Transcript Birth of Stars - High Energy Physics at Wayne State

Chapter 20:
The Birth of
Stars and
the
Discovery
of Planets
Outside the
Solar
System
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Questions About Star Formation
Are new stars still being created, or did creation
cease billions of years ago?
Where are new stars being created?
Are planets a natural result of star formation or
is our solar system unique in the universe?
How can we observe planets around distant
stars?
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Basics About Stars (Table 20.1)
Stable (main-sequence) stars maintain equilibrium by
producing energy through nuclear fusion in their
cores. Generating energy by fusion defines a star.
Hydrogen is being converted to helium, but eventually
the supply of hydrogen will run out.
Stars range in mass from about 1/12 Msun to 200 Msun.
Low mass stars are more common.
For main sequence stars, mass and luminosity are
related such that high mass stars have high luminosity
and low mass stars have low luminosity.
Galaxies, like the Milky Way, contain enough gas and
dust to create billions of new stars.
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Life Cycle of Stars: from Birth to Maturity
Stage 1: Giant Molecular Cloud – cold dust clouds in space
Clumps (dust bunnies) accrete matter from cloud to form
protostar
Stage 2: Protostar – energy generated by gravitational collapse
Stage 3: Wind formation – protostar produces strong solar winds
winds eject much of the surrounding cocoon gas and dust
winds blow mostly along the rotation axes
Stage 4: Main Sequence -- the new star becomes stable
Equilibrium: hydrogen fusion into helium in the core balances
gravity.
Stage continues until most of the hydrogen in the core is used
up.
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Stage 1: Giant Molecular Cloud
giant molecular cloud
large, dense gas cloud with dust
cold enough for molecules to form
thousands of giant molecular clouds in the
galactic disk
each giant molecular cloud contains vast
amount of material for star formation
about one million solar masses
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Star Formation (Reference Slide)
matter in part of giant molecular cloud begins to
collapse
tens to hundreds of solar masses
collapse can start by itself if matter is cool and
massive enough
shock waves can trigger collapse by compressing the
gas clouds into clump
the explosion of a nearby massive star – supernova
gravity from nearby stars or groups of stars
gravity pulls more matter to form sufficiently massive
clumps
whatever the reason, the result is the same: gas
clumps compress to become protostars
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Eagle
Nebula
M16
nearby star shines
light on gas cloud
revealing protostar
formation
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Evaporating Gas Globules
giant molecular cloud
shock wave creates clumps in giant
molecular cloud
clusters: many stars forming
simultaneously
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Stage 2: Protostars
gas clump collapses and heats
up as gas particles collide
gravitational energy is converted to
heat energy
heated clump produces infrared
and microwave radiation
at this stage the warm clump is
called a protostar
Rotating gas clump forms a disk
with the protostar in the center
other material in the disk may
coalesce to form another star or
planets
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Behind the visible
part of the Orion
Nebula is a much
denser region of
gas and dust that is
cool enough for
molecules to form.
Many stars are
now forming inside
it.
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trapesizium
cluster:
•stars that
provide much
of the energy
which makes
the brilliant
Orion Nebula
visible
•other stars
obscured by
nebula
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Observation of protostars
Infrared detectors
enable observation of
protostars.
Many stars forming in
the Nebula above and
to the right of the
Trapezium stars.
They can only be seen
in the infrared image.
Visible
Infrared
Images are from the Hubble Space Telescope
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Protostar
gravity pulls more
matter into clump
energy from falling
matter creates heat
protostar forms as
hot matter begins to
glow in infrared
protostar surrounded
by "cocoon" of dust
matter falling into a
rotating star tends to
pile up in a disk
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Social Stars
Young stars seem to be social
Fragmentation of the giant molecular cloud
produces protostars that form at about the same
time.
Stars are observed to be born in clusters.
Other corroborating evidence for this is that there
are no isolated young stars.
This observation is important because a valuable
test of the stellar evolution models is the
comparison of the models with star clusters.
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Stage 3: Wind Formation
strong stellar winds
winds eject much of
the surrounding gas
and dust
wind
Winds constrained to
flow preferentially
along the rotation
axes
proto-planetary
disk
With most of the
cocoon gas blown
away, the forming
star finally becomes
visible
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wind
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Jets
Jets from Stellar Wind
gravitational
contraction
continues
eventually enough
energy for stellar
wind to form jets
jets blow away
cocoon
fusion begins at end
of this stage
star reaches zero
age main sequence
when fusion starts
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Stage 4: Main Sequence
We define the star’s arrival on the main
sequence as the time when fusion begins.
Eventually becomes stable because hydrostatic
equilibrium is established.
It settles down to spend about 90% of its life as
a main sequence star.
Fusing hydrogen to form helium in the core.
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evolution to main sequence
•zero age main sequence
•point at which star begins fusing hydrogen into helium.
•moving to left – temperature is increasing
evolution to main sequence
ages of forming
stars in years as
they grow towards
main sequence
mass determines
position on main
sequence
Life Cycle of Stars: from Birth to Maturity (Recap)
Stage 1: Giant Molecular Cloud – cold dust clouds in space
Clumps (dust bunnies) accrete matter from cloud to form
protostar
Stage 2: Protostar – energy generated by gravitational collapse
Stage 3: Wind formation – protostar produces strong solar winds
winds eject much of the surrounding cocoon gas and dust
winds blow mostly along the rotation axes
Stage 4: Main Sequence -- the new star becomes stable
Equilibrium: hydrogen fusion into helium in the core balances
gravity.
Fusion continues until most of the hydrogen in the core is used
up.
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Summary of Birth Process
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Evolution to Main Sequence
•ages of forming stars in years as they grow towards main sequence
•zero age main sequence – ZAMS
•point at which star begins generating energy by fusion
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time to reach main
sequence stage
short for big stars
•as low as 10000 years
long for little stars
•up to 100,000,000
years for low mass
HR Diagram: Analogy to Weight vs Height for People
600
weight (pounds)
500
400
300
200
100
0
0
1
2
3
4
5
6
7
8
9
10
height (feet)
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Weight and Height changes as Age increases (Marlin Brando)
350
300
weight (pounds)
250
200
150
100
50
0
0
1
2
3
4
5
6
7
height (feet)
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Different paths for different body types (Woody Allen)
300
Brando
Allen
250
weight (pounds)
200
150
100
50
0
0
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2
3
4
height
(feet)
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20.3 Evidence that Planets Form
Around Other Stars
It is hard to see a planet orbiting another star.
Look for a disk of material before it clumps to form
planets -- big disk is more visible than small planet.
Look for evolution of disks -- evidence for clumping
into planets.
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proto-planetary
disk
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Proto-planetary Disks
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Dust Ring Around a Young Star
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Disk Around Epsilon Erdani
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20.4 Planets Beyond the Solar
System: Search and Discovery
If we can’t directly observe planets, can we
indirectly observe them?
3 methods have succeeded.
First method: doppler shift
A planet must orbit its star to be stable.
Search for the effect of the planet’s orbit on the star.
Both planet and star orbit around a common center
of mass.
The star “wobbles” a bit as the planet orbits it.
The wobble has the same period as the planet’s
orbit.
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Search for Doppler Shift
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Velocity from
measured
Doppler Shift vs. time -shows the star’s orbit
about unseen partner
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Second Method to Find Planets
Look for a small reduction of star light when an
orbiting planet moves between us and the star.
Works only when planet’s orbit is lined up properly.
Will block all visible wavelengths -- this is a cross check.
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Third Method to Find Planets
• Measure infrared (thermal) radiation of “hot” planet.
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As of June
2005, more than
155 extrasolar
planets found.
Systems of 2, 3,
and possibly
more planets
are seen.
Masses are
measured in
Jupiter-masses.
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Discovered Planets
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20.4.3 Explaining the Planets Seen
Now that we have a large sample of planetary
systems, astronomers can refine their models
of planet formation.
Almost all the planets are Jupiter-sized, and
many have highly eccentric orbits close to their
star. This is a surprise and is hard for the early
models to explain.
The formation of planetary systems is more
complex and chaotic than we thought.
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Planet Mass Distribution
Not many
brown-dwarf
sized
planets
(M>10MJup).
Jupiter-sized
planets are
common.
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Eccentric Orbits Are Common
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Planets and Star “Metallicity”
Planets are
more
common
around stars
with more
heavy
elements
(“metallicity”).
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