Astronomy Study Guide 2

Chapter 5 - The Formation of a Solar System

• Nebular Theory:
  The nebular theory suggests that the solar system formed from a giant cloud of gas and dust (a nebula) that gradually collapsed under gravity. As the cloud contracted, it spun faster, flattened into a disk, and formed the Sun and planets.

• Disk Flattening:
  As the rotating nebula collapsed, conservation of angular momentum caused it to flatten into a disk shape. This explains why planets orbit the Sun in roughly the same plane and direction. The faster rotational speed also influenced the solar system’s shape.

• Accretion:
  Accretion is the process by which dust and small particles in the protoplanetary disk collided and stuck together, gradually forming larger bodies like planetesimals and, eventually, planets.

• Why Terrestrial Planets Are Rocky & Jovian Planets Are Gas Giants:

  • Terrestrial planets (Mercury, Venus, Earth, Mars) formed closer to the Sun, where it was too hot for gases to condense, leaving behind dense, rocky materials.

  • Jovian planets (Jupiter, Saturn, Uranus, Neptune) formed farther out, where temperatures were colder, allowing gases like hydrogen and helium to condense and accumulate.

• Order of the Planets from the Sun:
  Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

• Meteoroids vs. Meteors vs. Meteorites:

  • Meteoroid: A small rock or particle in space.

  • Meteor: A meteoroid that burns up as it enters Earth's atmosphere (a "shooting star").

  • Meteorite: A meteoroid that survives its descent and lands on Earth's surface.

• Comet Origins:

  • Long-period comets come from the Oort Cloud, a distant region far beyond Pluto.

  • Short-period comets come from the Kuiper Belt, located just beyond Neptune’s orbit.

• Asteroids as Failed Planetesimals:
  Asteroids are rocky bodies that never combined into a full-sized planet due to gravitational disruptions, primarily from Jupiter.

• Planet vs. Dwarf Planet:

  • Planet: Orbits the Sun, has enough gravity to become nearly spherical, and has cleared its orbit of other debris.

  • Dwarf Planet: Meets the first two criteria but hasn’t cleared its orbit (e.g., Pluto).

Chapter 10 - The Sun as a Star

• Hydrostatic Equilibrium:
  This balance occurs when the inward pull of gravity is perfectly counteracted by the outward pressure from nuclear fusion, keeping the Sun stable.

• Layers of the Sun (from innermost to outermost):

  • Core: Extremely hot (about 15 million K); where nuclear fusion occurs.

  • Radiative Zone: Energy moves outward slowly via radiation; opaque to light.

  • Convective Zone: Energy transferred through convection (rising hot gas); less dense than the radiative zone.

  • Photosphere: The visible surface; about 5,800 K.

  • Chromosphere: A thin, reddish layer seen during solar eclipses.

  • Corona: The Sun's outer atmosphere; very hot (millions of K) and extends far into space.

• Proton-Proton Chain (Nuclear Fusion):
  The process where hydrogen nuclei combine to form helium, releasing energy that powers the Sun.

• Solar Thermostat:
  A self-regulating process in which any increase in temperature speeds up fusion, causing the Sun to expand slightly and cool down — and vice versa.

• Sun’s Magnetic Activity & Solar Phenomena:

  • Sunspots: Dark, cooler regions on the Sun caused by magnetic field disruptions.

  • Prominences: Loops of gas that follow magnetic field lines above the Sun’s surface.

  • Solar Flares: Sudden, intense bursts of radiation from the Sun’s surface.

• Solar Flares vs. Coronal Mass Ejections (CMEs):

  • Solar Flare: A brief, powerful release of energy.

  • CME: A massive burst of solar wind and magnetic fields that can cause auroras and power grid disruptions on Earth.

• Sunspot Cycle:
  A roughly 11-year cycle where the number of sunspots increases to a peak and then decreases again.

Chapters 6, 7 & 8 - The Planets

• Terrestrial Planet Layers:

  • Core: Dense metal core (mostly iron).

  • Mantle: Rocky layer surrounding the core.

  • Crust: Solid outer layer.

• Mercury’s Temperature Extremes:
  Mercury’s lack of an atmosphere allows daytime temperatures to reach 800°F (427°C) and nighttime temperatures to drop to -330°F (-201°C).

• Mercury’s Atmosphere (or lack thereof):
  Mercury’s weak gravity and proximity to the Sun prevent it from holding a substantial atmosphere.

• Venus’ Thick Atmosphere:
  Venus’ atmosphere is rich in carbon dioxide, trapping heat through a runaway greenhouse effect, making it the hottest planet in the solar system.

• Earth’s Magnetosphere:
  Earth’s magnetic field is generated by the movement of molten iron in its outer core. It protects the planet from harmful solar wind and radiation.

• Mars’ Weak Magnetic Field:
  Mars lacks a strong magnetic field because its core cooled and solidified faster than Earth’s, halting the dynamo effect.

• Primary Atmospheric Composition:

  • Venus: Carbon dioxide with clouds of sulfuric acid.

  • Mars: Carbon dioxide with traces of nitrogen and argon.

• Surface Features:

  • Mercury: Craters, cliffs, and smooth plains.

  • Venus: Volcanic plains, mountains, and few craters.

  • Mars: Large volcanoes (like Olympus Mons), canyons (like Valles Marineris), and dry riverbeds.

• Maria vs. Highlands on the Moon:

  • Maria: Dark, flat regions formed by ancient volcanic eruptions.

  • Highlands: Lighter, mountainous regions made of older, cratered rock.

• Planet & Moon Facts:

  • Venus rotates backward (retrograde rotation).

  • Europa (one of Jupiter’s moons) is a prime candidate for extraterrestrial life due to its subsurface ocean.

• Jovian Planet Composition:
  Jupiter, Saturn, Uranus, and Neptune are composed mostly of hydrogen and helium.

• Uranus & Neptune’s Blue Appearance:
  These planets contain methane, which absorbs red light and reflects blue light, giving them their distinct color.