Astronomy Unit 3 – Flashcards
Unlock all answers in this set
Unlock answersquestion
Why are Cepheid variables important?
answer
Cepheids are pulsating variable stars, and their pulsation periods are directly related to their true luminosities. Hence, we can use Cepheids as "standard candles" for distance measurements.
question
Based on current estimates of the value of Hubble's constant, how old is the universe?
answer
between 12 and 16 billion years old
question
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
question
When we observe a distant galaxy whose photons have traveled for 10 billion years before reaching Earth, we are seeing that galaxy as it was when the universe was
When
answer
4 billion years old
question
Why do virtually all the galaxies in the universe appear to be moving away from our own?
answer
Observers in all galaxies observe a similar phenomenon because of the universe's expansion.
question
Which region of the early universe was most likely to become a galaxy?
answer
a region whose matter density was higher than average
question
Which of the following does not provide strong evidence for the Big Bang theory?
answer
observations of the amount of hydrogen in the universe
question
Which of the following statements about the cosmic background radiation is not true?
answer
It is the result of a mixture of radiation from many independent sources, such as stars and galaxies.
question
How was Edwin Hubble able to use his discovery of a Cepheid in Andromeda to prove that the "spiral nebulae" were actually entire galaxies?
answer
From the period-luminosity relation for Cepheids, he was able to determine the distance to Andromeda and show that it was far outside the Milky Way Galaxy.
question
What is Hubble's law?
answer
The recession velocity of a galaxy is directly proportional to its distance from us.
question
Why did the era of nuclei end when the universe 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.
question
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.
question
What kinds of atomic nuclei formed during the era of nucleosynthesis?
answer
hydrogen and helium and trace amounts of deuterium and lithium
question
Which model of the universe gives the youngest age for its present size?
answer
a recollapsing universe
question
Why do we expect the cosmic background radiation to be almost, but not quite, the same in all directions?
answer
The overall structure of the universe is very uniform, but the universe must have contained some regions of higher density in order for galaxies to form.
question
Why can't we see past the cosmological horizon?
answer
Beyond the cosmological horizon, we are looking back to a time before the universe had formed.
question
Dark energy has been hypothesized to exist in order to explain
answer
observations suggesting that the expansion of the universe is accelerating.
question
Approximately how long did the era of nucleosynthesis last?
answer
5 minutes
question
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
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.
