Astronomy on the Web — Study Guide

Unit 1 — Ch 1–4 · Cosmic context & gravity (Test 1 · May 31 – Jun 3)

Ch 1 — A Modern View of the Universe

Cosmic address: Earth → Solar System → Milky Way Galaxy → Local Group → Local Supercluster → Observable Universe.

• Star — large glowing gas ball, generates heat & light by nuclear fusion.
• Planet — moderately large body orbiting a star; shines by reflected light. Rocky, icy, or gaseous.
• Dwarf planet — IAU: orbits the Sun, massive enough to be spherical (hydrostatic equilibrium), but has not cleared its orbital neighborhood.
• Moon / satellite — body that orbits a planet.
• Asteroid — small rocky body orbiting a star.
• Comet — small icy body orbiting a star.
• Solar (star) system — a star + all material orbiting it.
• Nebula — interstellar cloud of gas and/or dust.
• Galaxy — great island of stars held together by gravity, orbiting a common center.
• Universe — sum total of all matter and energy.

Looking back in time. Light travels at finite speed (≈300,000 km/s), so we see distant objects as they were in the past. Moon → 1 sec · Sun → 8 min · Sirius → 8 yr · Andromeda → 2.5 Myr.

Distance units. AU = Earth–Sun ≈ 150 million km (93 million mi). Light-year ≈ 10 trillion km ≈ 6 trillion mi. Parsec = 3.26 ly.

Scale of the universe. Milky Way ~100 billion galaxies; 10¹¹ stars/galaxy × 10¹¹ galaxies = 10²² stars (≈ grains of dry sand on all Earth's beaches).

How did we come to be? Big Bang → H + He. Heavier elements forged in stars, recycled into new systems including ours. On a 1-year cosmic calendar, all of human civilization is a few seconds; one lifetime is a fraction of a second.

Spaceship Earth. Earth rotates once/day; orbits Sun at 1 AU once/year with 23.5° axial tilt (Polaris). Sun orbits galactic center every ~230 Myr at ~70,000 km/h relative to local stars. Hubble showed all galaxies beyond the Local Group recede, and the more distant, the faster — universe expanding.

Ch 2 — Discovering the Universe for Yourself

2.1 Patterns in the night sky. Naked-eye Earth shows ~2,000 stars plus the Milky Way band. A constellation is a region of sky, not a physical group — the IAU officially divided the entire sky into 88 constellations in 1930. The bright stars of one constellation may actually be at vastly different distances; they just appear together from our viewpoint.

Celestial sphere. Imaginary dome with stars "fixed" on its inner surface. The ecliptic is the Sun's apparent annual path across the sphere. The Milky Way band is our edge-on view through the disk of our galaxy.

Local sky vocabulary.

• Zenith — point directly overhead.
• Horizon — all points 90° away from the zenith.
• Meridian — line through zenith connecting the N and S horizon points.
• Altitude (angle above horizon) and direction (along horizon) specify any object's local position.

Angular measure. Full circle = 360°. 1° = 60 arcminutes ('). 1' = 60 arcseconds ("). So 1° = 3,600". Hour version (RA): 24 h = 360°, so 1 h = 15°. Field "rules of thumb" at arm's length: pinkie ≈ 1°, three middle fingers ≈ 5°, fist ≈ 10°. Full Moon ≈ 0.5° = 1,800".

Angular size formula. angular size = physical size × (360° / 2π × distance). Same object farther away → smaller angular size.

Why stars rise and set. Earth rotates W→E, so sky appears to circle E→W. Stars near the north celestial pole are circumpolar (never set); stars near the south celestial pole are never visible from northern latitudes; everything else rises in the east and sets in the west.

Sky depends on latitude (not longitude). Altitude of the celestial pole equals your latitude. If Polaris sits 50° above your north horizon, you are at 50° N. Omaha is ≈41° N, so Polaris sits ~41° above the northern horizon.

Sky depends on time of year. As Earth orbits, the Sun appears to move eastward along the ecliptic, so the stars opposite the Sun at midnight change month to month.

Day length. Solar day = 24 h (noon to noon, relative to Sun). Sidereal day = 23 h 56 min (Earth's true rotation period, relative to stars). Difference: Earth has moved ~1° along its orbit, so it must rotate ~1° more to face the Sun again.

2.2 The reason for seasons. Earth–Sun distance varies only ~3% — not enough to cause seasons. The real cause is Earth's 23.5° axial tilt:

• Earth's axis points the same direction (toward Polaris) all year. As Earth orbits, the orientation of that axis relative to the Sun changes.
• When your hemisphere tilts toward the Sun: more direct sunlight + longer days = summer. Away from Sun: less direct + shorter days = winter.
• Both hemispheres are at the same Earth–Sun distance, but seasons are opposite — proof distance doesn't drive seasons.

Solstices and equinoxes.

• Summer solstice (~Jun 21) — Sun's highest path, rise + set farthest north of east. ~15 hr daylight at 40° N.
• Winter solstice (~Dec 21) — Sun's lowest path, rise + set farthest south of east. ~9 hr daylight at 40° N.
• Vernal equinox (~Mar 20) and autumnal equinox (~Sept 22) — Sun rises due east, sets due west, ~12 hr daylight everywhere.

Precession. Earth's axis wobbles like a top with a 26,000-year period. Tilt stays ~23.5° (so seasons keep working), but the axis points to different "north stars" over time. Polaris won't always be the North Star, and the vernal equinox has already shifted from Aries (when ancient sign mapping was set) into Pisces.

2.3 The Moon — phases and eclipses.

• Half the Moon is always lit by the Sun and half is dark. We see a changing combination of those faces as the Moon orbits Earth.
• 29.5-day synodic cycle: new → waxing crescent → 1st quarter → waxing gibbous → full → waning gibbous → 3rd quarter → waning crescent.
• Waxing = lit side growing; visible afternoon/evening, rises later each day. Waning = shrinking; visible late night/morning, sets later each day.
• Synchronous rotation: Moon rotates exactly once per orbit, so we always see the same face (no "permanent dark side" — just a permanent far side).

Two conditions for an eclipse: (1) full Moon (lunar) or new Moon (solar), AND (2) Moon at or near a node (where its orbit crosses the ecliptic plane). The Moon's orbit is tilted ~5° to the ecliptic, so most months no eclipse occurs. There are ~2 eclipse seasons per year.

• Lunar eclipse (at full Moon) — Earth's shadow falls on the Moon. Can be penumbral (subtle), partial, or total (Moon turns reddish from refracted sunlight).
• Solar eclipse (at new Moon) — Moon's shadow falls on Earth. Can be partial, total (rare — Moon perfectly covers Sun), or annular (Moon too far to fully cover, ring of Sun remains).
• Saros cycle — eclipses repeat in pattern every 18 yr 11⅓ days.

2.4 The ancient mystery of the planets. The 5 naked-eye planets:

• Mercury — hard to see, always near Sun (twilight only).
• Venus — extremely bright morning or evening "star."
• Mars — noticeably red.
• Jupiter — very bright.
• Saturn — moderately bright.

Planets normally drift slowly eastward against the stars night to night. Occasionally they appear to reverse (westward) for weeks — apparent retrograde motion. This is easy with heliocentrism (Earth "laps" the outer planet, or Mercury/Venus lap us) but hard for geocentrism. The Greeks rejected the heliocentric explanation largely because they could not detect stellar parallax — and they wrongly assumed the stars couldn't be far enough away to make parallax invisible.

Ch 3 — The Science of Astronomy

3.1 Ancient roots. All humans use everyday scientific thinking (observation + trial-and-error). Astronomy is the oldest science — practiced for calendars + agriculture, religion/ceremony, and navigation.

• Central Africa, 6500 BC — predicted seasons from crescent-Moon orientation.
• Egyptian obelisks — shadows tell time of day.
• Stonehenge, England (~1550 BC) — aligned with solstice sunrise.
• Templo Mayor (Mexico), Anasazi kiva (NM), Sun Dagger (SW US), Machu Picchu (Peru), Big Horn Medicine Wheel (Wyoming) — solar alignments worldwide.
• Polynesians — celestial navigation across open Pacific.
• China (1400 BC) — earliest records of supernovae on tortoiseshell inscriptions.
• Days of the week — named after Sun, Moon, and 5 visible planets (Saturn, Sun, Moon, Mars, Mercury, Jupiter, Venus → Sat, Sun, Mon, …).

3.2 Greek science. The Greeks were the first to build models of nature and demand that model predictions agree with observations — without resort to myth.

Eratosthenes measures the Earth (~240 BC). Noted that at Syene the noon Sun cast no shadow at the summer solstice, while at Alexandria it cast a 7° shadow. Distance Syene → Alexandria was ~5,000 stadia. So circumference = 5,000 × (360/7) ≈ 250,000 stadia ≈ 42,000 km — within ~5% of the modern value (~40,100 km).

Greek geocentric model. Plato + Aristotle: Earth at center; heavens must be "perfect" (perfect spheres + perfect circles only). This made apparent retrograde motion hard to explain.

Ptolemy (~140 AD). Built the most sophisticated geocentric model — accurate enough to stay in use for ~1,500 years. Translated to Arabic as the Almagest ("the greatest compilation"). Used three gimmicks to fit data while keeping circles:

• Deferent — large circle around Earth.
• Epicycle — small circle whose center moves along the deferent (creates retrograde loops).
• Equant — offset point around which the epicycle's motion is uniform.

The Muslim world preserved + extended Greek knowledge (al-Mamun's House of Wisdom, Baghdad, ~800 AD). When Constantinople fell in 1453, Eastern scholars carried this knowledge to Europe, helping ignite the Renaissance.

3.3 The Copernican revolution.

• Copernicus (1473–1543) — proposed Sun-centered model (published 1543). Allowed computing planetary distances in AU. But because he still used perfect circles, his predictions were no more accurate than Ptolemy's.
• Tycho Brahe (1546–1601) — compiled most accurate pre-telescope data ever made (~1 arcminute precision). Couldn't detect parallax, so kept Earth at center, but recognized other planets orbit the Sun (Tychonic hybrid). Hired Kepler.
• Johannes Kepler (1571–1630) — first tried circular orbits; an 8-arcminute discrepancy in Mars's position vs. Tycho's data forced him to ellipses. His famous line: "If I had believed we could ignore these eight minutes, I would have patched up my hypothesis accordingly. But since it was not permissible to ignore, those eight minutes pointed the road to a complete reformation in astronomy."

Kepler's three laws of planetary motion:

1. Law of ellipses. The orbit of each planet is an ellipse with the Sun at one focus (not the center). An ellipse looks like an elongated circle.
2. Law of equal areas. A line from the planet to the Sun sweeps equal areas in equal times. ⇒ Planets move faster when close to the Sun (perihelion), slower when far (aphelion).
3. Harmonic law. p² = a³ — orbital period in years squared equals the semi-major axis in AU cubed. Example: Jupiter at a = 5 AU → p² = 125 → p ≈ 11.2 years.

Galileo Galilei (1564–1642) — three Aristotelian objections to Copernicus, three Galilean responses.

• Objection 1: "If Earth moved, things in the air would be left behind." Galileo's ramp experiments showed objects keep moving unless a force slows them — basis of Newton's 1st law. A bird, an arrow, or an apple shares Earth's motion.
• Objection 2: "Heavens must be perfect — no irregular shapes allowed." Tycho's observations of comets + a supernova already challenged this. Galileo's telescope then revealed sunspots (the Sun isn't blemish-free), mountains and craters on the Moon (it's not a perfect sphere), and that the Milky Way resolves into countless individual stars.
• Objection 3: "If Earth orbits the Sun, we'd see stellar parallax." Galileo's telescope showed stars must be much farther than Tycho thought — so lack of detectable parallax stopped being a problem.

Galileo's two killer pieces of evidence for heliocentrism:

• Phases of Venus. Venus shows a full set of phases (crescent → gibbous → full), which is only possible if Venus orbits the Sun. The Ptolemaic geocentric model predicts Venus would only ever show crescent shapes.
• Moons of Jupiter. Four bright satellites orbiting Jupiter — direct proof that not everything orbits Earth.

In 1633 the Catholic Church ordered Galileo to recant. His book was on the Index of Forbidden Books until 1824; the Church formally vindicated him in 1992.

3.4 The nature of science. "Science" = Latin scientia, "knowledge." The idealized scientific method: observe → hypothesize → test → revise. Real science is messier (sometimes you "just look" first, sometimes you follow intuition), but it has three hallmarks:

1. Seeks explanations relying solely on natural causes (no divine intervention in scientific models).
2. Progresses by building + testing models that explain observations as simply as possible (Occam's razor).
3. A scientific model must make testable predictions that would force the model to be revised or abandoned if they fail.

Scientific theory — different from "theory" in everyday speech. In science, a theory is NOT a hypothesis (a guess). A theory must:

• Explain a wide variety of observations with a few simple principles.
• Be supported by a large, compelling body of evidence.
• Have not failed any crucial test of its validity.

Example test-question framing: Darwin's theory of evolution "meets all the criteria of a scientific theory" — meaning it has stood for >160 years of testing without failing, not that scientific opinion is split.

Ch 4 — Making Sense of the Universe: Motion, Energy & Gravity

Bennett's Ch 4 ties together three big ideas + their astronomical applications: Newton's laws of motion, conservation laws (energy + angular momentum), and universal gravity.

4.1 Describing motion.

• Speed — how fast (e.g. 60 km/h). Velocity = speed + direction (a vector).
• Acceleration = any change in velocity, including a change in direction at constant speed. A car turning a corner at 30 mph is accelerating.
• Acceleration of gravity at Earth's surface: g ≈ 9.8 m/s² (10 m/s² is fine for rough work). Any object in free fall (no air) accelerates at g regardless of mass — Galileo's leaning-tower/ramp result.
• Momentum = mass × velocity (p = mv). A vector.
• Force = any push or pull that can change momentum. A net force (the sum of all forces) is required to accelerate something.

Mass vs weight. Mass = amount of matter (kg) — same everywhere in the universe. Weight = the gravitational force on you right now (N) — depends on the local g. On the Moon (g ≈ 1.6 m/s²) you weigh ~1/6 of your Earth weight but your mass is unchanged. Astronauts in orbit are in continuous free fall, so they feel weightless even though Earth's gravity is still ~89% of surface strength at the ISS altitude.

4.2 Newton's three laws of motion.

1. Law of inertia. An object moves at constant velocity unless a net force acts on it. (At rest = velocity 0 is just a special case.) This is what Galileo's experiments showed and what overturned Aristotle's "things naturally come to rest."
2. F = m·a. Net force equals mass times acceleration. Equivalent form: F = Δp/Δt — net force is the rate of change of momentum.
3. Action–reaction. For every force there is an equal and opposite force. The Earth pulls you down with your weight; you pull the Earth up with the same force. A rocket pushes hot gas backward; the gas pushes the rocket forward.

4.3 Conservation laws in astronomy.

• Conservation of momentum. The total momentum of a system stays constant unless an external force acts. Why a rocket works.
• Conservation of angular momentum. L = m·v·r for a circular orbit. With no external torque, L is constant. Three astronomical payoffs:
  • Earth keeps spinning + orbiting without needing fuel (no significant torque acts on it).
  • An ice-skater pulling arms in spins faster — same L, smaller r ⇒ larger v. A collapsing gas cloud spins up the same way (so a forming star/disk flattens into a rotating protoplanetary disk).
  • Kepler's 2nd law (equal areas) is just conservation of angular momentum for an orbiting planet.
• Conservation of energy. Energy can change form but never be created or destroyed.

Three basic categories of energy:

• Kinetic energy — energy of motion. KE = ½·m·v². Thermal energy is the microscopic kinetic energy of jiggling atoms; temperature measures their average KE.
• Radiative energy — energy carried by light (photons).
• Potential energy — stored energy by virtue of position or arrangement. Two key kinds:
  • Gravitational PE — higher up = more PE. A falling rock converts grav PE → KE.
  • Mass-energy via E = mc² — mass itself is a huge reservoir of energy. Tiny mass deficits in fusion (4 H → He in the Sun) release enormous energy. This is how stars shine.

4.4 Newton's universal law of gravity.

F = G · (M₁ · M₂) / r²

• Every mass attracts every other mass.
• Force is proportional to both masses: double either mass → double the force.
• Force follows an inverse-square law with distance: double the distance → quarter the force; triple the distance → 1/9 the force.
• G = 6.674 × 10⁻¹¹ N·m²/kg² — gravity is intrinsically very weak; only huge masses (planets, stars) make it noticeable.

Why Kepler's laws work — Newton's payoff. Newton derived Kepler's three laws from his law of gravity. The general Newton form of Kepler's 3rd law is: p² = (4π² / G(M₁+M₂)) · a³ — so by measuring an orbit's period + size we can weigh the central body (this is how we get the mass of the Sun, Jupiter, Sgr A*, etc.).

Orbits. A bound orbit is an ellipse (or circle, a special ellipse). Add energy and it can become a parabola or hyperbola (unbound — escape trajectory).

• Escape velocity from Earth's surface ≈ 11.2 km/s. From the Sun's surface ≈ 618 km/s.
• A satellite "falls" continuously around Earth, missing the surface because Earth curves away at the same rate (Newton's cannonball thought experiment).

Tides. The Moon's gravity is slightly stronger on the near side of Earth than on the far side — that difference across Earth's diameter is the tidal force. Tidal force ∝ 1/r³ (steeper than gravity's 1/r²), so a nearby weaker source can dominate over a far stronger one.

• Two tidal bulges (near + far side of Earth) → ~two high tides + two low tides per day.
• Spring tides — biggest range — at new and full Moon, when Sun and Moon align (their tidal pulls add).
• Neap tides — smallest range — at first and third quarters, when Sun and Moon are at right angles (their tidal pulls partly cancel).
• Tidal friction slows Earth's rotation by ~1.7 ms/century; the Moon recedes ~3.8 cm/year (measured by laser ranging to Apollo retroreflectors).
• Tidal heating is what drives Io's volcanism + Europa's subsurface ocean (covered in Ch 10).

Unit 2 — Ch 5–7 · Light, telescopes, solar-system formation (Test 2 · Jun 14 – Jun 17)

Ch 5 — Light & Matter

• EM spectrum (long λ → short λ): radio · microwave · infrared · visible · UV · X-ray · gamma. c = λ·f.
• Photon energy E = h·f; high frequency = high energy.
• Three thermal radiation laws (blackbody):
  • Hotter objects emit more total power per area (Stefan–Boltzmann: ∝ T⁴).
  • Hotter objects peak at shorter wavelengths (Wien's law).
  • Doesn't depend on composition, only T.
• Spectra — continuous (hot dense), emission (hot thin gas → bright lines), absorption (cool gas in front of continuous source → dark lines). Lines reveal composition.
• Doppler shift — blueshift if approaching, redshift if receding. Δλ/λ = v/c.

Ch 6 — Telescopes

• Refractor (lens) vs reflector (mirror) — modern research is all reflectors (no chromatic aberration, can be huge).
• Light-gathering power ∝ aperture² · angular resolution ∝ λ/D (bigger telescope = sharper).
• Atmosphere problems — seeing (turbulence blurs images), absorption (blocks UV, X-ray, most IR), light pollution.
• Adaptive optics — deformable mirror corrects for turbulence in real time.
• Space telescopes — Hubble (vis/UV), JWST (IR), Chandra (X-ray), Spitzer (IR retired).
• Radio telescopes & interferometry — multiple dishes combine signal for huge effective aperture (VLA, ALMA, EHT).

Ch 7 — Our Solar System & Its Formation

• Patterns of motion — planets orbit in nearly the same plane, same direction, nearly circular orbits.
• Two planet families: terrestrial (Mercury, Venus, Earth, Mars — small, dense, rocky, few moons) vs jovian (Jupiter, Saturn, Uranus, Neptune — large, gaseous, ringed, many moons).
• Asteroid belt between Mars and Jupiter; Kuiper belt beyond Neptune; Oort cloud far out (comets).
• Nebular theory — solar system formed from a collapsing rotating cloud of gas + dust ≈4.6 Gyr ago. Conservation of angular momentum → flat disk → planetesimals → planets.
• Frost line — beyond ~3 AU it was cold enough for water/methane/ammonia ices, so jovian planets accreted huge ice-rock cores that captured gas.
• Heavy bombardment + late impacts shaped young planets. Radiometric dating of meteorites pins the age.

Unit 3 — Ch 9–11 · Planetary tour (Test 3 · Jul 5 – Jul 8)

Ch 9 — Terrestrial Planets

• Four geological processes: impact cratering · volcanism · tectonics · erosion. How active a world is depends on size (heat retention) + composition + distance from Sun.
• Mercury — heavily cratered, huge iron core, no atmosphere, extreme temperature swings.
• Venus — runaway greenhouse, 96% CO₂, surface ~470 °C, retrograde slow rotation, ~90 bar pressure.
• Earth — only known active plate tectonics, magnetic field, oxygen atmosphere, liquid water surface.
• Mars — past liquid water (river valleys, polar ice), thin CO₂ atmosphere now, Olympus Mons + Valles Marineris.
• Atmosphere outcomes — depends on gravity (escape speed), temperature, and outgassing/loss history.

Ch 10 — Jovian Planets

• Jupiter — H/He gas giant, Great Red Spot, 95+ moons, strongest planetary magnetosphere.
• Saturn — H/He, spectacular rings (ice particles), low density (0.7 g/cm³ — would float).
• Uranus / Neptune — ice giants (water/methane/ammonia mantles); Uranus spins on its side (98° tilt).
• Rings — at all 4 jovian planets; made of ice/rock; lie inside the Roche limit where tidal forces prevent moon formation.
• Notable moons — Io (volcanic, tidal heating from Jupiter), Europa (subsurface ocean), Titan (thick N₂ atmosphere, liquid CH₄ lakes), Enceladus (geyser plumes), Triton (retrograde, captured KBO).

Ch 11 — Asteroids, Comets, Dwarf Planets

• Asteroids — rocky remnants, mostly in main belt. C-type (carbonaceous), S-type (silicate), M-type (metallic).
• Comets — icy bodies from Kuiper belt or Oort cloud. Develop coma + tails (ion + dust) when near Sun. Tails point away from Sun (solar wind + radiation pressure).
• Meteoroid → meteor → meteorite — in space / in atmosphere / on ground.
• Dwarf planets — Pluto, Eris, Haumea, Makemake, Ceres. Spherical but didn't clear orbit.
• Mass-extinction impacts — Chicxulub ~66 Myr ago ended non-avian dinosaurs. Tunguska 1908.

Unit 4 — Ch 12–15 · Exoplanets, Sun, stars, star birth (Test 4 · Jul 19 – Jul 22)

Ch 12 — Other Planetary Systems

• Detection methods — Doppler (radial velocity wobble), transit (dimming when planet crosses), direct imaging, gravitational microlensing.
• Hot Jupiters — gas giants very close to their stars; surprising — explained by orbital migration.
• Kepler & TESS missions — thousands of confirmed exoplanets, many in habitable zones.

Ch 13 — The Sun

• Structure — core · radiative zone · convective zone · photosphere · chromosphere · corona.
• Energy — proton-proton chain fusion: 4 H → He + energy (E = mc²). Photons take ~170,000 yr to random-walk out of the core.
• Solar activity — sunspots (cool, magnetic), 11-year cycle, solar flares + CMEs.
• Solar neutrino problem — solved by neutrino oscillation (flavors change in transit).

Ch 14 — Stellar Properties

• Luminosity (true brightness, W) vs apparent brightness (W/m² at Earth). Inverse-square: b = L/(4πd²).
• Parallax — apparent shift of nearby star against distant background; d (pc) = 1/p (arcsec).
• Spectral classes — O B A F G K M (hottest → coolest). Mnemonic: "Oh Be A Fine Guy/Gal Kiss Me."
• H-R diagram — luminosity vs temperature. Main sequence (90% of stars, fusing H), giants, supergiants, white dwarfs.
• Mass determines a main-sequence star's whole life: high mass = hot, blue, luminous, short-lived.

Ch 15 — Star Birth

• Molecular clouds — cold dense regions where gravity overcomes pressure → collapse.
• Protostar — contracting cloud heats up; once core hits ~10 million K, H fusion ignites → star enters main sequence.
• Mass distribution at birth — many low-mass, few high-mass stars (IMF). Below ~0.08 M☉ = brown dwarf (no H fusion).

Unit 5 — Ch 16–19 · Star death, galaxies, cosmology (Test 5 · Jul 26 – Jul 29)

Ch 16 — Star Stuff (Death)

• Low-mass (≤8 M☉) — main sequence → red giant → He flash → horizontal branch → AGB → planetary nebula + white dwarf (carbon-oxygen core).
• High-mass (>8 M☉) — burns C, O, Si up to Fe; iron core collapses → Type II supernova → neutron star or black hole.
• Type Ia supernova — white dwarf in binary accretes mass past Chandrasekhar limit (~1.4 M☉) → runaway C fusion. Standard candle for cosmology.
• Nucleosynthesis — elements heavier than Fe made in supernovae and neutron-star mergers.

Ch 17 — Stellar Remnants & Black Holes

• White dwarf — Earth-sized C/O core, supported by electron degeneracy. Max mass = Chandrasekhar ≈1.4 M☉.
• Neutron star — ~10 km wide, ~1.4–3 M☉, supported by neutron degeneracy. Spinning + magnetized → pulsar.
• Black hole — gravity beats degeneracy; escape velocity > c. Schwarzschild radius R = 2GM/c² is the event horizon.
• Effects near BH — extreme gravitational time dilation, tidal spaghettification, accretion-disk X-ray emission.

Ch 18 — The Milky Way

• Structure — thin disk (young, metal-rich, Pop I), thick disk + halo (old, metal-poor, Pop II), central bulge with bar, dark-matter halo.
• Sun's position — ~26,000 ly from center, in the Orion arm.
• Rotation — flat curve at large radii ⇒ much more mass than visible ⇒ dark matter.
• Sgr A* — supermassive black hole at galactic center, ~4 million M☉ (confirmed via S-star orbits and EHT image).

Ch 19 — Galaxies, Hubble & Cosmology

• Hubble classification — spirals (S) · barred spirals (SB) · ellipticals (E) · irregulars.
• Hubble's law — v = H₀·d. H₀ ≈ 70 km/s/Mpc. The farther a galaxy, the faster it recedes.
• Big Bang — universe began ~13.8 Gyr ago, hot dense state, expanding ever since.
• CMB — cosmic microwave background, ~2.7 K blackbody glow from recombination ~380,000 yr after BB.
• Dark matter + dark energy — current Λ-CDM model: ~5% ordinary matter, ~27% dark matter, ~68% dark energy (drives accelerating expansion).