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Swinburne Online Education Exploring Stars and the Milky Way
Module :
Evolution of Stars
Activity :
From Starbirth
© Swinburne University of Technology
Summary:
In this Activity we learn how stars are created out of
interstellar gas and dust:
• the best conditions for star formation
•how stellar discs and jets are produced
• and quite a bit of interesting physics along the way.
Hit the Dust
If a nebula contains enough mass, it may begin to collapse because
of gravity. Whether it succeeds in collapsing depends on the mass:
pressure within the gas and dust opposes the collapse.
Low-mass cloud
High-mass cloud
gravity
gravity
pressure
pressure
Pressure “wins”,
cloud expands
Gravity “wins”,
cloud contracts
Dust is not enough
Molecular cloud collapse usually succeeds in creating low- to
medium- mass stars (like our Sun), but to create high-mass stars,
dust alone is not enough. There are three factors fighting against it.
The particles are in
motion (causing heat
and pressure)
Gravity works to
collapse the cloud,
but ...
magnetism
gravity
Magnetic forces from
moving charges act
against collapse
A rapidly rotating
disc tends to
spread out
A shocking affair
It is believed that most
molecular clouds
couldn’t contract enough
to form high-mass stars
under gravity alone.
However, shock waves
travelling through a
nebula can cause it to
bunch up in places,
sometimes enough for
gravity to be able to do
its work.
NGC 604, a huge, star-forming nebula in the
M33 Galaxy in the constellation Triangulum
Four shocks
Astronomers can think
of four events that
could cause a shock
wave to pass through a
molecular cloud.
An explosion (such as a
supernova) can emit a huge,
hot, fast-moving blast of gas
and dust that will crash into
anything nearby as it
expands.
NGC 6188, a region
of molecular dust
and hot young blue
OB stars
More shocks!
A shock wave
can travel around a
galaxy, creating
regions of denser
molecular cloud
A collision with
another molecular
cloud can really
create havoc in
both
Fusion starting in
a nearby star can
throw off a blast of
very hot hydrogen
gas (H II)
Group of new, big, hot OB stars
A domino effect
For example, a bunch of massive, hot
stars forming near a molecular cloud can
set the scene for the next generation of
stars.
First, the new stars produce strong
stellar winds which eat away and
compress the edges of the cloud.
Some of the stars even supernova.
Parts of the cloud then become so
dense that a cluster of new massive
stars forms, along with a new stellar
wind.
The stellar wind causes a
compression wave along the edge of
the rest of the cloud, and the process
begins again.
Molecular cloud
Gas gobblers
Once a group of these young stars forms within a
molecular cloud, the solar wind buffets all surrounding
objects - it clears a space all around the new star.
The molecular cloud
that is the Rosette
Nebula, 4500 ly from
Earth, was the
birthplace of the
young, hot, blue stars
in the centre.
Now, they are busy
modifying their home.
How exhausting
The stellar winds usually consist mostly of hydrogen (H II),
with some heavier elements . Although it’s not very dense,
this “exhaust” gas moves at hundreds of kilometres per
second and sweeps the molecular cloud away.
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Star-Forming Regions in our Galaxy
Of course, not all stars are young, hot massive stars. So
we had better look at stars more generally.
The Hubble space
telescope is now filling in
vast gaps in our
knowledge about the
formation of stars, by
giving us unprecedented
images of stars (and
probably planets) in the
process of forming in our
galaxy, the Milky Way.
One of the richest
stellar nurseries
in our part of the
Milky Way is
the Orion Nebula.
Here is the region of
the Orion Nebula
around the
Trapezium, a group
of four hot recentlyformed stars.
The Hubble Space Telescope has been used to make an
extensive study of star-forming regions in the constellation Orion,
which people in the Southern Hemisphere often call “the
Saucepan”.
Orion Nebula
The next slide is a
mosaic of 45 images
taken by the HST of 15
separate fields in the
centre of the Orion
nebula, which is located
in the middle of Orion’s
“sword” - the “handle of
the saucepan”.
At least 153
glowing
protoplanetary
discs have been
spotted around
young stars being
formed in this
region.
These disks may
become planets:
hence their name.
These protoplanetary discs are made of dust and gas.
They were first
discovered
with the Hubble
space
telescope in
1992 and called
proplyds:
probably
embryonic
solar systems
that may
eventually form
planets.
Here are Hubble
images of four
newly-discovered
proplyds around
young stars
in the Orion
Nebula.
The red glow in
the centre of
each disc is a
young, newlyformed star,
roughly one
million years old.
The largest disc in this photo is nearly edge-on, with a
diameter approximately the same as Pluto’s orbit.
Surrounding the disc is
diffuse hot gas which
has been evaporated
from the disk surface
by radiation from
nearby hot young
stars.
Follow this link to see a
Hubble animation
showing where these
proplyds are located in
the Orion Nebula.
Click here to view an animation
(/esmwmovs/h20.htm)
Here’s the centre
of the Trapezium
cluster, with four
massive energetic
stars evaporating
a number of
nearby proplyds
(the small white
blobs).
The star within
shelters the dust
behind it, so a
little remains
there like a tail.
Casting a shadow
It’s almost as if the proplyds
have shadows, made of dust.
They may have been sitting
there quietly minding their own
business when a new young
star popped up nearby ...
… and blasted them with
stellar wind!
YOW!
The Eagle Nebula
You can see pillars of dust being
formed this way in many nebulae
such as the Eagle Nebula. There
are also many proplyds there.
At the “top” of each pillar, inside,
there must be a young star or
protostar that has so far sheltered
the dust “below” it from a stellar
wind from “above”, which but in
time may be exposed completely.
Here is a Hubble
“close-up” of M16, a
star cluster in the
Eagle Nebula, which
contains these
magnificent “pillars”
on the edge of the
molecular cloud.
The pillars are
columns of gas &
dust, protected by
young protostars
cocooned in gaseous
envelopes, from
strong stellar winds
due to young hot
stars (out of the
picture, top right).
Gravity causes gravidity
Let’s look at how molecular
clouds become “gravid” - that
is, they become the site for
the formation of new stars.
A cold molecular cloud, only a
few degrees warmer than the
near-zero of space, begins to
collapse under its own
gravitational field.
Funny, that ..
When WE’re pregnant,
we get LARGER!
Potential, kinetic, angular
Momentum and energy have to be
conserved during all of this, so
various terrific changes take place as
the molecular cloud gets smaller.
• To conserve energy, the smaller the
cloud gets the hotter it gets (heat is
related to the kinetic energy of a gas).
• To conserve angular momentum,
the smaller it gets the faster it (or
parts of it) will spin.
Tell me
about
momentum
and energy
As a big cloud:
•high potential energy
•low kinetic energy
•low spin speed
As a smaller cloud:
•lower potential energy
•higher kinetic energy
•higher spin speed
Inside a cocoon of dust
Once the nebula
is sufficiently
contracted, you
have a rotating
disc with a
protosun in the
centre.
Young protosun is
forming in centre
Start with a vast,
rotating, contracting
cloud of gas,
dust & molecules
Why a disc?
Nebula contracts
to form a disc
Disc gets cooler as
you go further out
Our Solar System
This is why our own Sun, and its planets, are all orbiting in the same
direction.
It’s the direction that the original cloud of gas and dust was spinning.
Hot hot hot
As our cloud gets smaller
we get packed closer
… that we
escape!
… and sometimes we get
Now, as with the Sun in an earlier
so fast and furious
activity, the hotter the gas in the core
the more it will want to expand
against the force of gravity.
So there is a constant balance being
sought between the pull of gravity
inwards and the pressure of hot gas
outwards.
We move faster, too, so
we hit each other more
But if we are lucky, there will be
enough mass around to make sure
that gravity wins ...
Gravity
Hot enough!
If gravity is strong enough, then the
core gets hot enough and the gas is
under enough pressure ...
…for fusion reactions to start and
you have a baby star.
The baby star, or Young Stellar
Object, consists of a hot, tight
core and a dusty cloud.
61H+  4He++ + 2e+ + 2 + 2 + 21H+
6 hydrogen atoms fuse to become one helium nucleus,
two positrons, two neutrinos, two gamma rays
and two spare hydrogen atoms to keep the fusion going
Headstrong youth
Hot gas contains lots of
charged particles (ions)
The temperature of the gas
will not be high enough yet
for it to be visible to human
eyes, but infra-red monitors
will see it clearly, and can
pick up its hot centre.
The smaller the cloud
becomes, the more
concentrated the magnetic
effects will be.
Tell me
about
magnetism
Infra-red
radiation
Moving charge creates
a magnetic field
Easiest escape route
from the core
is along the “axis”
Stellar winds and jets
Gas and dust will be
radiated out from the
YSO, but because of
the disc of dust most of
this outflow will head
along the “axis” in two
“jets”.
There will also be two
cones of material
escaping from the disc
itself.
disc
core
Hot gas and radiation
is released by the disc
Twin exhausts
However because a lot
of the stellar wind is hot
and ionised (H II) it is
strongly funnelled by
magnetic effects along
the axis.
This explanation of
why there are two jets
is often called the twin
exhaust model.
Charged particles end
up spiralling along the
axis
The Red Rectangle
This well-established young
star is seen almost side-on
from Earth, so it clearly
shows the two cones of hot
dust emitted by its disc.
“That’s not red!”, you say.
The spectrum from the dust
shows a strong red line
indicating hydrogen stuck to
carbon, but the colour is
overpowered in the photo by
yellow light from the star.
axis
disc
Jets and a disc
disc
This photo of HH30, a
young star, taken by the
Hubble Space Telescope
axis
in 1995 clearly shows the
dark line of the disc
where it is most cool and
dense (we are looking at
the outer edge).
The red lines are the thin, fast jets of hot material, and the
paler areas show the glowing upper and lower parts of
the disc, and some of its escaping material.
What shape did you say?
There is some conjecture
that the disc might
actually be a slightly
different shape from a
simple disc that is
thickest in the middle.
Surfaces
Surfacesof
ofthe
thedisc?
disc?
It is thought that dust and gas further out from the centre
might take longer to drift towards the disc, and so the
surface should be concave (“innie”) rather than convex
(“outie”).
A pulsed jet
HH30 and other young stars
appear to be emitting their
exhaust in pulses.
This is possibly because
the star goes through a
regular series of phases:
chunks fall from the disc
towards the star, and the
material is expelled along
the jet; then there is a
pause while the disc
prepares the next batch.
See next slide
please!
HH30- Februayr 1994 and January 1995
This Activity has shown you a bit more about young
stars and some of their more spectacular characteristics
such as jets.
In the next Activity, we shall take a further look at the
life-cycle of a star.
Image Credits 1
MSSSO © M. Bessell (used with permission)
Lagoon Nebula
NGC 6188
Rosette Nebula
N2004
Orion nebula
Eagle Nebula
Trapezium
NGC 604, courtesy of Hui Yang at the University of Illinois, and NASA
http://oposite.stsci.edu/pubinfo/gif/NGC604.gif
Orion and the Aurora Australis, taken from the Space Shuttle, credit STS-59
Crew, NASA
http://antwrp.gsfc.nasa.gov/apod/ap990320.html
Red Rectangle AAO © David Malin (used with permission)
http://www.aao.gov.au/local/www/dfm/image/aat094.jpg
Image Credits 2
Hubble:
Proplyds in Orion
http://antwrp.gsfc.nasa.gov/apod/image/proplyds_hst.gif
Four protoplanetaries in Orion
http://oposite.stsci.edu/pubinfo/gif/OriProp4.gif
Orion Nebula Mosaic Credit Dave Johnston
http://www.cita.utoronto.ca/~johnston/orion.html#figures
Pillars of creation in a star-forming region, Star-birth clouds
http://www.stsci.edu/pubinfo/pr/95/44.html
Jets and a disk
http://oposite.stsci.edu/pubinfo/gif/JetDisk3.gif
Double jets of HH30 evolve with time
http://oposite.stsci.edu/pubinfo/gif/HH30.gif
Hit the Esc key (escape)
to return to the Index Page
Momentum and energy
Over the centuries, physicists and astronomers have
found that there are a few laws that they can rely on.
One of these is to do with the conservation of
momentum, and another is to do with the conservation
of energy.
Let’s start with the more familiar one first...
Energy
In physics, energy has pretty
much the same meaning that it
has in daily life.
The traditional definition is:
Energy is the capacity for
doing work.
An object has energy if it is
moving, or can cause something
else to move, or would rather like
to move (given half a chance).
?
There’s a lot more heat and
light in Summer, right?
Yep
And heat and light are
forms of energy, right?
You’re not wrong
And energy is the capacity
for doing work, isn’t it?
Sure is
So, how come everyone gets
holidays in Summer?
Potential Energy (PE)
Apple at rest:
no kinetic energy
lots of potential energy
This last situation is of particular concern to
astronomers (and other physicists), as most
astronomy is involved with the question of how and
why things are moving the way they are, and
changing the way they do.
If gravity (or another force) “wants” an object to
move, then we say that the object has potential
energy.
A slight change of circumstances (such pushing
this apple from behind) could see the object
suddenly lose its potential energy and have real
kinetic energy instead.
Far from Earth,
PE = GMm/R
where m = mass
M = mass of Earth
G = 6.7 x 10-11 N.m2.kg-2
and R = distance from
centre of Earth
Close to Earth’s surface,
PE = mgh
where m = mass, g = 9.8 ms-2
and h = height above ground
Kinetic Energy (KE)
The term kinetic comes from the Greek kinetos,
and means motion.
Any object with mass, in motion, has kinetic
energy.
The more mass the object has, the higher its
kinetic energy.
The faster it is going, the higher its kinetic
energy.
In fact, the kinetic energy is related to the square
of the speed.
If mass = m
and speed = v, then
kinetic energy = 1/2 m v2
Conservation of Energy
If potential energy is defined in this way, it is
found that energy is also conserved during
collisions and interactions.
Over the centuries, different types of energy
have been identified, and it is the work of
scientists, mathematicians and engineers to
study, predict and even use the various
possibilities.
For instance: humans convert the chemical
energy stored in coal, gas and oil to electrical
energy, and then to the energies of heat, light
and motion.
Lots of PE
No KE
Some PE
Some KE
No PE
Lots of KE
Oomph!
Oomph!
Momentum
Momentum is the amount of oomph something has because
• it has mass, and
Oomph!
• it is moving.
The more mass an object has, the more momentum it has.
The more speed an object has, the more momentum it has.
If mass = m
and speed = v, then
momentum = mv
Conservation of momentum
If there is a collision or interaction between two (or more) objects
then the total momentum before the event is the same as the total
momentum afterwards.
You have to take direction into account!
Initially, this ball has
a little bit of momentum
to the right
Finally, the small ball has
a bit of momentum
to the left
Initially, the two
together have a total
momentum to the left
Initially, this ball has
a lot of momentum
to the left
Finally, the two together
have a total momentum
to the left
Finally, the big ball has
a bit of momentum
to the left
Angular momentum
If something is spinning, it also has angular momentum
and that is conserved as well.
The angular momentum of an object depends on how
the mass is arranged about the axis it is spinning
around, and the speed at which it spins.
radius
r
m
Speed
v
For each part of an object,
angular momentum = mvr
where m = mass,
v = speed, and
r = distance from centre
The classic ice skater concept
If an iceskater is spinning
with his arms and a leg out,
he will spin slowly.
But if he pulls these limbs
closer to his body, he will
spin faster.
Angular momentum is conserved
Distance large,
speed small
axis
Distance small,
speed large
This is because the total sum, for each part
of his body, of the angular momentum
mass x speed x distance from axis
must stay the same.
If the distances of his hands, arms and
legs from the axis get smaller, the speeds
must get bigger to compensate!
Angular momentum in space
Distance small,
speed large
Distance large,
speed small
If a molecular cloud, for instance, contracts under
gravity, it will spin faster.
Back to
Potential
Energy
Back to
Potential
Energy
Magnetic fields in astronomy 1
Most of the bodies that
we observe in space are
rotating, some of them
very fast indeed.
If the inside of such a body
is fluid (as in a molecular
cloud, protostar, star or
planet such as the Earth) it
can slosh around in
complex and turbulent
motion.
Rotation causes eddies
inside the body
Magnetic fields 2
In warm or hot objects there
will be ions (charged
particles), and as these move
they create magnetic fields:
that is, the object develops a
North and South pole and you
can draw magnetic field lines
to show where a magnet
would move if you were to let it
go near the object.
N
S
S
N
The direction of the field depends on how the insides are sloshing, and
can change with time. While our Sun’s magnetic field reverses every
22 years, that of the Earth flips only once every million years or so.
Back to
“Headstrong
Youth”
Back to
“Headstrong
Youth”
Why a disc?
Rotation axis
Why does the dust contract
into a disc shape, and not
a sphere?
The answer is that the nebula is spinning.
This doesn’t greatly affect the movement of gas and dust
along the general direction of the rotation axis. If it is
attracted towards the centre by gravity, it can move there.
However if a particle moves inwards from the side (in
our picture) there is a problem.
As the distance from the axis decreases, conservation of
angular momentum means that the particle will spin
faster.
Distance large,
speed small
Distance small,
speed large
It takes a greater
force to keep a fast
spinning object
moving in a circle
than it does for a
slowly moving
object ...
… which is why if you
whirl a weight on a
string around your
head fast, you’d better
make sure that the
string is strong!
Slow spinning
gas
Fast spinning
gas
Relatively small
force needed to
keep it spinning
in circle
Relatively large
force needed to
keep it spinning
in a circle
So as gas and dust move in towards the rotation axis,
it spins faster and faster, and needs a greater and
greater force to keep it
from “spinning out”.
This force is provided
by gravity, but it has
its limits. The attractive
force of gravity that the
cloud can provide depends
on its mass and size ...
… but a point will be reached where, to move any closer
to the axis, the gas and dust would need a greater
attractive force than gravity in this situation can provide.
So the collapse of the cloud
is halted relatively soon in the
direction perpendicular to the
rotation axis,
but can continue along the axis,
turning a spherical cloud
gradually into a disk-shaped one.
For the maths enthusiasts:
For each part of a rotating object,
angular momentum = mvr
where m = mass of that part,
v = its orbital speed, and
r = distance from the rotation axis
and
An object (e.g a gas molecule or dust particle), mass
m, moving in a circle under the attractive force of
gravity at orbital speed v, and distance r from the
rotation axis, obeys:
mv2/r = GMm/r2 (the force due to gravity)
Back to
the
Cocoon
Back to
the
Cocoon