Cosmology and Stellar Evolution Practice Flashcards

A comprehensive set of practice questions covering the Big Bang timeline, cosmological evidence, stellar classification, and nucleosynthesis processes.

What are the core differences between the Steady State Model (SSM) and the Big Bang Theory (BBT) regarding the expansion of the universe?

The SSM proposes the universe has always existed and maintains constant matter density by continuously creating new matter as it expands. The BBT proposes the universe began ~$13.8\,billion$ years ago as a hot, dense singularity and its density decreases as it expands and cools.

What characterizes the Planck Epoch in the Big Bang timeline?

Occurring from $10^{-43} \rightarrow 10^{-36}\,s$ at temperatures $>10^{32}\,K$, all four fundamental forces were unified as one, and the universe was a singularity that current theories of physics cannot explain.

In what epoch did the universe undergo exponential expansion by a factor of at least $10^{26}$?

The Inflationary epoch, occurring between $10^{-36}\,s$ and $10^{-32}\,s$ at temperatures from $10^{28}$ to $10^{15}\,K$.

What is the significance of the expansion speed in the Quark epoch?

The electroweak force split into electromagnetic and weak forces, and particles like quarks, electrons, and neutrons appeared in large numbers, though it was still too hot for quarks to bind together.

How does the Hadron Epoch explain the presence of matter in the current universe?

During this era ($10^{-6} \rightarrow 1\,s$), an asymmetry existed where for every $1\,billion$ antimatter particles, there was $1\,billion\,and\,one$ matter particle, leaving matter behind after annihilation.

What were the predicted mass abundances of elements after Big Bang Nucleosynthesis (BBN)?

Hydrogen (~$75\%$), Helium-4 ($25\%$), and trace amounts of Lithium and Deuterium.

What event led to the creation of the Cosmic Microwave Background Radiation (CMBR)?

Recombination (or Decoupling) at ~$380,000\,years$, where the universe cooled to ~$3000\,K$, allowing electrons and nuclei to form neutral atoms and making the universe transparent to photons.

What are the three key pieces of evidence for the Big Bang Theory?

1. Hubble’s Law (galactic redshift). 2. Cosmic Microwave Background Radiation (CMBR). 3. Relative abundance of light elements ($75:25$ Hydrogen to Helium ratio).

State Hubble's Law as an equation and define its variables.

$v = H_0 \times d$, where $v$ is the recessional velocity, $d$ is the distance, and $H_0$ is the Hubble constant.

How did Hubble’s observations of galactic spectra lead to the discovery of an expanding universe?

Hubble observed that light from distant galaxies was red-shifted (Doppler effect), meaning they move away from the observer, and that the redshift increases with distance.

What is the formula to estimate the age of the universe from Hubble's constant?

$$\text{Age} = \frac{1}{H_0} \approx 13.8\,billion\,years$$

How is energy released in nuclear fusion reactions like those in stars?

Energy is released because the mass of the products is less than the mass of the reactants (mass defect), quantified by Einstein’s equation $E = \Delta mc^2$.

Define a Black Body.

An ideal object that absorbs all incident EMR (no reflection) and acts as an ideal emitter, emitting a continuous spectrum depending only on its temperature.

What are the two main points of Wien’s Law?

1. As temperature increases, the intensity of all wavelengths increases. 2. The peak wavelength ($\lambda_{peak}$) decreases, shifting the curve toward shorter wavelengths and higher energy.

Match the spectrum type to its appearance: Continuous, Emission, and Absorption.

Continuous: Unbroken band of all colors. Emission: Bright, colored lines on a dark background. Absorption: Continuous spectrum with dark lines missing at specific wavelengths.

List the spectral classification of stars from hottest to coolest.

O, B, A, F, G, K, M.

What is the formula for Luminosity ($L$) and what factors determine it?

$L \propto AT^4$. It is determined by the star's surface area ($A$) and its surface temperature ($T$).

What defines the Main Sequence stage on an H-R Diagram?

Stars spend the majority of their lifetime here fusing hydrogen into helium in their cores; higher mass results in higher luminosity and temperature but a shorter lifetime.

What happens when a star reaches the 'Turnoff Point' on a Main Sequence?

The star exhausts its hydrogen fuel and breaks away from the main-sequence line to become a red giant.

Difference between the Proton-Proton (P-P) Chain and the CNO Cycle?

The P-P chain dominates in low-to-middle mass stars ($\le 1.3M_{Sun}$) at lower ignition temperatures ($1.0 \times 10^7\,K$), while the CNO cycle dominates in stars with mass $> 1.3M_{Sun}$ at higher temperatures ($1.5 \times 10^7\,K$).

How does Carbon-12 ($^{12}_6C$) function in the CNO cycle?

It acts as a catalyst, being captured by a proton at the start and regenerated at the end of the cycle with no net change in its amount.

What is the Triple-Alpha Process?

The fusion of three Helium-4 (alpha) nuclei to form one Carbon nucleus, occurring in red giants when cores reach temperatures of ~$10^8\,K$.

Why does fusion typically stop once an iron ($Fe$) core is formed in supermassive stars?

Fusing elements heavier than iron consumes energy rather than releasing it (iron is the most stable configuration), so it cannot provide the pressure to balance gravity’s inward pull.

How are elements heavier than iron produced?

They are forged during the massive energy release of a supernova explosion, which also disperses these elements into space.

Convert the energy released per P-P chain ($26.73\,MeV$) to Joules.

$\Delta E = \Delta m \text{ (in kg)} \times c^2$. For a mass defect of $(4.032 - 4.003)\,u$, $\Delta E = 4.34 \times 10^{-12}\,J$.