Milky Way Galaxy: Structure, Composition, and Cosmology
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44 Terms
What type of galaxy is the Milky Way?
Barred spiral galaxy (SBb/SBc)
How many stars are in the Milky Way?
~100 billion (10¹¹)
What are the three main components of the Milky Way?
Disk, bulge, halo
Where does most star formation occur in the Milky Way?
In the spiral arms (disk)
How do disk stars move?
Nearly circular orbits in the galactic plane with slight vertical motion
How do bulge stars move?
Random, elliptical orbits near the center
How do halo stars move?
Highly elliptical, randomly oriented orbits
Which region of a galaxy has the most ordered motion?
Disk
What is the star-gas-star cycle?
Stars form, evolve, return material, and new stars form
What are elements heavier than helium called in astronomy?
Metals
Why are younger stars more metal-rich?
They form from gas enriched by previous generations of stars
Which component of a galaxy forms first: the halo or the disk?
Halo
Why does a disk form from a collapsing cloud?
Conservation of angular momentum
Why do halo stars have low metallicity?
They formed early before heavy elements existed
Why might a galaxy not form a disk?
No initial rotation
How can elliptical galaxies form from spirals?
Galaxy mergers
What is at the center of the Milky Way?
Supermassive black hole
What is the mass of the black hole at the center of the Milky Way?
~4 million solar masses
What is the key evidence for the supermassive black hole at the galactic center?
Fast orbits of nearby stars
What are the main galaxy types?
Spiral, barred spiral, elliptical, irregular
What color indicates active star formation in a galaxy?
Blue
Where are elliptical galaxies most common?
Dense clusters
What is a standard candle?
An object with known luminosity
What are Cepheid variables used for?
Measuring distances to nearby galaxies
What is the best standard candle for very distant galaxies?
Type Ia (white dwarf) supernovae
What is Hubble's Law?
v = H₀ × d
What does Hubble's Law imply about the universe?
The universe is expanding
What is the relationship between a galaxy's distance and its speed?
Farther galaxies move away faster
What does the Big Bang describe?
Origin of the universe from a hot, dense state
What formed during Big Bang nucleosynthesis?
Hydrogen and helium nuclei
What is the Cosmic Microwave Background (CMB)?
Leftover radiation from the early universe
What are the three key pieces of evidence for the Big Bang?
Expansion, CMB, and hydrogen/helium abundance
What percentage of the universe is dark energy?
~68%
What percentage of the universe is dark matter?
~27%
What percentage of the universe is ordinary matter?
~5%
What is dark matter?
Invisible matter detected via gravity
What do flat rotation curves show?
More mass than visible matter
What is gravitational lensing?
Light bending due to mass
Why is hot gas considered evidence for dark matter?
It needs extra gravity to stay bound
What is dark energy?
Unknown energy causing accelerated expansion
What observation revealed the existence of dark energy?
Type Ia supernovae
Why is the universe considered flat?
Total density is approximately equal to critical density
What is the most likely fate of the universe?
Continued accelerating expansion
What is the Big Freeze?
A cold, dark, isolated universe