Telescopes Yerkes Refractor Arecibo Radio Disk Mauna Kea Hubble Space Telescope
The Earth’s Shroud • The Earth’s atmosphere acts to “screen” out certain kinds, or bands, of light. • Visible light and radio waves penetrate the atmosphere easiest; the IR somewhat. Most other bands are effectively blocked out. • Consequently, telescopes are built at high altitude or placed in space to access these otherwise inaccessible bands.
Flux of Light Light carries energy (e.g., perceived warmth from sunlight) How does this energy propagate through space? And how does that relate to the apparent brightness of a source? “Flux” describes how light spreads out in space: with L=luminosity (or power), and d = distance, flux is Watts/square meter = J/s/m2
Kirchoff’s Laws • A hot solid, liquid, or dense gas produces a continuous spectrum of emission. • A thin gas seen against a cooler background produces a bright line or emission line spectrum. • A thin gas seen against a hotter source of continuous radiation produces a dark line or absorption line spectrum.
Blackbodies • A common approximation for the continuous spectrum produced by many astrophysical objects is that a blackbody (or Planckian). • A blackbody (BB) is a perfect absorber of all incident light. • BBs also emit light!
Atomic Physics • Atoms composed of protons, neutrons, and electrons • p and n in the nucleus • e resides in a “cloud” around the nucleus • mp/mn~1 • mp/me~2000
Interaction of Matter and Light • Absorption: Occurs when a photon of the correct energy moves an electron from a lower orbit to an upper orbit. • Emission: Occurs when an electron drops from an upper orbit to a lower one, thereby ejecting a photon of corresponding energy • Ionization: Occurs when a photon knocks an electron free from the atom • Recombination: Capture of a free electron
The Gross Solar Spectrum Blackbody-like Blackbody deviations
Doppler Shift The Doppler effect is a change in l, n, E of light when either or both the source and detector are moving toward or away from one another. So, this is a relative effect.
Brief Overview of Stellar Evolution • Pre-Main Sequence (really short time): The phase in which a protostar forms out of a cloud of gas that is slowly contracting under gravity • Main Sequence (long time): The phase in which a star-wannabe becomes hot enough to initiate and maintain nuclear fusion of hydrogen in its core to become a true star. • Post-Main Sequence (sorta short time): H-burning ceases, and other kinds of burning may occur, but the star is destined to become a White Dwarf, Neutron Star, or Black Hole
Observational Clues from the Solar System: Orbits of planets lie nearly in ecliptic plane The Sun’s equator lies nearly in the ecliptic Inner planets are rocky and outer ones gaseous All planets orbit prograde Sun rotates prograde Planet orbits are nearly circular Big moons orbit planets in a prograde sense, with orbits in equatorial plane of the planet Rings of Jovians in equatorial planes S.S. mass in Sun, but angular momentum in planet orbits Formation of Stars and Planets
Solar Nebula Theory Immanuel Kant (German): 1775, suggested that a rotating cloud that contracts under gravity could explain planetary orbit characteristics Basic Modern View – • Oldest lunar rocks ~4.6 Gyr • Planets formed over brief period of 10-100 Myr • Gas collects into “disk”, and cools leading to formation of condensates • Growth of planetesimals by collisions • Build up minor bodies and small rocky worlds • Build up Jovian cores that sweep up outer gases
The Chaotic Early Solar System • Recent computer models are challenging earlier views that planets formed in an orderly way at their current locations • These models suggest that the jovian planets changed their orbits substantially, and that Uranus and Neptune could have changed places • These chaotic motions could also explain a ‘spike’ in the number of impacts in the inner solar system ~3.8 billion years ago The Moon and terrestrial planets were bombarded by planetesimals early in solar system history.
Cosmic Billiards • The model predicts: • After formation, giant planet orbits were affected by gravitational ‘nudges’ from surrounding planetesimals • Jupiter and Saturn crossed a 1:2orbital resonance (the ratio of orbital periods), which made their orbits more elliptical. This suddenly enlarged and tilted the orbits of Uranus and Neptune • Uranus / Neptune cleared away the planetesimals, sending some to the inner solar system causing a spike in impact rates 100 Myr 880 Myr 20 AU planetesimals 883 Myr ~1200 Myr N U S J The early layout of the solar system may have changed dramatically due to gravitational interactions between the giant planets. Note how the orbits of Uranus and Neptune moved outwards, switched places, and scattered the planetesimal population.
The Big Picture • The current layout of our solar system may bear little resemblance to its original form • This view is more in line with the “planetary migration” thought to occur even more dramatically in many extrasolar planet systems • It may be difficult to prove or disprove these models of our early solar system. The many unexplained properties of the nature and orbits of planets, comets and asteroids may provide clues. Artist’s depiction of Neptune orbiting close to Jupiter (courtesy Michael Carroll)
Density and Composition Ex: Moon – r(surf) ~ 2800 and <r> ~ 3300 Earth – r(surf) ~ 2800 but <r> ~ 5500