Download Birth - Wayne State University Physics and Astronomy

The Birth of Stars
&
the Discovery of Planets Outside the
Solar System
29 March 2005
AST 2010: Chapter 20
1
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?
29 March 2005
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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
The ability to generate energy by fusion defines a star
Each second in the Sun, about 600 million tons of hydrogen
undergo fusion into helium, with about 4 million tons
turning to energy in the process
This rate of hydrogen use means that eventually the Sun (and
all other stars) will run out of central fuel
Stars’ masses range from 1/12 MSun to 200 MSun
Low-mass stars are far more common
For main-sequence stars, the most massive (spectral type
O) are also the most luminous and have the highest
surface-temperature, whereas the least massive (spectral
type M or L) are the least luminous and the coolest
A galaxy of stars, such as the Milky Way, contains
enormous amounts of gas and dust, enough to make
billions of stars like the Sun
29 March 2005
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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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Molecular Clouds
Vast clouds of gas dot the Milky Way
A giant molecular cloud is
a large, dense cloud of gas
cold enough (10-20 K) for molecules to form
Each giant molecular cloud contains a vast amount of
material from which a star may be assembled
The cloud may have a mass as large as 3 million times the
Sun’s mass
Within the clouds are lumps, regions of high density
and low temperature
These conditions are believed to be just what is required to
make new star
The combination of high density and low temperature makes it
possible for gravity to overcome pressure
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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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Pillars of
high-density
dust in the
central
regions of
the M16
nebula
29 March 2005
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Dense Globules in M16 Nebula
One of the pillars
in M16 appears to
have very dense
globules
These structures
have been termed
evaporating gas
globules (e.g.g.s)
They may harbor
embryonic stars
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Evaporating Gas Globules
Giant Molecular Cloud
A shock wave creates
clumps in a 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
29 March 2005
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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
29 March 2005
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Observation of protostars
Infrared detectors
enable observation
of protostars
Many stars appear
to be forming in the
Nebula above and
to the right of the
Trapezium stars
They can only be
seen in the infrared
image
29 March 2005
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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29 March 2005
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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
29 March 2005
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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
29 March 2005
wind
AST 2010: Chapter 20
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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
fusion starts
when star reaches
zero age main
sequence
29 March 2005
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29 March 2005
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Stage 4: Main Sequence
We define the stars arrival on the main
sequence as the time when fusion
begins
Eventually become stable because
hydrostatic equilibrium has been
established
They settle down to spend about 90% of
their lives as main sequence stars
Fusing hydrogen to form helium in the core
29 March 2005
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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.
29 March 2005
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Summary of Birth Process
29 March 2005
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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
10,000 years
long for little
stars
up to 100
million years
for low mass
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HR Diagram: Analogy to Weight versus 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 Change as Age Increases (Marlon 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
1
2
3
4
5
6
7
height (feet)
29 March 2005
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Evidence that Planets Form around Other Stars
It is very hard to see a planet orbiting
another star
Planets around other stars may be
detected indirectly
One way is to look for disks of material
from which planets might be condensing
A big disk is more visible than a small planet
Look for evolution of disks -- evidence for
clumping into planets.
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Disks around Protostars
Four disks
around stars
in the Orion
Nebula
The red glow
at the center
of each disk
is believed to
be a young
star, no more
than a million
years old
29 March 2005
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Dust Ring around a Young Star
A debris disk has
been found around a
star called HR 4796A
The star is estimated
to be young, about 10
million years old
If there are newly
formed planets
around the star, they
will concentrate the
dust particles in the
disk into clumps and
arcs
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Disk around Epsilon Erdani
Evidence for a clumpy disk has been found around a
nearby star named Epsilon Eridani
The star is surrounded by a
donut-shaped ring of dust
that contains some bright
patches
The bright spots might be
warmer dust trapped
around a planet that formed
inside the donut
Alternatively, the spots could
be a concentration of dust
brought together by the gravitational influence of
planet orbiting just inside the ring
29 March 2005
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Planets Beyond the Solar System: Search and Discovery
If we can’t directly observe planets, can we indirectly
observe them?
Kepler’s and Newton’s laws apply
A planet usually orbits a star
Both the planet and the star orbit around a common
center of mass
The planet’s motion has an effect on the star’s motion
As a result, the star wobbles a bit
From the observed motion and period of the wobble,
the mass of the unseen planet can be deduced using
Kepler’s laws
It is a planet if its mass is less than 1/100 the Sun’s
mass (or about 10 times Jupiter’s mass)
29 March 2005
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Doppler Method for Detecting Planets
The star slightly wobbles due to the
motion of the unseen companion planet
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The plot of the velocity
from measured
Doppler shift versus time
shows the star’s orbit
about unseen partner
Second technique:
The light curve (
luminosity versus time)
shows shows planet’s
transit
29 March 2005
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To date, more
than 100 star
systems with
“planets” have
been found
Systems of 2,
3, and possibly
more “planets”
have been seen
The masses of
the “planets”
are measured in
Jupiter-masses.
29 March 2005
Discovered Planets
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Some Properties of the First 100 Extrasolar Planets Found
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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 difficult for the early
models to explain
The formation of planetary systems is
more complex and chaotic than we
thought
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