Astronomy Ch 18 Quiz Q&As – Flashcards
Unlock all answers in this set
Unlock answersquestion
In the Milky Way galaxy, gas and dust are found
answer
everywhere.
question
A "fuzzy" dark or light patch in the sky is called a
answer
nebula.
question
What do "forbidden" lines reveal about interstellar space?
answer
The density or pressure of this gas is much lower than can be produced in a laboratory.
question
Why are dark dust clouds largely misnamed?
answer
They contain much more gas than dust.
question
What are the very cold (about 20-K), dense clouds of gas thought to be the most massive objects in the Galaxy called?
answer
molecular clouds
question
What feature of interstellar dust is inferred by the polarization of starlight?
answer
its shape
question
Which of the following describes the shape of dust particles, based on polarization of light?
answer
rodlike
question
Unlike atoms, molecules can:
answer
both A and B (Vibrate and rotate)
question
The density of interstellar dust is very low, yet it still blocks starlight because
answer
the dust particles are about the same size as the light waves they absorb.
question
Which statement about the dark nebulae is true?
answer
They can be penetrated only with longer wavelengths such as radio and infrared.
question
A large gas cloud in the interstellar medium that contains several type O and B stars would appear to us as
answer
an emission nebula.
question
Some regions along the plane of the Milky Way appear dark because
answer
stars in that region are hidden by dark dust particles.
question
The Local Bubble was probably created by:
answer
a nearby supernova perhaps 300,000 years ago, brighter than the Full Moon.
question
Which of the following have not yet been observed in space?
answer
DNA
question
When an electron in H changes its spin from the same to the opposite direction as the proton, it
answer
emits a radio wave photon.
question
How does emission of long wavelength radio radiation from neutral hydrogen help us overcome the shortcomings of visible light in mapping out distant parts of our galaxy?
answer
It allows us to detect interstellar matter in regions where there is no light, allowing us to "see" what is located at all points in the galaxy, not just in the bright regions lit by hot, young stars.
question
What information does 21 cm radiation provide about the gas clouds?
answer
their density their motion their distribution their temperature --all of these
question
The ________ of light passing through thin dust clouds lets us map the Galaxy's magnetic field.
answer
polarization
question
Complex molecules in space are found:
answer
inside dense dust clouds.
question
Which statement is true about the interstellar medium?
answer
We know more about the gas than the dust.
question
Emission nebula take up about how much of the Milky Way galaxy?
answer
1%
question
Why is 21-cm radiation so important to the study of interstellar matter and the Galaxy?
answer
Emitted by hydrogen, it passes through interstellar dust and lets us to map the entire Galaxy.
question
What color is a H I region?
answer
colorless; it emits in the radio region
question
Which of these is not a consequence of dust in the interstellar medium?
answer
red light from the emission nebulae
question
Spectra of stars often show narrow absorption lines from interstellar matter. What type of interstellar matter produces these?
answer
neutral gas
question
Contrast the sizes of molecular clouds with emission nebulae. (SA)
answer
The emission nebula are only a few light years across, limited to those regions made hot enough to glow visibly by OB associations like the Orion Nebula. The giant molecular clouds are often hundreds of light years across, vast cold star forming regions that only occasionally are lit up enough to show hot spots like M-42 or other emission nebula, which are the tips of these cosmic icebergs.
question
The spectra of interstellar gas clouds show that they have the same basic composition as
answer
stars.
question
The most common molecule in a molecular cloud is:
answer
molecular hydrogen, H2.
question
Spectra of stars often show narrow absorption lines from interstellar matter. What type of interstellar matter produces these?
answer
neutral gas
question
The "Local Bubble" is:
answer
about 100 pcs. across and transparent to extreme ultraviolet radiation.
question
The overall dimming of starlight by interstellar matter is called
answer
extinction.
question
In which wavelengths have the births of new stars been best mapped recently?
answer
infrared
question
What is the primary visible color of an emission nebula?
answer
red due to ionized hydrogen atoms
question
A reflection nebula is caused by
answer
starlight scattered by dust particles.
question
What two things are needed to create an emission nebulae?
answer
hot stars and interstellar gas, particularly hydrogen
question
Most interstellar clouds are
answer
much bigger than our solar system.
question
Which statement is true about the interstellar medium?
answer
We know more about the gas than the dust.
question
Why are so few molecules observed in visible light? Where are most observed, and why? (SA)
answer
Few molecules, such as TiO, are durable enough to exist in the photospheres of the coolest stars and thus be visually detected. Most less durable molecules can exist only at much colder temperatures, where they give off infrared, microwave, or even radio emissions from their rotational and vibrational transitions.
question
Interstellar absorption lines are narrow primarily because:
answer
the matter is at a low temperature, atoms are almost still.
question
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
answer
comets.
question
Which is the least dense?
answer
interstellar dust
question
Unlike the ultraviolet photons originally emitted by the embedded stars of an emission nebula, the photons emitted by the recombination of electrons with atoms
answer
escape the nebula.
question
How does emission of long wavelength radio radiation from neutral hydrogen help us overcome the shortcomings of visible light in mapping out distant parts of our galaxy? (SA)
answer
It allows us to detect interstellar matter in regions where there is no light, allowing us to "see" what is located at all points in the galaxy, not just in the bright regions lit by hot, young stars.
question
Explain how "forbidden" lines appear in spectra of emission nebulae. (SA)
answer
These lines correspond to excited states that require a very long time to decay and emit a photon. In Earth labs, the gas density cannot be made low enough to allow this transition before a collision knocks the atom into another state. In the nebula, though, the density is so low that the transition has a chance to proceed.
question
Complex molecules in the interstellar medium are found
answer
inside dense dust clouds.
question
What effect does even thin clouds of dust have on light passing through them?
answer
It dims and reddens the light of all more distant stars.
question
The gas density in an emission nebula is typically about how many particles per cc?
answer
hundred
question
We look toward a cool star. How can we determine that there is an interstellar dust cloud between us and the star? (SA)
answer
The temperature of the star is determined by spectral type and photometrically using the B and V filters. If there is no intervening dust then the two temperature determinations will agree. If there is dust, the photometrically determined temperature will appear lower than the temperature indicated by the spectral type because the starlight has been reddened. Dust cannot affect the spectral type.
question
The average temperature of the typical dark dust cloud is about
answer
100 K.
question
Ionization of hydrogen in HII regions is most visible at:
answer
656.3 nm, Balmer alpha line for 3-2 electron transition.
