Astronomy a beginners guide to the universe 8th CHaisson mcmillan chapter 11

Astronomy
A BEGINNER’S GUIDE
TO THE UNIVERSE
EIGHTH EDITION

CHAPTER 11

The Interstellar Medium
Lecture Presentation

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Chapter 11 The Interstellar Medium

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Units of Chapter 11

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Interstellar Matter
Star-Forming Regions
Dark Dust Clouds

Formation of Stars Like the Sun
Stars of Other Masses
Star Clusters
Summary of Chapter 11

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11.1 Interstellar Matter

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The interstellar medium consists of gas and dust.
Gas is atoms and small molecules, mostly hydrogen
and helium.

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Dust is more like soot or smoke, larger clumps of
particles.

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Dust absorbs light and
reddens light that gets through.

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This image shows distinct reddening of stars near the

edge of the dust cloud.

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11.1 Interstellar Matter

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Dust clouds absorb blue light preferentially;
spectral lines do not shift.

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11.2 Star-Forming Regions

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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

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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

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Nebula is a general term used for fuzzy
objects in the sky.

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Dark nebula: dust cloud
Emission nebula: glows, due to hot
stars

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Reflection nebula: light from imbedded
star bounces off of cloud particles

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11.2 Star-Forming Regions

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Emission nebulae generally glow red—this is the Hα 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

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How nebulae work

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11.2 Star-Forming Regions

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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

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Emission nebulae are made of hot, thin gas, which exhibits distinct emission lines.

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11.3 Dark Dust Clouds

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Average temperature of dark dust clouds is a few tens
of kelvins.

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These clouds absorb visible light (a), and emit radio
wavelengths (b).

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11.3 Dark Dust Clouds

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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

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The Horsehead Nebula is a particularly distinctive dark dust cloud.

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11.3 Dark Dust Clouds

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Interstellar gas emits low-energy radiation due to a transition in the hydrogen atom.

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11.3 Dark Dust Clouds

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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.

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Here, the red and green lines correspond to
different rotational transitions.

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11.3 Dark Dust Clouds

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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

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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

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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

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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

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Stage 1:

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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

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Stage 2:

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Individual cloud fragments begin to collapse. Once the density is high enough, there is no
further fragmentation.

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Stage 3:

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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

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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

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Stage 4:

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The core of the cloud is now a
protostar and makes its first
appearance on the H–R diagram.

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