What evidence supports the theory that there is a black hole at the center of our galaxy?
answer
The motions of the gas and stars at the center indicate that it contains a million solar masses within a region only about 1 parsec across
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What kinds of objects lie in the disk of our galaxy?
answer
gas and dust, old K and M stars, open clusters, O and B stars (all of the above)
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Compared with our Sun, most stars in the halo are?
answer
old, red, and dim and have fewer heavy elements
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Harlow Shapley concluded that the Sun was not in the center of the Milky Way Galaxy by?
answer
mapping the distribution of globular clusters in the galaxy.
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Where does most star formation occur in the Milky Way today?
answer
in the spiral arms.
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Why are Cepheid variables important?
answer
directly related to their true luminosities. hence, we can use cepheids as "standard candles" for distance measurements.
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What makes white-dwarf supernovae good standard candles?
answer
they are very bright, so they can be used to determine the distances to galaxies billions of light-years away and they should all have approximately the same luminosity.
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What do scientists mean by the critical density of the universe?
answer
has enough mass to contract again and a universe that will continue to expand forever.
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What is the most accurate way to determine the distance to a nearby galaxy?
answer
using cepheid variables
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Dr. X believes that the Hubble constant is Ho=55km/s/Mpc. Dr. Y believes it is Ho=80km/s/Mpc.
answer
Dr. X believes that the universe is older than Dr. Y believes
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What is a galactic wind?
answer
hot gas erupting into intergalactic space from a large superbubble
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Why is a dense cloud more likely to produce an elliptical galaxy than a spiral galaxy?
answer
the higher gas density forms stars more efficiently, so all the gas is converted into stars before a disk can form
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How many more stars does a starburst galaxy form, in one year, than the Milky Way?
answer
about a hundred
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Which of the following cannot be true of the very first stars formed in the Universe?
answer
they may have had rocky planets around them
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I observe a galaxy that is 100 million light-years away: what do i see?
answer
the light from the galaxy as it was 100 million years ago and it is redshifted
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Why isn't space expanding within systems such as our solar system or the milky way?
answer
their gravity is strong enough to hold them together against the expansion of the universe
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What evidence suggests that the Milky Way contains dark matter?
answer
We observe clouds of atomic hydrogen far from the galactic center orbiting the galaxy at unexpectedly high speeds, higher speeds than they would have if they felt only the gravitational attraction from objects that we can see.
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What is the evidence for an accelerating universe?
answer
White-dwarf supernovae are slightly dimmer than expected for a coasting universe.
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Why do we believe 90 percent of the mass of the Milky Way is in the form of dark matter?
answer
the orbital speeds of stars far from the galactic center are surprisingly high, suggesting that these stars are feeling gravitational effects from unseen matter in the halo.
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What does the universe look like on very large scales?
answer
Galaxies appear to be distributed in chains and sheets that surround great voids.
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What is Einstein's cosmological constant?
answer
a repulsive force that counteracts gravity and was introduced to allow for a static universe
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The Planck era refers to the time period?
answer
before the Planck time
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How many forces operated in the universe during the GUT era?
answer
two, gravity and the GUT force
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Why can't current theories describe what happened during the Planck era?
answer
we do not yet have a theory that links quantum mechanics and general relativity.
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Why did the era of nuclei end when the univers was about 300,000 years old?
answer
the universe had expanded and cooled to a temperature of about 3,000 K, cool enough for stable, neutral atoms to form
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Olbers' paradox is an apparently simple question, but its resolution suggests that the universe is finite in age. What is the question?
answer
Why is the sky dark at night?
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What are the two key observational facts that led to widespread acceptance of the Big Bang model?
answer
the cosmic background radiation and the high helium content of the universe.
question
Where do the photons in the cosmic background radiation originate?
answer
the end of the era of nuclei
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