Quarks to Cosmos Midterm 2
LECTURE 9 — Stars & the Sun
Q: What are the stellar spectral classes and what does the order mean?
A: O, B, A, F, G, K, M. Ordered by decreasing temperature. Originally alphabetical by spectral line strength.
Q: What is the Sun’s spectral class?
A: G2V.
Q: Where are red giants, main sequence, and white dwarfs on the HR diagram?
A: Red giants = upper right; Main sequence = diagonal; White dwarfs = lower left.
Q: What determines if a star is on the main sequence?
A: Hydrogen fusion in the core.
Q: How does luminosity depend on mass?
A: L ∝ M^3–4.
Q: How does lifetime depend on mass?
A: Lifetime ∝ 1 / M^2.5.
Q: What are the regions of the Sun?
A: Core (fusion), Radiative zone, Convective zone, Photosphere, Chromosphere, Corona.
Q: What is hydrostatic equilibrium?
A: Gravity inward balanced by pressure outward.
Q: What powers the Sun?
A: Proton-proton chain: H → He + energy. Equation: E = mc².
Q: Other products of the pp-chain?
A: Neutrinos.
Q: How many solar neutrinos reach Earth?
A: ~10^38 per second.
Q: Mass lost by Sun per second?
A: ~4 × 10^9 kg/s.
Q: Age of the Sun?
A: ~4.6 billion years.
LECTURE 10 — Standard Model & Neutrinos
Q: What is the Standard Model?
A: Theory of fundamental particles and forces (except gravity).
Q: How many generations of matter?
A: Three; differ in mass and stability.
Q: Force carriers?
A: Photon (EM), W/Z (weak), gluon (strong), Higgs.
Q: Difference between quarks and leptons?
A: Quarks feel strong force; leptons do not.
Q: What is a neutrino?
A: Neutral, tiny mass, interacts only via weak force.
Q: What makes an electron neutrino?
A: Created with electrons in weak interactions.
Q: Solar neutrino problem?
A: Too few detected neutrinos. Explanation: neutrino oscillations.
Q: Experiment that solved it?
A: Sudbury Neutrino Observatory (SNO).
LECTURE 11 — Stellar Evolution (Sun)
Q: What balances hydrostatic equilibrium on the main sequence?
A: Gravity vs fusion pressure.
Q: What element accumulates in the Sun's core during the main sequence?
A: Helium (does not burn yet).
Q: Does a solar-mass star stay the same during the main sequence?
A: No; it brightens and expands.
Q: What happens as the Sun leaves the main sequence?
A: Core contracts, outer layers expand → red giant.
Q: What happens to a star's appearance during red giant phase?
A: Bigger, cooler, redder, more luminous.
Q: What is the helium flash?
A: Sudden He fusion in a degenerate core.
Q: Why doesn’t the core expand during the helium flash?
A: Supported by electron degeneracy pressure (not temperature-dependent).
Q: What is the triple-alpha process?
A: 3 He → C.
Q: What builds up in the core during steady helium burning?
A: Carbon and oxygen.
Q: End of AGB phase produces what two things?
A: Planetary nebula + white dwarf.
Q: What stabilizes a white dwarf?
A: Electron degeneracy pressure.
Q: What process heavy stars undergo that the Sun will not?
A: Fusion to iron and core collapse. Threshold: >8 solar masses.
LECTURE 12 — Novae, Supernovae, Heavy Stars
Q: What is needed for a nova?
A: White dwarf + mass transfer.
Q: What is a Roche lobe?
A: Region where a star’s gravity dominates; overflow causes accretion.
Q: What is the Chandrasekhar limit?
A: ~1.4 solar masses; max mass of white dwarf.
Q: What happens if a white dwarf exceeds this limit?
A: Type Ia supernova.
Q: Difference between heavy and light star evolution?
A: Heavy stars fuse up to iron; light stars stop at carbon/oxygen. Boundary: 8 solar masses.
Q: What accumulates in the core of a heavy star?
A: Iron (because fusion beyond iron absorbs energy).
Q: Nova vs supernova?
A: Nova = surface explosion; supernova = catastrophic core collapse.
Q: Type I vs Type II supernova?
A: Type I: no hydrogen lines. Type II: hydrogen present.
Q: What stops core collapse supernova?
A: Neutron degeneracy pressure.
Q: Most stable element?
A: Iron-56.
Q: Alpha, beta, neutron decay?
A: Alpha: He nucleus emitted. Beta: neutron ↔ proton + e ± neutrino. Neutron: neutron emitted.
Q: s-process?
A: Slow neutron capture; stops at lead; occurs in AGB stars.
Q: r-process?
A: Rapid neutron capture; forms very heavy elements; occurs in neutron star mergers and supernovae.
Q: Remnants of type Ia and type II supernova?
A: Type Ia: no remnant. Type II: neutron star or black hole.
Q: Size, mass, density of neutron star?
A: ~10 km radius, ~1.4–2 solar masses, ~10^17 kg/m³.
LECTURE 13 — Pulsars & Neutron Stars
Q: What is an LGM?
A: "Little Green Men” nickname for first pulsar signals.
Q: Why do pulsars pulse?
A: Rotating beams from magnetic poles.
Q: What is a millisecond pulsar?
A: Very fast pulsar; “recycled” by accretion.
Q: Mass required to produce a pulsar?
A: >8 solar masses initially.
Q: What prevents neutron star collapse?
A: Neutron degeneracy pressure.
Q: Maximum neutron star mass?
A: ~2–3 solar masses.
Q: Escape velocity of Earth?
A: ~11 km/s.
Q: Escape velocity of neutron star?
A: ~0.3c.
Q: What is a GRB?
A: Gamma-ray burst.
Q: Why were GRBs confusing initially?
A: Unknown distance; extremely energetic.
Q: What is an accretion disk?
A: Rotating material falling into a compact object.
Q: Kilonova?
A: Short-lived EM burst from neutron star merger.
Q: Collapsar?
A: Massive star collapsing to black hole.
LECTURE 14 — Black Holes
Q: What happens when neutron degeneracy fails?
A: Collapse to a black hole.
Q: What is the Schwarzschild radius?
A: Radius of a black hole's event horizon.
Q: Schwarzschild radius of Earth and Sun?
A: Earth ~9 mm; Sun ~3 km.
Q: What is gravitational redshift?
A: Light loses energy escaping gravity → shifts to red.
Q: What do we see when an object falls into a black hole?
A: Appears to freeze and redshift at horizon.
Q: What is Cygnus X-1?
A: First strong stellar black hole candidate.
Q: What is LIGO?
A: Laser interferometer detecting gravitational waves.
Q: What is GW170817?
A: First neutron star merger detected by gravitational waves.
Q: Jupiter becomes black hole — change?
A: No change in gravity outside; same mass → same orbit.
Q: Types of black holes?
A: Stellar (few solar masses), Intermediate (10^2–10^4), Supermassive (10^6–10^10).
Q: What is Sgr A*?
A: Supermassive black hole at Milky Way center.
LECTURE 15 — Galaxies & Dark Matter
Q: What is a spiral nebula?
A: Old term for galaxies.
Q: Herschel’s model?
A: Milky Way map using star counts.
Q: What is a Cepheid variable?
A: Pulsating star with period-luminosity relationship.
Q: How do we calibrate Cepheids?
A: Measure distances to nearby Cepheids.
Q: Who proposed dark matter?
A: Fritz Zwicky.
Q: Who confirmed it via rotation curves?
A: Vera Rubin.
Q: Three pieces of evidence for dark matter?
A: Rotation curves, gravitational lensing, cluster dynamics.
Q: What is the Bullet Cluster?
A: Colliding clusters showing dark matter’s separation from normal matter.
Q: Why dark matter isn’t ordinary matter?
A: Does not emit, absorb, or scatter light.
Q: MACHO vs WIMP?
A: MACHO = massive compact object; WIMP = weakly interacting particle.
LECTURE 16 — Cosmology & Expansion
Q: What is Hubble’s Law?
A: v = H₀ d.
Q: What determines expansion rate in GR?
A: Total energy density (matter, radiation, dark energy).
Q: Does the Hubble constant change over time?
A: Yes; expected to slow in matter era, accelerate with dark energy.
Q: Can objects move faster than light due to expansion?
A: Yes, because space itself expands.
Q: What does z > 1 mean?
A: Large redshift; object very distant.
Q: Standard candles for most distant measures?
A: Type Ia supernovae.
Q: Einstein’s “biggest mistake”?
A: Adding the cosmological constant to force a static universe.
Q: Consequence of expansion?
A: Universe dynamic, not static.