Transcript Starbirth - Lincoln-Sudbury Regional High School

Swinburne Online Education Exploring Stars and the Milky Way
Module
: 6:
Module
Evolution of Stars
Activity 2:
From Starbirth
© Swinburne University of Technology
Summary
In this Activity, we will learn how stars are created from
interstellar gas and dust. In particular, we will discuss:
• the best conditions for star formation;
• how stellar disks and jets are produced; and
• some of the interesting physics that is necessary to
describe star formation.
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
opposes the gravitational collapse.
Low-mass cloud
High-mass cloud
gravity
gravity
pressure
pressure
Pressure “wins”,
cloud expands
Gravity “wins”,
cloud contracts
Gravity is not enough
The collapse of a molecular cloud usually succeeds in creating low- to
medium-mass stars (like our Sun), but to create high-mass stars,
gravity 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
disk tends to
spread out
A shocking affair
Most molecular clouds
cannot contract enough
to form high-mass stars
under gravity alone.
However, shock waves
travelling through a
nebula can produce
overdense regions,
sometimes enough to
trigger a localised
collapse.
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 create
havoc in both
clouds
The start of fusion
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
Let’s think through a possible scenario in
which star formation can occur: a group of
massive, hot stars forming near a molecular
cloud.
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 at the edge of the remaining cloud,
and the process begins again.
Molecular cloud
Gas gobblers
Once a group of these young stars forms within a molecular
cloud, the stellar wind buffets all surrounding objects - it clears a
space all around the new star.
The molecular cloud that is the
Rosette Nebula, 4500 light years
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 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. Let’s
now look at stars more generally.
The Hubble Space
Telescope is currently
filling in vast gaps in our
knowledge about the
formation of stars by
giving us unprecedented
images of stars (and
probably planets) that are
in the process of forming
in the Milky Way.
One of the richest
stellar nurseries
in our neighbourhood
of the Milky Way is
the Orion Nebula.
Here is the region of
the Orion Nebula
around the
Trapezium, a group
of four, hot, nascent
stars.
Star formation in Orion
The Hubble Space Telescope has been used extensively to study
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 disks
have been detected
around young stars
being formed in this
region.
These disks may
become planets,
hence their name.
Protoplanetary disks are made of dust and gas.
The ones you
see here were
first discovered
with the HST in
1992.
They are also
called proplyds,
embryonic solar
systems that
may eventually
form planets.
Here are HST
images of four
newly-discovered
proplyds around
young stars
in the Orion
Nebula.
The red glow in
the centre of
each disk is a
young, newlyformed star,
roughly one
million years old.
The largest disk in this photo is nearly edge-on, with a
diameter approximately the same as Pluto’s orbit.
Surrounding the disk is
diffuse hot gas which
has been evaporated
from the disk surface
by radiation from
nearby hot young stars.
Click here to see a
Hubble animation
showing where these
proplyds are located in
the Orion Nebula.
Here we see the
centre of the
Trapezium cluster,
with four, massive,
energetic stars
evaporating a
number of nearby
proplyds (the small
white blobs).
The star within the
proplyd shelters the
dust behind it,
producing the
apparent tail effect.
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.
Inside the top of each pillar,
there should be a young star
or protostar that has so far
sheltered the dust below it
from the stellar wind coming
from above. In time, the dust
may be exposed completely.*
* Note that the directions in this description only apply to the
photograph - not the physical object!
Here is a Hubble close-up
of M16, the Eagle Nebula,
which contains these
magnificent pillars.
The pillars are columns of
gas and dust, protected
from the strong winds of
young hot stars (out of the
picture, top right) by young
protostars cocooned in
gaseous envelopes.
However, Hubble observations were done at visible wavelengths, so we couldn’t
actually “see” inside the cloud.
This region has recently been observed with the VLT* at near-infrared wavelengths,
which can penetrate the dense dust and reveal the newly born stars. Very young,
relatively massive stars are found at the tips of two of the main three pillars, and
about a dozen lower mass young stars are observed within the “evaporating
gaseous globules” (the denser knots seen as small bumps) in one of the pillars.
This observation confirms that the pillars are indeed regions of star formation.
*The European Southern Observatory Very Large Telescope (VLT)
at the Paranal Observatory (Atacama, Chile) .
Gravity causes gravidity
Let’s look at how molecular
clouds become gravid - that
is, how 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 cloud spins faster as it
Tell me
collapses.
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, a
rotating disk with
a protostar in the
centre remains.
Young protostar is
forming in centre
Start with a vast,
rotating, contracting
cloud of gas,
dust & molecules
Why a disk?
Nebula contracts
to form a disk
Disk gets cooler as
you go further out
Our Solar System
This is why our own Sun and the Solar System planets
are all spinning in the same direction. It’s the direction
that the original cloud of gas and dust was spinning.
The hotter the gas in the core of
the protostar, the more it will try 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.
But if we are lucky, there will be
enough mass around to make
sure that gravity wins ...
… that we
escape!
… and sometimes we get
so fast and furious
We move faster, too, so
we hit each other more
Gravity
Hot hot hot
As our cloud gets smaller
we get packed closer
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 (YSO), consists of a
hot, tight core and a dusty
cloud.
1 +
4
++
6 H  He
+ 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
telescopes will see it clearly,
and can detect 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 rotational
axis
Stellar winds and jets
Gas and dust will be
radiated from the YSO
but the disk of dust
disk
channels most of this
outflow along the axis in
two jets.
core
There will also be two
cones of material
escaping from the disk
itself.
Hot gas and radiation
is released by the disk
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
Jets and a disk
This photo of HH30, a
young star, taken by the
Hubble Space Telescope
in 1995 clearly shows the
dark line of the disk
where it is most cool and
dense (we are looking at
the outer edge).
disk
axis
The red lines are the thin, fast jets of hot material, and the
paler areas show the glowing upper and lower parts of
the disk, and some of its escaping material.
What shape did you say?
There is some conjecture
that the disk might
actually be a slightly
different shape from a
simple disk.
Surfaces
Surfacesof
ofthe
thedisk?
disk?
It is thought that dust and gas further out from the centre
might take longer to drift towards the disk, 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 may be because the
star goes through a regular
series of phases: chunks of
matter fall from the disk
towards the star, and the
material is expelled along
the jet; then there is a
pause while the disk
prepares the next batch.
HH30
In this HST image,
we see the motion of
distinct clumps of
matter in the HH30
jet over a period of
just 11 months.
Two views of HH47. Top: Multi-colour.
Right: As seen in the S II emission
line. The protostar is at top right. The
jet, moving at 125 km s-1 from top
right to bottom left, is sporadic and
ends in a bow shock region.
Summary
In this Activity, we had a closer look at how star
formation follows the collapse of a cold, molecular
cloud. This process is a struggle between the
competing forces of gravity and pressure, with magnetic
and rotational effects playing a secondary role.
Shockwaves are often involved, and are necessary if
high mass star formation is to occur.
We then looked at some observations of starbirth
regions in our Galaxy, identifying characteristic features
such as proplyds and jets. In the next Activity, we shall
follow a complete life-cycle of a star.
Image Credits
MSSSO © M. Bessell (used with permission)
Lagoon Nebula
NGC 6188
Rosette Nebula
NGC 2004
Orion nebula
Eagle Nebula
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
http://antwrp.gsfc.nasa.gov/apod/image/orion_aurora_sts59.gif
AAO © David Malin (used with permission)
http://www.aao.gov.au/local/www/dfm/image/aat094.jpg
Image Credits
HST: 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
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
HH47 in S II
http://huey.jpl.nasa.gov/~bode/mprl/SII.model.gif
Hit the Esc key (escape)
to return to the Module 6 Home 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...
Sir Isaac Newton
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 interest 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
Close to Earth’s surface,
changing the way they do.
PE = mgh
If gravity (or another force) “wants” an object to where m = mass, g = 9.8 ms-2
move, then we say that the object has potential and h = height above ground
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.
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
Finally, the two together
have a total momentum
to the left
Initially, this ball has
a lot of 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 large,
speed small
Distance small,
speed large
If a molecular cloud, for instance, contracts under
gravity, it will spin faster.
Return to Activity
Magnetic fields in astronomy
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
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.
Return to Activity
Why a disk?
Rotation axis
Why does the dust contract
into a disk 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 moves 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)
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