Transcript Chapter 11 The Interstellar Medium

Chapter 11 The Interstellar Medium
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
Summary of Chapter 11
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.
Dust clouds absorb blue
light preferentially; spectral
lines do not shift.
11.2 Star-Forming Regions
This is the central section of the Milky Way
Galaxy, showing several nebulae, areas of star
formation.
These nebulae are very large and have very low
density; their size means that their masses are
large despite the low density.
“Nebula” is a general term used for fuzzy objects
in the sky.
Dark nebula:
dust cloud
Emission
nebula:
glows, due to
hot stars
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.
How nebulae work
There is a
strong
interaction
between the
nebula and
the stars
within it; the
fuzzy areas
near the
pillars are
due to
photoevaporation.
Emission nebulae are made of hot, thin gas,
which exhibits distinct emission lines.
11.3 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).
This cloud is very dark, and can be seen only by
its obscuration of the background stars. This
image is the same cloud, but in the infrared.
The Horsehead Nebula is a particularly
distinctive dark dust cloud.
Interstellar gas emits low-energy radiation, due
to a transition in the hydrogen atom.
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.
11.3 Dark Dust Clouds
These are carbon monoxide-emitting clouds in the outer
Milky Way, probably corresponding to regions of star
formation.
21-cm radiation has yielded
important information about
A. the density of helium in the
universe.
B. the physical structure of our
galaxy.
C. the prevalence of water in the
universe
D. .the spin-flip propensities of
methyl alcohol (ch2oh) .
Dark clouds are best studied
through examination of
A. interstellar absorption lines in
the spectra of distant stars.
B. Balmer emission lines.
C. radio waves emitted by
molecules.
D. ultraviolet radiation emitted by
the gas.
As an object contracts, its rate
of rotation
A. stays the same.
B. slows down.
C. speeds up.
D. may or may not change.
We cannot see the nucleus of
our galaxy because
A. over 32,000 light years, the
photons are too diffuse for us
to receive a coherent picture.
B. it has been consumed by a
gigantic black hole.
C. it is obscured by clouds of
dust and gas.
D. it spins too fast.
Interstellar 21-cm radiation is
emitted by
A. water.
B. methyl alcohol.
C. helium.
D. hydrogen.
What effect does interstellar dust
have on the magnitudes and
colors of a star?
A. Dims the star only.
B. Makes the star appear redder
only.
C. Makes the star dimmer and
redder.
D. No change.
How can interstellar dust be
detected?
A. Dark regions of fewer stars
in the milky way.
B. Stars that look redder than
their spectral type.
C. Bluish nebulas around hot
stars.
D. All of the above.
E. None of the above.
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.
Stars go through a number of stages in the
process of forming from an interstellar cloud.
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.
When looking at just a few atoms, the
gravitational force is nowhere near strong
enough to overcome the random thermal motion.
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.
Stage 4:
The core of the
cloud is now a
protostar, and
makes its first
appearance on
the H–R diagram.
Planetary formation has begun, but the protostar
is still not in equilibrium – all heating comes from
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.
These jets are being emitted
as material condenses onto
a protostar.
These protostars are in Orion.
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.
If the mass of the original nebular fragment is
too small, nuclear fusion will never begin. These
“failed stars” are called brown dwarfs.
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.
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.
This is a globular cluster – note the absence of
massive main-sequence stars, and the heavily
populated red giant region.
These images are believed to show a star cluster
in the process of formation within the Orion
Nebula.
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.
When a star first appears on the
H-R diagram, it moves
A. up.
B. down.
C. to the right.
D. to the left.
A forming star is first
detectable as
A. a new star in an ordinary
field of stars.
B. a bright region in an
otherwise dark cloud.
C. an infrared emitting region
in an interstellar cloud.
D. a contracting cloud of gas.
A star reaches the main
sequence when
A. it starts to collapse.
B. it is a proto star.
C. nuclear reactions start.
D. it starts to shine.
When a star first appears on
the H-R diagram it is
A. cool and faint.
B. cool and bright.
C. hot and faint.
D. on the main sequence.
As a star is forming by the
condensing of gases, the
gases
A. cool as they fall.
B. heat up as they fall.
C. stay about the same
temperature.
D. any of the above, depending
upon the mass involved.
As a new star evolves from cool
dust and gas to a hot star, the peak
wavelength of its spectrum of
electromagnetic radiation will
A. increase from visible to infrared
wavelengths
B. .remain the same.
C. change from the infrared to the
visible wavelengths.
D. change from the ultraviolet to the
visible range.
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 cloud of dust and gas.
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.