Astro Lecture 4 Flashcards
What did Einstein propose in 1905 to explain the photoelectric effect?
Einstein proposed that light consists of discrete entities called photons that have both wave-like and particle-like characteristics.
What are the primary subtypes of waves?
Pressure/density waves (e.g., sound), displacement waves (e.g., vibrating string), surface waves (some ocean waves), electromagnetic waves (light), and gravitational waves.
What is the relationship between wavelength (λ), frequency (ν), and the speed of light (c)?
λ = c/ν, or equivalently, c = λν.
What are the components of an electromagnetic wave and how are they oriented?
Electric and magnetic fields that oscillate perpendicular to each other and to the direction of motion.
What was the ‘ultraviolet catastrophe’ in classical physics?
Classical wave theory of light predicted atoms couldn’t exist and that there would be vastly too much ultraviolet light emitted - contradicting observations.
Why does photosynthesis only work with specific frequencies of light?
Because light consists of photons with discrete energy levels determined by frequency, rather than being a continuous wave that could deliver the same power at any frequency.
What are the two main settings where elements form in the universe?
The early universe during Big Bang Nucleosynthesis and in stars through stellar nucleosynthesis.
When did Big Bang Nucleosynthesis occur?
Approximately 3 minutes after the Big Bang.
Why did Big Bang Nucleosynthesis stop after producing mainly hydrogen and helium?
The universe expanded and cooled too quickly, becoming too cold for further nuclear reactions to occur.
What are the conditions in the core of our Sun?
Temperature of approximately 15 million K, pressure of about 250 billion atmospheres, and density roughly 9 times that of depleted uranium.
What is the significance of iron in stellar nucleosynthesis?
Iron is the most energetically stable nucleus; forming elements lighter than iron releases energy (exothermic), while forming elements heavier than iron requires energy input (endothermic).
How is energy released during nuclear fusion calculated?
Using Einstein’s equation $E = mc^2$, where the energy released equals the mass difference between reactants and products multiplied by the speed of light squared.
What is the fusion sequence that builds carbon in stars?
Three helium nuclei (³He) fuse to form carbon-12.
How are elements heavier than iron formed?
Primarily in supernovae explosions, where extreme energy and neutron-rich environments enable rapid neutron capture.
What is cosmic ray spallation?
A process where high-speed cosmic rays strike objects and produce a spray of lighter nuclei, forming some rare light elements.
What is the cycle of element dispersal in the universe?
Supernovae scatter elements → enriched nebulae form → new stars and planets condense → eventually enabling life.
Why are visible light wavelengths concentrated where they are in the spectrum?
Because the Sun’s blackbody radiation (at 5800K) peaks in the visible range, and our eyes evolved to detect the most abundant wavelengths.
How does the elemental composition of humans differ from that of the Sun?
Humans are primarily composed of oxygen, carbon, hydrogen, and nitrogen, while the Sun is overwhelmingly hydrogen and helium.
Why is carbon preferred over silicon in living organisms despite silicon’s abundance?
Because carbon chemistry is more flexible, allowing for the complex molecules needed for life.
What insight does the elemental ratio from Big Bang Nucleosynthesis provide?
It tells us about conditions in the universe just 3 minutes after the Big Bang, serving as our earliest probe of the early universe.
