Astronomy 241: Foundations of Astrophysics I. The Solar System
Astronomy 241 is the first part of a year-long Prerequisites: Physics 170, Physics 272 introduction to astrophysics. It uses basic (or concurrent), and Math 242 or 252A. classical mechanics and thermodynamics to Contact: Professor Joshua Barnes analyze the structure and evolution of the ([email protected]; 956-8138) Solar System. www.ifa.hawaii.edu/~barnes/ast241
Tuesday, August 21, 2012 Astrophysics BS Degree Proposal in ASTR PHYS MATH CHEM
preparation FALL 241 251A 161, 171 or 181 year 1 161L, 171L, or 181L
SPRING 170 242 252A 162 year 1 170L 162L
Foundations of Astrophysics I: FALL 241 272 243 253A The Solar System year 2 272L
Foundations of Astrophysics II: SPRING 242 274 244 Galaxies & Stars year 2 274L
Observational Astronomy & FALL 300 310 311 (or 307?) Laboratory year 3 300L 350
SPRING 301 311 Observational Project year 3 450
FALL 423 480 Stellar Astrophysics year 4 495
SPRING 496 481 Senior Project I, II year4 485
1 of: ASTR 320, 426, or 430
Tuesday, August 21, 2012 Units
Text uses MKS units (meter, kilo-gram, second); e.g.
G ≃ 6.674 × 10-11 m3 kg-1 s-2 (gravitational constant).
Astronomers also use non-standard units:
AU ≃ 1.496 × 1011 m (“average” Earth-Sun distance)
30 M⊙ ≃ 1.989 × 10 kg (Sun’s mass)
yr ≃ 3.156 × 107 s (Earth’s orbital period)
Tuesday, August 21, 2012 Order of Magnitude & Dimensional Analysis: An Example
Given that Jupiter’s average density is slightly greater than water, estimate the orbital period of a satellite circling just above the planet.
-3 -10 3 -1 -2 ρJ ~ 1000 kg m G ~ 10 m kg s
-7 -2 GρJ ~ 10 s
-1/2 3.5 (GρJ ) ~ 10 s ~ 1 hour
(More accurate calculation gives 2.9 hours.)
Tuesday, August 21, 2012 Precision
For most calculations, uncertainties of ~1% are OK. This requires two or three significant figures.
e.g., let X = a × 10b
magnitude of a 10% uncertainty 1% uncertainty |a| ≥ 5 one sig. fig. two sig. fig. |a| < 5 two sig. fig. three sig. fig.
• Don’t exaggerate the precision of your results! • Remember some operations magnify uncertainty; e.g., if X has 1% uncertainty, X3 has 3% uncertainty.
Tuesday, August 21, 2012 Astronomy 241: Foundations of Astrophysics I
1. Solar System Overview
1. Constituents of the Solar System
2. Rotation and Revolution
3. Formation Scenario
Tuesday, August 21, 2012 Overview: Structure
100 AU
S
20,000 AU 5 AU
Inner system: Outer system: Oort Cloud: terrestrial planets, jovian planets and comets. asteroids. moons, “KBOs”.
Tuesday, August 21, 2012 Overview: Two Types of Planets
Terrestrial Planets Jovian Planets small size & mass large size & mass high density low density
rock & metal H, He, H2O, CH4, NH 3, . . . solid surface no solid surface few moons, no rings many moons & rings close to sun, warm far from sun, cold
Tuesday, August 21, 2012 A Brief Tour: The Sun
• Largely H and He • Over 99.9% of solar system’s mass • Self-regulating nuclear furnace
Tuesday, August 21, 2012 A Brief Tour: Mercury
• Large iron core and desolate rock crust
Tuesday, August 21, 2012 A Brief Tour: Venus
• Earth’s “twisted sister” • Extreme greenhouse effect
Tuesday, August 21, 2012 A Brief Tour: Earth
• Liquid surface water!
• Complex and dynamic atmosphere
• Remarkably large moon
Tuesday, August 21, 2012 A Brief Tour: Mars
• Cold & dry, but water flowed long ago • Complex and interesting topography
Tuesday, August 21, 2012 A Brief Tour: Jupiter
• Largest gas-giant planet — no solid surface!
• Many different moons, some as big as planets
Tuesday, August 21, 2012 A Brief Tour: Saturn
• Spectacular ring system • Titan, a moon with atmosphere
Tuesday, August 21, 2012 A Brief Tour: Uranus
• Water and other hydrogen molecules
• Extreme axis tilt
Tuesday, August 21, 2012 A Brief Tour: Neptune
• Most distant major planet
• Large moon with “backwards” orbit
Tuesday, August 21, 2012 A Brief Tour: Pluto and Other “Icy Dwarfs”
• Tiny compared to major planets
• Ices of H2O & other H-rich compounds
• Many still to be discovered!
Tuesday, August 21, 2012 A Brief Tour: Small Objects
Asteroids: • small (< 1000 km) • rock & metal (?) • inner system
Comets: Dust & Debris: • small (~ 10 km) • meteoroid streams • ice & rock on cometary orbits • Kuiper belt & Oort cloud • zodiacal dust
Tuesday,Dwarf August 21, 2012 planets: • small (~ 1000 km) • ice & rock • Kuiper belt A Brief Tour: The Solar Wind
• Outflow of charged particles
• Speeds of 300 to Earth’s Magnetosphere 800 km s-1
Spectacular CMEs and proton showers, LASCO C3 (Apr. 1-27, 2001)
• Interacts with planetary magnetic fields
Tuesday, August 21, 2012 Diurnal Motion
Due to rotation of Earth.
Sidereal Day: 86,162 s (≃ 23h56m)
Solar Day: 86,400 s (= 24h)
Tuesday, August 21, 2012 Planetary Motion
The Sun & Moon always move west- to-east relative to the stars.
The planets usually move west-to-east as well, but some- times they move the other way!
Tuesday, August 21, 2012 Planetary Motion: Mars Goes Retrograde
Mars at Opposition
All planets undergo retrograde motion.
Tuesday, August 21, 2012 Copernican model
1. Heavenly motions are uniform, eternal, and circular or compounded of several circles.
2. The center of the universe is near the Sun.
3. Around the Sun, in order, are Mercury, Venus, Earth and Moon, Mars, Jupiter, Saturn, and the fixed stars.
4. The Earth has three motions: daily rotation, annual revolution, and annual tilting of its axis.
5. Retrograde motion of the planets is explained by the Earth's motion.
6. The distance from the Earth to the Sun is small compared to the distance to the stars.
Tuesday, August 21, 2012 Revolution and Rotation
All planets and most moons revolve in the same direction and stay close to the same plane.
The sun and most planets also rotate in that direction.
This provides a clue to the Solar System’s formation.
Tuesday, August 21, 2012 Formation Scenario
Tuesday, August 21, 2012 Collapse: From Cloud to Disk
1. A gas cloud starts to collapse due to its own gravity.
2. It spins faster and heats up as it collapses.
3. Vertical motions die out, leaving a spinning disk.
4. The solar system still spins in the same direction.
Tuesday, August 21, 2012 Collapse: Angular Momentum and Energy
1. Angular momentum conservation causes the cloud to spin faster as it contracts: (rotation speed) ∝ 1 (cloud diameter) Collapse stops when the cloud spins at orbital speed.
2. Energy conservation causes the cloud to heat up:
potential kinetic thermal energy energy energy gravitational collapse gas shocks
Tuesday, August 21, 2012 Planet Formation: The Frost Line
The disk was hot at the center, and cool further out.
Inside the frost line, only Outside, hydrogen compounds rocks & metals can condense. can also condense.
The frost line was ~4 AU from the Sun — between the present orbits of Mars and Jupiter.
Tuesday, August 21, 2012 Planet Formation: Terrestrial Planets
1. Within the frost line, bits of rock and metal clumped together to make planetesimals. 2. As the planetesimals grew, they became large enough to attract each other. 3. Finally, only a few planets were left.
Tuesday, August 21, 2012 Planet Formation: Jovian Planets
1. Outside the frost line, icy planetesimals were very common, forming planets about 10 times the mass of Earth.
2. These planets attracted nearby gas, building up giant planets composed mostly of H and He.
3. The disks around these planets produced moons.
Tuesday, August 21, 2012