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Chapter 18 Practice

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The "Local Bubble" was probably created by a nearby supervona in the distant past.
True
A large gas cloud in the interstellar medium that contains several type O and B stars would appear to us as
an emission nebula
Spectra of stars often show narrow absorption lines from interstellar matter. What type of interstellar matter produces theses?
Neutral Gas
Why are dark dust clouds largely misnamed?
They contain much more gas than dust.
In which wavelengths have the births of new stars been best mapped recently?
infrared
Which of the following describes the shape of dust particles, based on polarization of light?
rodlike
Most interstellar clouds are:
much bigger than our solar system
What feature of interstellar dust is inferred by the polarization of starlight?
its presence in molecular
Interstellar absorption lines are narrow primarily because:
the matter is at a low temperature, and atoms are almost still
Complex molecules in the interstellar medium are found
primarily in the dense dust clouds
The density of interstellar dust is very low, yet it still blocks starlight because
the dust particles are about the same size as the light waves they absorb
The most common molecule in a molecular cloud is:
molecular hydrogen, H2
Unlike atoms, molecules can:
vibrate and rotate
The overall dimming of straight by interstellar matter is called:
extinction
Neutral hydrogen is most obvious in the electromagnetic spectrum:
a 21 cm in the radio region
When an electron in H changes its spin from the same to the opposite discretion as the proton, it:
emits an X-ray photon
Charles Messier mapped the night sky and identified many objects now known to be emission nebulae in his search for objects that might be confused with:
comets
What are the very cold (about 20 K), dense clouds of gas thought to be the most massive objects in the Galaxy called?
molecular clouds
Unlike the ultraviolet photons originally emitted by the embedded stars of an emission nebula, the photons emitted by the recombination of electrons with atoms:
escape the nebula
Which of the following has NOT yet been observed in space
DNA
Emission nebula take up about how much of the Milky Way Galaxy
1%
Why is 21-cm radiation so important to the study of interstellar matter and the Galaxy?
Emitted by hydrogen, it passes through interstellar dust and lets us map the entire Galaxy
What do "forbidden" lines reveal about interstellar space?
There are unknown elements in space that do not correspond to elements found on Earth
What information does 21 cm radiation provide about the gas clouds
all of these
In the Milky Galaxy, gas and dust are found
everywhere
Ionization of hydrogen in HII regions is most visible at:
656.3 nm, Balmer alpha line for 3-2 election transition
Due to absorption of shorter wavelengths by interstellar dust clouds, distant stars appear:
redder
Interstellar dust clouds are best observed at what wavelength?
radio and infrared
What effect do even thin clouds of dust have on light passing through them?
It dims and reddens the light of all more distant stars
The gas destiny in an emission nebula is typically about how many particles per cc?
hundred
Some regions along the plane of the Milky Way appear dark because:
stars in that region are hidden by dark dust particles
Complex molecules in space are found:
inside dense dust clouds
The relative density of dust to gas is least in which place?
The atmosphere of Earth
What color is an H I region?
colorless; it emits in the radio region
Molecular hydrogen is not very useful for mapping molecular clouds because:
molecular hydrogen is essentially invisible in the radio portion of the spectrum
Interstellar gas is composed primarily of
90% hydrogen, 9% helium by weight
The "Local Bubble" is:
about 100 pcs. across and transparent to extreme ultraviolet radiation.
The polarization of light passing through the dust grains shows that:
The dust grains are elongated in shape and the dust grains are aligned by a week interstellar magnetic field
Astronomers use roman numerals to indicate the ionization state of a gas. Ionized hydrogen is indicated by which of the following symbols?
H II
The average temperature of the typical dark dust cloud is about:
100 K.
The "Local Bubble" was probably created by:
the magnetic fields of a Milky Way in our spiral arm
Molecular hydrogen can best be detected by ultraviolet observations.
True
The temperature inside an H II region is colder than inside an H I region.
False
The dark dust clouds can be explored better with infrared than with visible light.
True
Emission nebulae appear red because they are associated with cool red giant stars.
False
Emission nebulae are blue
False
H II region are usually found isolated from other stars or types of nebulae
False
Dark dust clouds radiate mainly in the radio portion of the electromagnetic spectrum.
True
Reflection nebulae usually appear red
False
There is as much mass in the voids between the stars as in the stars themselves
True
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