History Of Astronomy Chapter 10

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1. What is the "Island Universe" theory?

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It was proposed that elliptical nebulae are disk shaped star systems (Island Universes) and that we live on one such system

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2. Why did the Ancient Greeks use the word "galaxy" to describe the bright strip of light across the sky? What do we call this strip of light today? What part of our galaxy does this strip represent?

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The bright band of light seen across the night sky was named “galaxy” by the Ancient Greeks (Galaxy: “milky” ) Today we call it the milky way. Kant correctly explained: if we live in a disk shaped star system then the Milky Way is simply our view through the disk’s length

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3. What did William, Caroline and John Herschel produce together? What was Herschel attempting to prove from this research? Why was he not able to prove this?

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They produced a catalogue of over 5000 nebulae. They attempted to identify whether nebulae are distant star systems or nearby glowing gas clouds. Since there are a large variety of nebulae, they couldn’t come to a general conclusion about their composition.

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4. What did Herschel's map of the visible stars reveal about the shape of our galaxy?

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It revealed that our star system is disk-shaped.

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5. What did Lord Rosse discover when he looked at the elliptical-shaped nebulae through his Leviathan telescope?

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He discovered that many elliptical nebulae are spiral nebulae

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6. What is a variable star? What characterizes the Cepheid variables? Explain briefly why Cepheids pulsate.

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Variable stars are stars that fluctuate in brightness at regular time intervals. Cepheids are luminous variable stars which pulsate rapidly (at intervals of days to weeks). A Cepheid has a highly opaque surface, its outward photons push the surface outwards, which temporarily expands the star and reduces its opacity

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7. What are the SMC and LMC? What did Henrietta Leavitt discover when she observed Cepheids in the SMC? Why was this discovery so valuable? Who used her discovery to determine the boundaries of our galaxy?

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Large Magellanic Cloud and Small Magellanic Cloud. By assuming the SMC Cepheid's are equally distant from Earth, Leavitt found: the brighter the Cepheid, the longer its period (P). Therefore, by constructing the Period-Luminosity relation for Cepheid's with known distances, a measurement of a Cepheid's P yields its luminosity (and therefore its distance, which is the distance to its star system.

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8. What are the 3 components of our galaxy, according to Shapley's model? Where is the Sun located in this model? Where is the Galaxy's centre? In what 2 ways is this model incorrect?

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It is 300 000 ly in diameter, all nebulae are systems within our galaxy, our sun resides in the outskirts of its disk, the Milky Way centre is in Sagittarius

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9. Why was Shapley convinced that the spiral nebulae are within our galaxy's boundaries?

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He believed that if they weren’t contained in our galaxy, then we would be able to see them all around the sky. If they were beyond our boundaries, why would they be affected by where the Milky Way disk is?

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10. What is the Zone of Avoidance? What was Shapley's explanation for it? What was Curtis'? Who was correct?

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The Zone of avoidance is the middle band where our galaxy is contained and there are no spiral nebulae, Shapely believed that they were pushed from our disk by radiation pressure from its high density of stars. Curtis was convinced that our galaxy was a spiral galaxy and the other spirals were extra-galactic. Curtis was correct. Curtis correctly proposed: If the Mily Way’s disk contains a band of dust, then if spirals are outside our Galaxy, they aren’t seen in the Zone of Avoidance because they are obscured by our dust.

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11. What observation led Curtis to suspect that the disks of galaxies are filled with interstellar dust? Where does this dust come from?

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Curtis completed a high resolution survey of spiral nebulae, revealing obscuring matter (dust) in their disks. This dust comes from the ejecta of dying stars

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12. What did Vesto Slipher observe about the spectra of spirals?

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He observed large redshifts in the absorption lines of spirals, suggesting recessional speeds as high as 1000s of km/sec

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13. In the Shapley-Curtis Debate, what did Shapley argue? What did Curtis argue? What 4 pieces of evidence did Curtis present?

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Shapley argued that our galaxy is so enormas that it must be the entire universe and spiral nebulae are contained within it. Curtis argued that the spiral nebulae were island universes like our own galaxies. His four pieces of evidence were:
1. Spiral radical velocities are much larger than star velocities
2. Spiral spectra are indicative of star systems,
3. The faintness of what appear to be novae in the spirals suggest that they are distant star systems,
4. Spirals show evidence of obscuring matter in their disks; if the Milky Way is such a spiral, this explains the Zone of Avoidance

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14. Who finally resolved the Shapley-Curtis debate? Explain how he did this. What was NASA's tribute to this discovery?

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Edwin Hubble ended the debate. He found a Cepheid in M31, with the PL relation he found a distance to M31 of ~1 million ly, this proved that spirals are galaxies beyond the Milky Way

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15. Describe (briefly) the 4 types of galaxies in Hubble's Classification system? Which are the most numerous? Which are the least numerous? What are the two types of spiral galaxies? What type of galaxy is the Milky Way?

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1. Spiral (S) : ~30 of known galaxies, 60% of which are barred (SB; eg. The Milky Way). A flattened disk containing spiral structure and possibly a centre bulged. Barred spiral (SB- spiral with central bar. Sub classified as Sa – tightly
wound with bright bulge or Sc – loosely wound with faint bulge. Theory that they come from the merging of two galaxies
2. Lenticulars (S0): ~ 15% of known galaxies – a flattened disk and possibly a central bulge, but no spiral structure
3. Ellipticals (E): ~5% of known galaxies, an elongated spheroid, designated a number, the higher the number the more elongated the elliptical believed to be formed when two merging galaxies are equal in mass
4. Irregulars (I): ~50% of known galaxies the small dwarf irregulars (dIs) are by far the most numerous type of galaxy in the Universe

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16. What is the Local Group? How does the size of the Milky Way and M31 compare to other galaxies in the Local Group? How did the Milky Way and M31 get this way?

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The local group is a cluster of galaxies that contains about 30 galaxies, they are much larger than the other galaxies in the local group. They grew to this size through galactic cannibalism

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17. What is the Local Supercluster?

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Galaxy groups cluster into superclusters containing thousands of galaxies. The local group is part of the local supercluster, known as the Virgo supercluster

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18. What is Hubble's Law? What does the slope of Hubble's Law tell us? What does the inverse of the slope tell us?

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The larger a galaxy’s redshift (or recessional velocity), the further its distance. The rate at which the universe is spreading out is given by the slope of Hubble’s law. This is called Hubble’s Constant (H0). The inverse slope tells us the age of the universe

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19. Prior to the discovery of Hubble's Law, who had found theoretically that the Universe is expanding?

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Einstein theoretically found that it was expanding. He did not believe this was possible so he added a cosmological constant to “fix” this equation.

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20. Describe what Einstein referred to as the "biggest blunder" of his career.

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When Hubble showed the Universe is expanding, Einstein called his constant the biggest blunder of his career

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21. Describe the 4 possible fates of the Universe according to Einstein's cosmological equation, including the density and Cosmological Constant associated with each fate.

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1. If density of the universe (p) = critical density required for gravity to halt the initial expansion momentum (Pcrit) and cosmological constant=0 then the universe’s expansion will halt (big freeze)
2. If density of the universe > critical density required for gravity to halt the initial expansion momentum and cosmological constant = 0, the univer’s expansion will halt and contract (Big crunch)
3. If density of the universe < critical density required for gravity to halt the initial expansion momentum and cosmological constant= 0, universe will expand forever (big chill)
4. If density of universe is < or = to critical density required for gravity to halt the initial expansion momentum and cosmological constant >0, the universe will expand at an accelerating rate forever (Big rip)

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22. What is the Big Bang theory? What is the Steady State theory? What is the Steady State explanation for Hubble's Law?

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The big bang theory claims that the Universe began from a single point and is expanding with time, decreasing in density
Steady State: the density of the Universe is constant: as galaxies recede from each other, new galaxies take their place. The steady state explanation for Hubble’s Law proposed that just because galaxies are receding from each other it does not mean that density is decreasing, the new galaxies would preserve the density.

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23. Describe the 2 pieces of observational evidence for the Big Bang.

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1. Comparing of distant galaxies with nearby galaxies, showed that spacing of distant galaxies was more dense. Radio surveys of the most distant galaxies revealed that they are closer together than the nearby galaxies. This showed that the universe is expanding
2. Remnants of big bang were actually seen

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24. Describe the observational evidence that the expansion of the Universe is accelerating. What does this tell us about Hubble’s Constant?

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The Supernova cosmology Project : 1990s-2010s: a survey of extragalactic supernovae was used to measure distances to the most distant galaxies in order to determine the rate of the Universe’s expansion in the past
The results: Hubble’s Constant (H0) was lower in the past and therefore the universe is expanding at an accelerating rate toward a Big Chill or Big Rip

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25. What does a map of the CMB (Cosmic Microwave Background) give us a picture of, and why?

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It gives us the locations where structures of matter were forming after the Big Bang, it shows us galaxy formation

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26. What did WMAP tell us about the composition of the Universe? What effect is "dark energy" believed to be having on the Universe? How is dark energy now accounted for in Einstein's cosmology equation?

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Normal matter comprises 27% of the Universe, the other 73% is dark energy, which may be responsible for the Universe’s accelerated expansion. It is denoted by Einstein’s cosmological constant

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27. Describe the ways that dark matter has been detected in galaxies and in intracluster space.

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Outer stars in galaxies rotate faster than predicted by a galaxy's visible mass, suggesting the presence of intergalactic DM.
Gravitational lensing of background galaxies suggests the presence of dark matter in galaxies halos as well as in intracluster dark matter (i.e., in the space between the galaxies in clusters).