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Question
Of normal matter, about 25% of it by mass is still primordial helium even today.Answer
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Related questions
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Describe the reasoning leading to the idea that the number of technological civilizations in the galaxy is equal to the average lifetime of a technological civilization.
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Name each of the factors in the Drake equation.
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Irregulars typically have very few blue stars.
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Quasar is short for what? Is this true of all of them?
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Contrast the sizes and membership of the Local Group and Virgo Cluster.
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Distinguish an E0 from an E7 galaxy.
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Hubble divided the spiral galaxies up in what six subtypes?
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If the central engine for our Milky Way is a black hole of 3 million solar masses, then it has an event horizon about ________ kilometers across.
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The minimum period of light variations are used to find the ________ of the central engine in galactic cores.
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Active galaxies emit most of their energy in ________ wavelengths than visible.
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A galaxy with a blueshift is moving ________ Earth.
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The Tully-Fisher relation uses the rotational speed of the galaxy to measure its ________.
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Hubble divided the spirals into normal and ________ categories.
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Galaxies that look like huge globular clusters are Hubble type ________.
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Our own Milky Way's Hubble class, according to the best data, is ________.
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The spiral arms are blue from the presence of Population ________ stars in them.
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When mass is accreted into a black hole, what fraction of the total mass-energy can be radiated away?
A) 0.007%
B) 0.08%
C) 1.4%
D) 20%
E) 50%
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Why is the energy source for active nuclei like Seyferts thought to be compact?
A) The sources appear to be single stars in photos.
B) Their energy appears to be non-stellar synchrotron radiation.
C) The light can vary over short time intervals.
D) We know the masses of the black holes that lurk there, and can find their radii.
E) They are all strong radio sources, with assigned frequencies by their sizes.
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In active galaxies, their central engines may be temporarily fed by
A) a sudden surge of star formation.
B) a series of supernovae around the core.
C) the sudden collapse of the core into a supermassive black hole.
D) a close encounter with a neighbor galaxy.
E) the fusion of helium into carbon in their cores.
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Describe the cycle of life and death we observe in the spiral arms.
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Compare the way an optical and radio astronomer would map the Galaxy.
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The Copernican Principle notes that we are nothing special, not at the center of anything. Apply it to the work of Harlow Shapley.
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How does metal content vary between young disk stars and older halo stars? Why?
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Why aren't there young stars in the halo?
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What advantage do radio astronomers have over optical ones in mapping the entire Milky Way?
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How did Herschel try to map our Galaxy 200 years ago, and why did he fail?
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Counting only the material between us and the galactic Center, we measure a mass of ________ billion Suns.
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The ________ is the spherical outer portion of the Galaxy, including the orbits of the globular clusters and most dark matter.
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Virtually the entire disk of the Galaxy has been mapped using the ________ wavelength.