Astronomy Review Flashcards: Solar System, Stars, Galaxies, and Cosmology

17.1 The Solar System

• Planets visible to unaided eye: Mercury, Venus, Mars, Jupiter, Saturn. Uranus technically visible under perfect conditions but very faint without telescope.

• Distinguishing planets from stars

  • Unaided eye: planets shine with steady light; stars twinkle. Planets appear to move relative to background stars over time.

  • With telescope: planets appear as small disks; stars remain point-like sources even at high magnification.

• Largest and smallest planets; nearest to the Sun

  • Largest: Jupiter. Smallest: Mercury. Nearest Sun: Mercury.

• Why planets shine

  • Planets reflect sunlight; they do not generate their own light.

• Satellites

  • Mercury and Venus have no satellites.

• Mass distribution in the Solar System

  • Mass is overwhelmingly in the Sun: Sun contains >99.8% of the total Solar System mass.

• Surface gravity on planets (Earth’s gravity as baseline)

  • Weigh less than on Earth on Mars and Mercury; weigh more on Jupiter, Saturn, Uranus, Neptune. Venus is close to Earth in weight.

• Earth viewed from Mars (Earth’s appearance as it orbits the Sun)

  • Earth would show phases (crescent → gibbous → full) as seen from Mars, similar to Moon phases for observers on Mars.

17.2 Comets

• Orbits vs planets

  • Planets: nearly circular orbits; Comets: highly elliptical orbits.

• Tails and sun proximity

  • Tails form in vicinity of the Sun as ices sublimate; solar wind and radiation pressure push gas/dust away, creating the tail.

• Observation through head and tail

  • When near Earth, stars are visible through both head and tail; implies comets are diffuse, not solid bodies; collision with Earth is unlikely (low danger).

17.3 Meteors

• Perseid meteor shower (August)

  • Not all Perseids have 1-year orbital periods; Earth passes through debris left by comet Swift–Tuttle; meteoroids have a range of orbital periods.

• Meteorites in collections

  • Over 90% of meteorites found after known falls are stony; most meteorites in museums are iron.

  • Reason: stony meteorites are common and resemble Earth rocks; iron meteorites are durable and easily identified/preserved.

• Atmosphere absence implications

  • If Earth had no atmosphere: comets would still be visible (reflect sunlight); meteors would not, as meteors require atmospheric entry to produce light from friction.

• Why most meteorites are found in Antarctica

  • Factors: high contrast against ice; ice flow concentrates meteorites in blue-ice regions; very dry, cold conditions preserve them from weathering.

17.4 Mercury

• Habitability considerations

  • Mercury unlikely to host life due to extreme temperature fluctuations, thin atmosphere, and lack of liquid water.

• Mercury’s day-night cycle and resonance

  • Mercury’s rotation ~59 Earth days per sidereal day; solar day length ~176 Earth days between sunrises due to a 3:2 spin-orbit resonance: Mercury rotates three times for every two orbits around the Sun.

17.5 Venus

• Why inferior planets (Mercury and Venus) are seen near the Sun

  • Orbits inside Earth's; always near the Sun in our sky; visible only near sunrise or sunset.

• Brightness of Venus

  • Venus is the brightest planet; at its brightest it can be over 15× brighter than Sirius (the brightest star).

• Phases of Venus and Mercury

  • Venus and Mercury show phases because their orbits are interior to Earth's; observed phase progression (crescent to full) as they orbit the Sun.

• Venus brightness vs disk appearance

  • Venus appears brighter in crescent phase because it is closer to Earth; full-disk phase occurs when farther away.

• Crater history and resurfacing on Venus

  • Fewer ancient craters than expected; resurfacing: catastrophic volcanic event ~300–600 Myr ago erased older craters.

• Why Venus heat is extreme

  • Proximity to Sun; runaway greenhouse effect from dense CO2 atmosphere.

17.6 Mars

• Why Venus brighter than Mars

  • Closer to Sun; closer to Earth; high reflectivity (albedo ~0.75 for Venus vs ~0.17 for Mars; Venus’ clouds are highly reflective).

• Meteoroid threat comparison (Earth vs Mars)

  • Mars more likely to be struck by meteoroids due to a very thin atmosphere offering little protection; Earth's atmosphere burns up most meteoroids.

• Evidence for ancient running water

  • Regions with many ancient craters imply old surfaces; running water would erode craters over time, so those craters indicate water was present long ago but not anymore.

17.8 Asteroids

• Distinction: asteroids vs meteoroids

  • Asteroid: rocky body in space orbiting the Sun; generally larger (meters to hundreds of km).

  • Meteoroid: smaller piece of an asteroid or comet (typically < a few meters).

• Origin of asteroids

  • Remnants from early Solar System that failed to form a planet, likely due to Jupiter’s gravity.

• Why few asteroids are spherical

  • Not enough mass for gravity to shape them into spheres; largest asteroid (Ceres) is roughly spherical.

• Evidence of asteroid impact on Earth

  • Chicxulub crater linked to dinosaur extinction; near-Earth object tracking is ongoing to assess future impact threats.

17.9 Jupiter

• Jupiter’s composition

  • Density ~1.33 g/cm^3; primarily hydrogen and helium; not rock/iron.

• The Great Red Spot

  • A persistent, high-pressure storm, analogous to a hurricane but enormous and long-lasting.

• Ancient Earth–Europa discussion

  • Earth likely never had Europa-like conditions (global ice ocean); however, life existed billions of years ago on Earth when oceans covered much of the planet.

• Io’s interior heating

  • Caused by tidal heating from gravitational flexing by Jupiter and neighboring moons (Europa, Ganymede);

  • Frictional heating drives strong volcanic activity.

17.10 Saturn

• Similarities and differences with Jupiter

  • Similarities: gas giants, hydrogen/helium composition; many satellites; ring systems; rapid rotation.

  • Differences: Saturn less massive/dense; ring system more prominent and spectacular.

• Rings as particles

  • Rings are likely composed of many small ice/rock particles rather than a solid sheet or diffuse gas; differential orbital speeds prevent a single solid disk; presence of “spokes” supports particle nature.

• Titan’s atmosphere and other satellites

  • Titan has a substantial atmosphere; other satellites (e.g., Io) can have very thin atmospheres (SO2 on Io).

17.11 Uranus, Neptune, Pluto, and More

• Planet most similar to Earth in size/mass and surface conditions

  • In size/mass: Venus; in surface conditions: Mars (rocky, thin atmosphere).

• Plate tectonics elsewhere

  • No evidence for plate tectonics on other planets.

• Planets with rings (besides Saturn)

  • Jupiter, Uranus, Neptune have rings; they are smaller and darker than Saturn’s and harder to see from Earth.

17.12 Phases of the Moon

• Why we always see the same hemisphere

  • Tidal locking: Moon’s rotation period equals its orbital period around Earth.

• Moon’s light and the Sun

  • Moon reflects sunlight; it does not produce its own light.

• Moonrise and phases

  • A full Moon rises at sunset; a Moon rising at midnight would be in the third quarter phase.

• Cycle duration

  • New moon to full moon ≈ two weeks (half of the ~29.5-day synodic month).

• Moon’s size and orbit relative to other satellites

  • The Moon is not the largest; Ganymede is larger; many smaller satellites exist.

• Lunar day/night and orbit

  • Lunar day/night each last about two Earth weeks; Moon’s orbital period relative to stars ≈ 27.3 days; Moon drifts eastward relative to the stars.

• Daily angular motion

  • The Moon moves about rac{360^ ext{o}}{27.3 ext{ days}} imes 1 ext{ day} ext{ ≈ } 13.2^ ext{o}/ ext{day}

17.13 Eclipses

• How the Moon’s size relates to eclipses

  • If the Moon were smaller, total solar eclipses would not occur because the Moon could not completely obscure the Sun; total lunar eclipses would still occur because Earth’s shadow would still cover the Moon.

• Why eclipses don’t happen every month

  • The Moon’s orbit is tilted by about 5° to the ecliptic; eclipses occur only when the Moon crosses the ecliptic at new or full phase.

• Solar vs lunar eclipse phases

  • Solar eclipse occurs at new Moon; lunar eclipse occurs at full Moon.

17.14 Lunar Surface and Interior

• Interior vs surface temperatures and activity

  • Moon’s interior is cooler and largely solid; limited seismic activity (moonquakes) supports a rigid interior.

• Maria and cratering history

  • Maria are dark, smooth plains formed by ancient lava flows; craters mostly from meteoroid impacts.

• Moonquakes and internal heat

  • Moonquakes are weaker and less frequent than Earthquakes, consistent with a cooler interior.

18.1 The Telescope; 18.2 The Spectrometer; 18.3 Spectrum Analysis

• Why large telescopes are valuable

  • Collect more light (fainter objects) and resolve close objects better; cameras enable long exposures beyond human eye limitations.

• Why stars twinkle

  • Atmospheric turbulence refracts starlight; planets appear as disks, reducing scintillation.

• Objects appearing as disks in a telescope

  • Planets appear as disks; stars remain point-like with telescopes.

• The sun’s spectrum and the sun’s composition

  • Composition inferred from dark absorption lines in the Sun’s spectrum; elements absorb at unique wavelengths.

• Star photospheres and dark lines

  • Dark lines originate in the relatively cool outer layers (photosphere) absorbing light at specific wavelengths.

18.4 Properties of the Sun

• Why the corona is usually invisible

  • Much dimmer than the photosphere; visible during total eclipses or with coronagraphs.

• Photosphere vs corona radiation

  • Photosphere provides most radiation due to higher density, despite corona being hotter, its low density limits total emission.

• Photon transport from core to surface

  • Photons undergo random-walk diffusion through dense plasma; can take tens of thousands of years to reach surface.

• Sun’s composition

  • Most abundant: hydrogen; second most abundant: helium.

• Helium discovery

  • Helium discovered in the Sun before on Earth; named after Helios.

18.5 The Aurora; 18.6 Sunspots; 18.7 Solar Energy

• Aurorae

  • Caused by charged particles from the Sun (solar wind) interacting with Earth’s atmosphere; concentrated at polar regions due to Earth’s magnetic field.

• Solar wind composition

  • Ions (mainly protons and electrons) streaming from the Sun.

• Sunspots

  • Cooler regions on the photosphere; appear darker due to being ~1500 K cooler than surroundings; involve an ~11-year solar cycle.

• Effects of solar storms

  • Can affect Earth’s magnetic field, power grids, radio, and satellites.

• Solar interior temperature

  • Core temperature ~15 million K; fusion converts hydrogen to helium; no solid core.

• Solar energy source and solar evolution

  • Hydrogen to helium fusion in core; Sun’s mass ~2×10^30 kg; annual mass loss and energy output discussed; Proxima Centauri distance ~4.24 light-years.

• Distance and luminosity relationship

  • Apparent brightness and luminosity relate to distance via b = rac{L}{4\,\pi d^2} (text presents the relation as b = 4\pi d^2 L, likely a misprint; the standard relation is the former).

• Standard candles: Cepheid variables

  • Period-luminosity relationship allows distance measurement.

18.8 Stellar Distances; 18.9 Variable Stars; 18.11 Stellar Properties; 18.12 H–R Diagram; 18.13 Stellar Evolution; 18.14 Supernovas; 18.15 Pulsars; 18.16 Black Holes

• Distances via luminosity and brightness

  • Compare intrinsic luminosity (L) with apparent brightness (b) to infer distance.

• Cepheids and distance measurement

  • Use period to infer luminosity, then determine distance.

• Stellar masses via binaries

  • Kepler’s Third Law applied to binary systems yields stellar masses.

• Stellar energy sources and life tracks on H–R diagram

  • Main sequence: hydrogen burning; higher mass → hotter, more luminous, shorter lifetimes; giant/supergiant branches; white dwarfs are ending stages; supernovae leave remnants (neutron stars, black holes).

• Common stellar endpoints

  • White dwarfs (Earth-sized), neutron stars (~10 km radius), black holes (event horizon defines size; gravitational collapse of massive stars).

19.1 The Milky Way; 19.2 Stellar Populations; 19.3 Radio Astronomy; 19.4 Galaxies; 19.5 Cosmic Rays; 19.6 Red Shifts; 19.7 Quasars; 19.8 Dating the Universe; 19.9 After the Big Bang; 19.10 Origin of the Solar System; 19.11 Exoplanets; 19.12 Interstellar Travel

• The Milky Way

  • Our galaxy is a large spiral; Sun is in the central disk.

  • Center of the disk thought to house a supermassive black hole.

• Major motions of Earth through space

  • Rotation about its axis; revolution around the Sun; Solar System’s orbit around the Galactic center; Milky Way’s motion through the universe.

• Globular clusters and Population II stars

  • Globular clusters are old, metal-poor, orbit the halo; Population II stars are old, metal-poor; located in the halo.

• Population I vs Population II

  • Population I: young, metal-rich stars in the disk/spiral arms; Population II: old, metal-poor in halo/globular clusters.

• Interstellar and intergalactic gas

  • Gas (mostly hydrogen) concentrated in the thin disk and spiral arms; neutral and molecular forms.

• Radio astronomy

  • Radio telescopes collect radio waves; do not magnify; larger telescopes improve sensitivity and resolution.

• Galaxies and large-scale structure

  • Galaxies clumped into clusters/superclusters, with filamentary structures and voids; rotation curves reveal dark matter

• Dark matter and dark energy

  • Dark matter inferred from rotation curves; located in halos around galaxies/clusters; dark energy inferred from accelerated expansion (Type Ia supernovae).

• The expanding universe and Big Bang evidence

  • Hubble’s law: v = H0 d; cosmic redshifts; CMB as relic radiation; light element abundances; cosmic timeline ~13.8 billion years.

• Exoplanets and planetary systems beyond the Sun

  • Planets form around other stars; detection challenging due to small size and faintness; exoplanets common but direct imaging is difficult.

• Interstellar travel

  • Many stars likely have planets; travel times to nearby stars are prohibitive with current tech; detection of exoplanets is challenging due to small sizes and brightness contrasts.

18.12–18.16 (Key equations and concepts)

• Luminosity-distance relation (standard form): b = rac{L}{4\pi d^2}

• Stefan–Boltzmann law: L = 4\pi R^2 \sigma T^4

• Doppler shift and binary stars: periodic redshift/blueshift in spectra indicates orbital motion.

• Hubble's law: v = H_0 d

• Distance to Cepheids via period-luminosity relation (specific functional form depends on calibration).

• Photosphere temperature and spectral classification connect to color: blue-hot; red-cool.

Connections, implications, and big ideas

• The Sun as a typical star: though ordinary, understanding its structure illuminates stellar physics for other stars.

• From small bodies to large-scale structure: asteroids, comets, and meteoroids provide clues to Solar System formation; galaxies and dark matter/energy reveal cosmic evolution.

• Tools of astronomy: telescopes (both optical and radio), spectrometry, and distance indicators (standard candles, parallax, Cepheids) are central to extracting physical properties.

• Observational evidence and scientific method: multiple lines of evidence (orbital dynamics, spectroscopy, CMB, supernovae) converge on the current cosmological model including dark matter and dark energy.

• Ethical/philosophical implications: exploration and detection of exoplanets and potential life raise questions about planetary protection and the search for life beyond Earth.

Quick reference: key formulas

• Luminosity–radius–temperature:
  L = 4\pi R^2 \sigma T^4

• Brightness–distance relationship (used for distance estimation):
  b = \frac{L}{4\pi d^2}

• Hubble’s law (expanding universe):
  v = H_0 d

• Cepheid period–luminosity relationship: (calibrated form depends on observations; used as standard candles)

• Distance from flux and luminosity (alternative forms): d = \sqrt{\frac{L}{4\pi b}}