Chapter 11 The Interstellar Medium

Copyright © 2010 Pearson Education, Inc.Copyright © 2010 Pearson Education, Inc.

Chapter 11The Interstellar

Medium

Copyright © 2010 Pearson Education, Inc.

http://apod.nasa.gov/apod/image/1110/Ngc896dBig_fera.jpg


Units of Chapter 11Interstellar MatterStar-Forming RegionsDark Dust CloudsThe Formation of Stars Like the SunStars of Other MassesStar Clusters


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.


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 scatter or block the shorter wavelengths.


Interstellar MatterThe 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.


Interstellar Matter

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


Question 2

When a star’svisible 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.


Question 2

When a star’svisible 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.


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.


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.


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


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.


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.


Star-Forming RegionsThese nebulae are very large and have very low density; their size means that their masses are large despite the low density.


Star-Forming RegionsThis is the central section of the Milky Way Galaxy, showing several nebulae, areas of star formation.


Question 5a) 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.

The reddish color of emission nebulae indicates that


Question 5a) 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.

The reddish color of emission nebulae indicates that

Glowing hydrogen gas emits red light

around the Horsehead

nebula.


Star-Forming RegionsEmission 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.


Star-Forming RegionsHow nebulae work


Star-Forming RegionsThere is a strong interaction between the nebula and the stars within it; the fuzzy areas near the pillars are due to photoevaporation.


Star-Forming RegionsEmission nebulae are made of hot, thin gas, which exhibits distinct emission lines.


Tarantula Nebula


Dark Dust CloudsAverage temperature of dark dust clouds is a few tens of kelvins.These clouds absorb visible light (left), and emit radio wavelengths (right).


Dark Dust CloudsThis cloud is very dark, and can be seen only because of the background stars.


Dark Dust CloudsThe Horsehead Nebula is a particularly distinctive dark dust cloud.


Dark Dust CloudsInterstellar gas emits low-energy radiation, due to a transition in the hydrogen atom.


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.


Question 6

21-centimeter radiation is important because

Cool atomic hydrogen gas produces 21-cm radio radiation

as its electron “flips” its direction of spin.

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.


Dark Dust CloudsThis 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)


Dark Dust CloudsThese are CO (carbon monoxide) emitting clouds in the outer Milky Way, probably corresponding to regions of star formation.


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.


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.


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.

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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Stars go through a number of stages in the process of forming from an interstellar cloud.


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.


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.



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.



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.



The Orion Nebula is thought to contain interstellar clouds in the process of condensing, as well as protostars.

Orion Nebula Mosaic



Stage 4:The core of the cloud is now a protostar, and makes its first appearance on the H–R diagram.



These jets are being emitted as material condenses onto a protostar.



These protostars are in Orion.



Planetary formation has begun, but the protostar is still not in equilibrium – all heating comes from the gravitational collapse.



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.



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.


Stars of Other MassesThis 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.


Stars of Other MassesIf the mass of the original nebular fragment is too small, nuclear fusion will never begin. These “failed stars” are called brown dwarfs.


Star ClustersBecause 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.


Star ClustersThis 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.


Star ClustersThis is a globular cluster – note the absence of massive main-sequence stars, and the heavily populated red giant region.


Cluster Location


Question 9

a) OB associations.b) molecular cloud complexes.c) aggregates.d) globular clusters.e) hives.

Very young stars in small clusters of 10-100 members are known as


Question 9

Very young stars in small clusters of 10-100 members are known as

NGC 3603 is a newborn cluster of hot young blue Type O and B stars – a perfect OB association.

a) OB associations.b) molecular cloud complexes.c) aggregates.d) globular clusters.e) hives.


Star ClustersThese images are believed to show a star cluster in the process of formation within the Orion Nebula.


Question 10

All stars in a stellar cluster have roughly the same

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.


Star ClustersThe 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


Question 11

Stars are often born within groups known as

a) clans.b) spiral waves.c) aggregates.d) clusters.e) swarms.


Question 11

The Pleiades – a nearby open cluster – is a group of relatively

young stars about 400 light-years from the Sun.

Stars are often born within groups known as

a) clans.b) spiral waves.c) aggregates.d) clusters.e) swarms.


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.


Question 12

Globular clusters are typically observed

Globular clusters orbit the center of the Milky Way, and are usually seen above or below the galactic plane far from our Sun.

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.

Chapter 11 The Interstellar Medium

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