Astronomy Ch 18 Quiz Q&As – Flashcards
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What are the very cold (about 20-K), dense clouds of gas thought to be the most massive objects in the Galaxy called?
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molecular clouds
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What feature of interstellar dust is inferred by the polarization of starlight?
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its shape
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Which of the following describes the shape of dust particles, based on polarization of light?
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rodlike
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Unlike atoms, molecules can:
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both A and B (Vibrate and rotate)
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The density of interstellar dust is very low, yet it still blocks starlight because
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the dust particles are about the same size as the light waves they absorb.
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Which statement about the dark nebulae is true?
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They can be penetrated only with longer wavelengths such as radio and infrared.
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A large gas cloud in the interstellar medium that contains several type O and B stars would appear to us as
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an emission nebula.
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Some regions along the plane of the Milky Way appear dark because
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stars in that region are hidden by dark dust particles.
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The Local Bubble was probably created by:
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a nearby supernova perhaps 300,000 years ago, brighter than the Full Moon.
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Which of the following have not yet been observed in space?
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DNA
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When an electron in H changes its spin from the same to the opposite direction as the proton, it
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emits a radio wave photon.
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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?
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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.
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What information does 21 cm radiation provide about the gas clouds?
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their density their motion their distribution their temperature --all of these
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The ________ of light passing through thin dust clouds lets us map the Galaxy's magnetic field.
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polarization
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Complex molecules in space are found:
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inside dense dust clouds.
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Which statement is true about the interstellar medium?
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We know more about the gas than the dust.
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Emission nebula take up about how much of the Milky Way galaxy?
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1%
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Why is 21-cm radiation so important to the study of interstellar matter and the Galaxy?
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Emitted by hydrogen, it passes through interstellar dust and lets us to map the entire Galaxy.
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What color is a H I region?
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colorless; it emits in the radio region
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Which of these is not a consequence of dust in the interstellar medium?
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red light from the emission nebulae
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Spectra of stars often show narrow absorption lines from interstellar matter. What type of interstellar matter produces these?
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neutral gas
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Contrast the sizes of molecular clouds with emission nebulae. (SA)
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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.
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The spectra of interstellar gas clouds show that they have the same basic composition as
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stars.
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The most common molecule in a molecular cloud is:
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molecular hydrogen, H2.
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Spectra of stars often show narrow absorption lines from interstellar matter. What type of interstellar matter produces these?
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neutral gas
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The "Local Bubble" is:
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about 100 pcs. across and transparent to extreme ultraviolet radiation.
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The overall dimming of starlight by interstellar matter is called
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extinction.
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In which wavelengths have the births of new stars been best mapped recently?
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infrared
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What is the primary visible color of an emission nebula?
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red due to ionized hydrogen atoms
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A reflection nebula is caused by
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starlight scattered by dust particles.
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What two things are needed to create an emission nebulae?
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hot stars and interstellar gas, particularly hydrogen
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Most interstellar clouds are
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much bigger than our solar system.
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Which statement is true about the interstellar medium?
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We know more about the gas than the dust.
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Why are so few molecules observed in visible light? Where are most observed, and why? (SA)
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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.
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Interstellar absorption lines are narrow primarily because:
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the matter is at a low temperature, atoms are almost still.
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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
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comets.
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Which is the least dense?
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interstellar dust
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Unlike the ultraviolet photons originally emitted by the embedded stars of an emission nebula, the photons emitted by the recombination of electrons with atoms
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escape the nebula.
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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)
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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.
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Explain how "forbidden" lines appear in spectra of emission nebulae. (SA)
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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.
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Complex molecules in the interstellar medium are found
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inside dense dust clouds.
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What effect does even thin clouds of dust have on light passing through them?
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It dims and reddens the light of all more distant stars.
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The gas density in an emission nebula is typically about how many particles per cc?
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hundred
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We look toward a cool star. How can we determine that there is an interstellar dust cloud between us and the star? (SA)
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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.
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The average temperature of the typical dark dust cloud is about
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100 K.
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Ionization of hydrogen in HII regions is most visible at:
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656.3 nm, Balmer alpha line for 3-2 electron transition.
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