500Surface temperature (K)
Lum
inos
ity (
com
pare
d to
the
Sun
)Lu
min
osity
(co
mpa
red
to t
he S
un)
10-4
1
10-2
SunSunAA
BB
CC
Answer: None of the above.
Question: Where do you think such a molecular cloud would be on this H-R diagram? Near A? B? C?
young protostar
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.
Temperature
Lum
inos
ity
Main Sequence
Because it is faint, it is near the bottom of the luminosity scale.
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?
Temperature
Lum
inos
ity
Main Sequence
young protostar
Answer: the protostar moves up and to the left as it becomes brighter and hotter.
young protostar
Temperature
Lum
inos
ity
Main Sequence
hotter
brighter
Smaller and …?The protostar in its cocoon of gas and dust continues to collapse under gravity. It becomes smaller, and therefore hotter. Hotter
= brighter?Hotter
= brighter?Smaller
= dimmer?Smaller
= dimmer?Question: Will the luminosity of the protostar increase, or decrease? Luminosity depends on both the size of an object and its temperature...
Temperature
Lum
inos
ity
Main Sequence
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.
young protostar
Hotterand dimmer
Hotterand dimmer
T-Tauri starsHow do we know all this?
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.
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!
young protostar
Temperature
Lum
inos
ity
Main Sequence
The contraction continues, with an increase in the core temperature of the protostar.
Hydrogen fusion!
Hydrogen fusion!
Suddenly one day the density and temperature of hydrogen nuclei in the core are enough to overcome their electrostatic repulsion, and ...
ZAMSThe protostar is no longer a protostar, but now is a real hydrogen-burning 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? googoo
baba
“Nine months”?More likeMILLIONSof years!
Temperature
Lum
inos
ityMain Sequence
0.5 MS
100
1 MS
30
2 MS
8
5 MS
0.7
15 MS 0.16
It took our own Sun 30,000,000 years to reach the main sequence!
Joining the ZAMS
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.
Time to reachmain sequenceTime to reach
main sequence
Mass of starMass of star
Larger goes fasterHuge, massive stars form very quickly, while little stars take a long time to form.
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.
0102030405060708090
100
0.5 1 2 5 15
Mass (in solar masses)Mass (in solar masses)
Mill
ions
of
year
sM
illio
ns o
f ye
ars
Zip!
Dum de dum...
snore
Temperature
Lum
inos
ity
Main Sequence
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.
Membership is exclusive
No way, Tiny Tot!
mutter grumble
Electrostatic loathingAs 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
Yeuchhh! Yourcharge is positive!
Yeuchhh! Yourcharge is positive!
Eeeeek!So is yours!
Eeeeek!So is yours!
Stop mucking about.Get in there and fuse!Stop mucking about.Get in there and fuse!
Wild horses couldn’tdrag me in there ...
Wild horses couldn’tdrag me in there ...
Gravity could …but there just ain’t enough
Gravity could …but there just ain’t enough
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.
From gravity to P&T to fusionWe’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.
Yeuchhh! Yeuchhh!
Eeeeek!Eeeeek!
Not quite thereGliese 623aGliese 623a
Gliese 623ba brown dwarfGliese 623b
a brown dwarf
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 B40 times heavier
than Jupiter
Gliese B40 times heavier
than Jupiter
JupiterJupiter
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 SunOur Sun
Gliese AGliese A
5.2 AU
40 AU
Not to scaleNot to scale
Not so hotWe 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.
Hello thereHello thereYOW!YOW!
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
Let’s leave the protostars that are too light to join the main sequence and look at the other possibility.
Temperature
Lum
inos
ity
Main Sequence
Aaaargh!Too bright!
heavy sigh...
What about stars that are too massive to ever reach the main sequence?
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 can win
gravity
pressure
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
What mess?
CLEAN UP THAT MESS!
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!
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 viewHere 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 overviewLet’s finish this Activity with a review of what you have learned:
Temperature
Lum
inos
ity
Main Sequence
The large, cool, darkmolecular cloud
begins to contract
The large, cool, darkmolecular cloud
begins to contract
It becomes hotterand brighter
It becomes hotterand brighter
… but alsosmaller
… but alsosmallerIf it has enough mass
fusion begins andit reaches the ZAMS
If it has enough massfusion begins and
it reaches the ZAMS
If it is too massiveit becomes unstableIf it is too massive
it becomes unstable
If it’s not massiveenough it won’tbe a star at all
If it’s not massiveenough it won’tbe a star at all
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 CreditsMSSSO © M. Bessell (used with permission)
Large Magellanic Clouds,
Eta Carinae
AAO © D. Malin (used with permission)Star trails in the Southern Cross
Gliese 623Bhttp://antwrp.gsfc.nasa.gov/apod/image/9911/gl623_hst_big.jpg
Gliese 229Bhttp://antwrp.gsfc.nasa.gov/apod/image/gl229b_hst.gif
Eta Carinaehttp://antwrp.gsfc.nasa.gov/apod/image/etacarinae_hst2.gif
Hit the Esc key (escape) to return to the Module 7 Home Page
Stimulated emissionWhen atoms or molecules get into excited states, they don’t always decay back to the ground state quickly.
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.
This is called stimulated emission.
Hey, you in themetastable state!
Wake up!Aren’t you supposed
to be emitting aphoton like me?
Hey, you in themetastable state!
Wake up!Aren’t you supposed
to be emitting aphoton like me?
Oh, yeah!Thanks for
reminding me!
Oh, yeah!Thanks for
reminding me!
The maser process
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.
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) .
1. Take a molecular cloudcontaining water molecules1. Take a molecular cloud
containing water molecules
4. Let the water beirradiated with some of
the infrared photons
4. Let the water beirradiated with some of
the infrared photons
3. Let the dustabsorb most of the gamma rays
from the hot youngstars and reradiateinfrared radiation
3. Let the dustabsorb most of the gamma rays
from the hot youngstars and reradiateinfrared radiation
2. Add a newly-formingnice and hot OB star
(or group of stars)in a cocoon of dust
2. Add a newly-formingnice and hot OB star
(or group of stars)in a cocoon of dust
The rest of the recipeThe water molecule has one of these “sleepy” metastable energy levels. When the molecular cloud is bathed in infrared photons from the nearby dust-enshrouded young stars, an unusually large number of water molecules can end up in the metastable energy levels.
5. The water molecule is excited by an infrared photon into a
higher energy level
5. The water molecule is excited by an infrared photon into a
higher energy level
6. The molecule decaysto a meta-stable state
6. The molecule decaysto a meta-stable state
ground state ground state
meta-stable state
meta-stable state
excited state excited state
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. infrared
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”.
ground state ground state
meta-stable state
meta-stable state
excited state excited state
5. The water molecule is excited by an infrared photon into a
higher energy level
5. The water molecule is excited by an infrared photon into a
higher energy level
6. The molecule decaysto a meta-stable state
6. The molecule decaysto a meta-stable state
infrared
The way that the infrared photons aid in populating the meta-stable state in the water molecules is called pumping.
7. … until a “reminder”microwave comes past 7. … until a “reminder”microwave comes past 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.
ground state ground state
meta-stable state
meta-stable state
excited state excited state
Get moving! Releasethat microwave!
Get moving! Releasethat microwave!
5. The water molecule is excited by an infrared photon into a
higher energy level
5. The water molecule is excited by an infrared photon into a
higher energy level
6. The molecule decaysto a meta-stable state
6. The molecule decaysto a meta-stable state
microwave
The thermal distribution of photonscoming from the dust surroundingthe young stars will peak in the infrared, but will include higher and lower energy photons, including some in the microwave region.
7. … until a “reminder”microwave comes past 7. … until a “reminder”microwave comes past
ground state ground state
meta-stable state
meta-stable state
excited state excited state
This is essentially the same process of stimulated emission that makes lasers work, except there it is visible light photonsthat are emitted, not microwave photons.
5. The water molecule is excited by an infrared photon into a
higher energy level
5. The water molecule is excited by an infrared photon into a
higher energy level
6. The molecule decaysto a meta-stable state
6. The molecule decaysto a meta-stable state
microwave
Copy-catsSo 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 relevantmolecular emission line by factors of up to 10 billion (1010).
Repeated all through
cloud
Repeated all through
cloud
Heaps ofmicro-waves
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.
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