Transcript 11 The Interstellar Medium

Chapter 11
The Interstellar
Medium
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Units of Chapter 11
Interstellar Matter
Star-Forming Regions
Dark Dust Clouds
The Formation of Stars Like the Sun
Stars of Other Masses
Star Clusters
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Question 1
Some regions of
the Milky Way’s
disk appear dark
because
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a) there are no stars there.
b) stars in that direction are obscured
by interstellar gas.
c) stars in that direction are obscured
by interstellar dust.
d) numerous black holes capture all
the starlight behind them.
Question 1
Some regions of
the Milky Way’s
disk appear dark
because
a) there are no stars there.
b) stars in that direction are obscured
by interstellar gas.
c) stars in that direction are obscured
by interstellar dust.
d) numerous black holes capture all
the starlight behind them.
Dust grains are about the same size as visible light, and they can
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Inc.
scatter
or block the shorter wavelengths.
Interstellar Matter
The interstellar medium consists of gas and dust.
Gas is atoms and small molecules, mostly
hydrogen and helium.
Dust is more like soot or
smoke; larger clumps of
particles.
Dust absorbs light, and
reddens light that gets
through.
This image shows distinct
reddening of stars near
the edge of the dust
cloud.
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Interstellar Matter
Dust clouds absorb blue
light preferentially; spectral
lines do not shift.
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Question 2
When a star’s
visible light
passes through
interstellar dust,
the light we see
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a) is dimmed and reddened.
b) appears to twinkle.
c) is Doppler shifted.
d) turns bluish in color.
e) ionizes the dust and creates
emission lines.
Question 2
When a star’s
visible light
passes through
interstellar dust,
the light we see
a) is dimmed and reddened.
b) appears to twinkle.
c) is Doppler shifted.
d) turns bluish in color.
e) ionizes the dust and creates
emission lines.
The same process results
in wonderful sunsets, as
dust in the air scatters the
Sun’s blue light, leaving
dimmer, redder light.
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Question 3
Astronomers use
the term nebula
to refer to
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a) outer envelopes of dying stars that drift
gently into space.
b) remnants of stars that die by supernova.
c) clouds of gas and dust in interstellar space.
d) distant galaxies seen beyond our Milky Way.
e) All of the above are correct.
Question 3
Astronomers use
the term nebula
to refer to
a) outer envelopes of dying stars that drift
gently into space.
b) remnants of stars that die by supernova.
c) clouds of gas and dust in interstellar space.
d) distant galaxies seen beyond our Milky Way.
e) All of the above are correct.
Nebula refers to any
fuzzy patch – bright or
dark – in the sky.
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Star-Forming Regions
“Nebula” is a general term used for fuzzy objects
in the sky.
Dark nebula: dust
cloud
Emission nebula:
glows, due to hot
stars
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Question 4
Interstellar gas
is composed
primarily of
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a) 90% hydrogen, 9% helium, and 1%
heavier elements.
b) molecules including water and CO2.
c) 50% hydrogen, 50% helium.
d) hydrogen, oxygen, and nitrogen.
e) 99% hydrogen, and 1% heavier elements.
Question 4
Interstellar gas
is composed
primarily of
a) 90% hydrogen, 9% helium, and 1%
heavier elements.
b) molecules including water and CO2.
c) 50% hydrogen, 50% helium.
d) hydrogen, oxygen, and nitrogen.
e) 99% hydrogen, and 1% heavier elements.
The composition of
interstellar gas mirrors that
of the Sun, stars, and the
jovian planets.
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Star-Forming Regions
These nebulae are very large and have very low
density; their size means that their masses are
large despite the low density.
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Star-Forming Regions
This is the central section of the Milky Way
Galaxy, showing several nebulae, areas of star
formation.
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Question 5
The reddish color of
emission nebulae
indicates that
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a) gas and dust is moving away
from Earth.
b) hydrogen gas is present.
c) dying stars have recently
exploded.
d) cool red stars are hidden inside.
e) dust is present.
Question 5
The reddish color of
emission nebulae
indicates that
a) gas and dust is moving away
from Earth.
b) hydrogen gas is present.
c) dying stars have recently
exploded.
d) cool red stars are hidden inside.
e) dust is present.
Glowing
hydrogen gas
emits red light
around the
Horsehead
nebula.
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Star-Forming Regions
Emission nebulae generally glow red – this is
the Hα line of hydrogen.
The dust lanes visible in the previous image
are part of the nebula, and are not due to
intervening clouds.
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Star-Forming Regions
How nebulae work
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Star-Forming Regions
There is a strong
interaction
between the nebula
and the stars
within it; the fuzzy
areas near the
pillars are due to
photoevaporation.
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Star-Forming Regions
Emission nebulae are made of hot, thin gas,
which exhibits distinct emission lines.
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Tarantula Nebula
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Dark Dust Clouds
Average temperature of dark dust clouds is a few
tens of kelvins.
These clouds absorb visible light (left), and emit
radio wavelengths (right).
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Dark Dust Clouds
This cloud is very dark, and can be seen only
because of the background stars.
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Dark Dust Clouds
The Horsehead Nebula is a particularly
distinctive dark dust cloud.
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Dark Dust Clouds
Interstellar gas emits low-energy radiation, due
to a transition in the hydrogen atom.
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Question 6
21-centimeter
radiation is
important
because
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a) its radio waves pass unaffected through
clouds of interstellar dust.
b) it arises from cool helium gas present
throughout space.
c) it can be detected with optical telescopes.
d) it is produced by protostars.
e) it reveals the structure of new stars.
Question 6
21-centimeter
radiation is
important
because
a) its radio waves pass unaffected through
clouds of interstellar dust.
b) it arises from cool helium gas present
throughout space.
c) it can be detected with optical telescopes.
d) it is produced by protostars.
e) it reveals the structure of new stars.
Cool atomic hydrogen gas
produces 21-cm radio radiation
as its electron “flips” its
direction of spin.
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Dark Dust Clouds
This is a contour map of H2CO near the M20
Nebula. Other molecules that can be useful for
mapping out these clouds are carbon dioxide
and water.
Here, the red and
green lines
correspond to
different rotational
transitions.
(frequencies)
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Dark Dust Clouds
These are CO (carbon monoxide) emitting clouds
in the outer Milky Way, probably corresponding to
regions of star formation.
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Question 7
Complex
molecules in
space are
found
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a) in the photospheres of red giant stars.
b) primarily inside dense dust clouds.
c) in the coronas of stars like our Sun.
d) scattered evenly throughout interstellar
space.
e) surrounding energetic young stars.
Question 7
Complex
molecules in
space are
found
a) in the photospheres of red giant stars.
b) primarily inside dense dust clouds.
c) in the coronas of stars like our Sun.
d) scattered evenly throughout interstellar
space.
e) surrounding energetic young stars.
A radio telescope image of the outer portion of the Milky Way,
revealing molecular cloud complexes.
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The Formation of Stars Like the
Sun
Star formation happens when part of a dust
cloud begins to contract under its own
gravitational force; as it collapses, the center
becomes
hotter and
hotter until
nuclear fusion
begins in the
core.
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The Formation of Stars Like the
Sun
When looking at just a few atoms, the
gravitational force is nowhere near strong
enough to overcome the random thermal motion.
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The Formation of Stars Like the
Sun
Stars go through a number of stages in the
process of forming from an interstellar cloud.
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Question 8
How do single
stars form within
huge clouds of
interstellar gas
and dust?
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a) Clouds fragment into smaller objects,
forming many stars at one time.
b) One star forms; other matter goes into
planets, moons, asteroids, & comets.
c) Clouds rotate & throw off mass until only
enough is left to form one star.
Question 8
How do single
stars form within
huge clouds of
interstellar gas
and dust?
a) Clouds fragment into smaller objects,
forming many stars at one time.
b) One star forms; other matter goes into
planets, moons, asteroids, & comets.
c) Clouds rotate & throw off mass until only
enough is left to form one star.
The theory of star formation predicts stars in a cluster
would form about the same time.
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The Formation of Stars Like
the Sun
Stage 1:
Interstellar cloud starts to contract, probably
triggered by shock or pressure wave from nearby
star. As it contracts, the cloud fragments into
smaller pieces.
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The Formation of Stars Like the
Sun
Stage 2:
Individual cloud fragments begin to collapse.
Once the density is high enough, there is no
further fragmentation.
Stage 3:
The interior of the fragment has begun heating,
and is about 10,000 K.
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The Formation of Stars Like
the Sun
The Orion Nebula is thought to contain
interstellar clouds in the process
of condensing,
as well as
protostars.
Orion Nebula Mosaic
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The Formation of Stars Like the
Sun
Stage 4:
The core of the cloud is
now a protostar, and
makes its first
appearance on the H–R
diagram.
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The Formation of Stars Like
the Sun
These jets are being emitted
as material condenses onto
a protostar.
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The Formation of Stars Like the
Sun
These protostars are in Orion.
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The Formation of Stars Like
the Sun
Planetary formation has begun, but the protostar
is still not in equilibrium – all heating comes from
the gravitational
collapse.
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The Formation of Stars Like the
Sun
The last stages can be
followed on the H–R
diagram:
The protostar’s
luminosity decreases
even as its
temperature rises
because it is
becoming more
compact.
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The Formation of Stars Like the
Sun
At stage 6, the core reaches 10 million K, and
nuclear fusion begins. The protostar has
become a star.
The star continues to contract and increase in
temperature, until it is in equilibrium. This is
stage 7: the star has reached the main
sequence and will remain there as long as it
has hydrogen to fuse in its core.
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Stars of Other Masses
This H–R diagram
shows the evolution
of stars somewhat
more and somewhat
less massive than the
Sun. The shape of the
paths is similar, but
they wind up in
different places on
the main sequence.
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Stars of Other Masses
If the mass of the original nebular fragment is
too small, nuclear fusion will never begin. These
“failed stars” are called brown dwarfs.
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Star Clusters
Because a single interstellar cloud can produce
many stars of the same age and composition,
star clusters are an excellent way to study the
effect of mass on stellar evolution.
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Star Clusters
This is a young star cluster called the Pleiades.
The H–R diagram of its stars is on the right. This
is an example of an open cluster.
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Star Clusters
This is a globular cluster – note the absence of
massive main-sequence stars, and the heavily
populated red giant region.
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Cluster Location
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Question 9
Very young stars in small
clusters of 10-100 members
are known as
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a) OB associations.
b) molecular cloud complexes.
c) aggregates.
d) globular clusters.
e) hives.
Question 9
Very young stars in small
clusters of 10-100 members
are known as
a) OB associations.
b) molecular cloud complexes.
c) aggregates.
d) globular clusters.
e) hives.
NGC 3603 is a newborn cluster of hot young blue
Type O and B stars – a perfect OB association.
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Star Clusters
These images are believed to show a star cluster
in the process of formation within the Orion
Nebula.
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Question 10
All stars in a stellar cluster
have roughly the same
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a) temperature.
b) color.
c) distance.
d) mass.
e) luminosity.
Question 10
All stars in a stellar cluster
have roughly the same
a) temperature.
b) color.
c) distance.
d) mass.
e) luminosity.
Stars in the Pleiades cluster vary in temperature, color, mass,
and luminosity, but all lie about 440 light-years away.
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Star Clusters
The presence of massive, short-lived O and B
stars can profoundly affect their star cluster,
as they can blow away dust and gas before it
has time to collapse.
This is a simulation
of such a cluster.
Carina Nebula
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Question 11
Stars are often born
within groups known as
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a) clans.
b) spiral waves.
c) aggregates.
d) clusters.
e) swarms.
Question 11
Stars are often born
within groups known as
a) clans.
b) spiral waves.
c) aggregates.
d) clusters.
e) swarms.
The Pleiades – a nearby open
cluster – is a group of relatively
young stars about 400 light-years
from the Sun.
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Question 12
Globular clusters are
typically observed
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a) in the plane of our Galaxy.
b) above or below the plane of
our Galaxy.
c) near to our Sun.
d) in the hearts of other
galaxies.
Question 12
Globular clusters are
typically observed
a) in the plane of our Galaxy.
b) above or below the plane of
our Galaxy.
c) near to our Sun.
d) in the hearts of other
galaxies.
Globular clusters orbit the center of the Milky Way, and are usually
seen above or below the galactic plane far from our Sun.
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