Chapter 11 The Interstellar Medium 2017 Pearson Education Inc Units of Chapter 11 Interstellar Matter StarForming Regions Dark Dust Clouds Formation of Stars Like the Sun Stars of Other Masses ID: 634312
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Chapter 11 The Interstellar Medium
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Units of Chapter 11
Interstellar Matter
Star-Forming Regions
Dark Dust Clouds
Formation of Stars Like the Sun
Stars of Other MassesStar ClustersSummary of Chapter 11
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11.1 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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11.1 Interstellar Matter
Dust clouds absorb blue light preferentially; spectral lines do not shift.
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11.2 Star-Forming Regions
This is the central section of the Milky Way Galaxy, showing several nebulae, areas of star formation.
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11.2 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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11.2 Star-Forming Regions
Nebula
is a general term used for fuzzy objects in the sky.
Dark nebula
: dust cloudEmission nebula: glows, due to hot stars
Reflection nebula
: light from imbedded star bounces off of cloud particles
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11.2 Star-Forming Regions
Emission nebulae generally glow red—this is the H
α
line of hydrogen.
The dust lanes visible in Figure 11.7 are part of the nebula, and are not due to intervening clouds.
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11.2 Star-Forming Regions
How nebulae work
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11.2 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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11.2 Star-Forming Regions
Emission nebulae are made of hot, thin gas, which exhibits distinct emission lines.
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11.3 Dark Dust Clouds
Average temperature of dark dust clouds is a few tens of kelvins.
These clouds absorb visible light
(a),
and emit radio wavelengths
(b).
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11.3 Dark Dust Clouds
The central portion of this cloud is very dark
and can be seen only by its obscuration of the background stars. Nearby are reflection and emission nebulae; M4 is a globular star cluster.
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11.3 Dark Dust Clouds
The Horsehead Nebula is a particularly distinctive dark dust cloud.
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11.3 Dark Dust Clouds
Interstellar gas emits low-energy radiation due to a transition in the hydrogen atom.
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11.3 Dark Dust Clouds
This is a contour map of H
2
CO 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.
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11.3 Dark Dust Clouds
These are carbon monoxide–emitting clouds in the outer Milky Way, probably corresponding to regions of star formation.
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11.4 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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11.4 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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11.4 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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11.4 The Formation of Stars Like the Sun
Stage 1
:
Interstellar cloud starts to contract, probably triggered by shock or pressure wave from a nearby star. As it contracts, the cloud fragments into smaller pieces.
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11.4 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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11.4 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
.
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11.4 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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11.4 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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11.4 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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11.4 The Formation of Stars Like the Sun
At
stage 6
, the core reaches 10 million kelvins, 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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11.4 The Formation of Stars Like the Sun
These jets are being emitted as material condenses onto a
protostar
.
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11.4 The Formation of Stars Like the Sun
These
protostars
are in Orion.
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11.5 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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11.5 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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11.6 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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11.6 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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11.6 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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11.6 Star Clusters
These images are believed to show a star cluster in the process of formation within the Orion Nebula.
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11.6 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.
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Summary of Chapter 11
Interstellar medium is made of gas and dust.
Emission nebulae are hot, glowing gas associated with the formation of large stars.
Dark dust clouds, especially molecular clouds, are very cold. They may seed the beginnings of star formation.
Dark clouds can be studied using the 21-cm emission line of molecular hydrogen.
Star formation begins with fragmenting, collapsing clouds of dust and gas.
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Summary of Chapter 11, cont.
The cloud fragment collapses due to its own gravity, and its temperature and luminosity increase. When the core is sufficiently hot, fusion begins.
Collapsing cloud fragments and
protostars
have been observed.
Mass determines where a star falls on the main sequence.One cloud typically forms many stars, as a star cluster.
© 2017 Pearson Education, Inc.