Transcript Test 2 Overview

Test 2: Overview
The total energy radiated from entire surface every second is called the
luminosity. Thus
Luminosity = (energy radiated per cm2 per sec) x (area of surface in cm2)
For a sphere, area of surface is 4pR2, where R is the sphere's radius.
The "Inverse-Square" Law Applies to Radiation
Each square gets 1/4
of the light
Each square gets 1/9
of the light
apparent brightness a 1
D2
D is the distance between
source and observer.
The frequency or wavelength of a wave depends on the
relative motion of the source and the observer.
Types of Spectra
1. "Continuous" spectrum - radiation
over a broad range of wavelengths
(light: bright at every color).
2. "Emission line" spectrum - bright at
specific wavelengths only.
3. Continuous spectrum with
"absorption lines": bright over a broad
range of wavelengths with a few dark
lines.
Kirchhoff's Laws
1. A hot, opaque solid, liquid
or dense gas produces a
continuous spectrum.
2. A transparent hot gas
produces an emission line
spectrum.
3. A transparent, cool gas
absorbs wavelengths from a
continuous spectrum,
producing an absorption line
spectrum.
The pattern of emission (or absorption) lines is a fingerprint of the
element in the gas (such as hydrogen, neon, etc.)
For a given element, emission and absorption lines occur at the same
wavlengths.
Sodium emission and absorption spectra
Stellar Spectra
Spectra of stars are different mainly due to temperature and composition differences.
'Atmosphere', atoms and
ions absorb specific
wavelengths of the blackbody spectrum
Interior, hot and
dense, fusion
generates radiation
with black-body
spectrum
Star
The Nature of Atoms
The Bohr model of the Hydrogen atom:
electron
_
_
+
+
proton
"ground state"
an "excited state"
Ground state is the lowest energy state. Atom must gain energy to
move to an excited state. It must absorb a photon or collide with
another atom.
But, only certain energies (or orbits) are allowed:
_
_
_
+
a few energy levels of H atom
The atom can only absorb photons with exactly the right
energy to boost the electron to one of its higher levels.
(photon energy α frequency)
When an atom absorbs a photon, it moves to a higher energy state briefly
When it jumps back to lower energy state, it emits a photon in a random direction
Ionization
Hydrogen
_
+
Energetic UV
Photon
_
Helium
+
Energetic UV
Photon
+
_
"Ion"
Atom
Two atoms colliding can also lead to ionization.
Radio Window
13
Optical Telescopes - Refracting vs. Reflecting
Refracting telescope
Focuses light with a lens (like a camera).
<-- object (point of light)
image at focus
Problems:
- Lens can only be supported around edge.
- "Chromatic aberration".
- Some light absorbed in glass (especially UV, infrared).
- Air bubbles and imperfections affect image quality.
Chromatic Aberration
Lens - different colors focus at different places.
white light
Mirror - reflection angle doesn't depend on color.
Reflecting telescope
Focuses light with a curved mirror.
<-- object
image
- Can make bigger mirrors since they are supported from behind.
- No chromatic aberration.
- Reflects all radiation with little loss by absorption.
Image of Andromeda galaxy with
optical telescope.
Image with telescope of twice the
diameter, same exposure time.
Resolving Power of a Mirror
(how much detail can you see?)
Andromeda Galaxy: (a) 10 arcminutes,
(b) 1 arcminute, (c) 5 arcseconds, and
(d) 1 arcsecond
fuzziness
you would
see with
your eye.
detail you
can see
with a
telescope.
Seeing
*
Air density varies => bends light.
No longer parallel
Parallel rays enter
atmosphere
dome
No blurring case.
Rays brought to
same focus.
Blurring. Rays
not parallel. Can't
be brought into
focus.
CCD
*
Sharp image
on CCD.
Blurred
image.
Radio Telescopes
Large metal dish acts as a mirror for radio
waves. Radio receiver at focus.
Surface accuracy not so important, so easy
to make large one.
But angular resolution is poor. Remember:
Jodrell Bank 76-m (England)
angular resolution α
wavelength
mirror diameter
D larger than optical case, but wavelength much larger (cm's to m's),
e.g. for wavelength = 1 cm, diameter = 100 m, resolution = 20".
Interferometry
A technique to get improved angular resolution using an array of
telescopes. Most common in radio, but also limited optical interferometry.
D
Consider two dishes with separation D vs. one dish of diameter D.
By combining the radio waves from the two dishes, the achieved
angular resolution is the same as the large dish.
Orbits of Planets
All orbit in same direction.
Most orbit in same plane.
Elliptical orbits, but low eccentricity for most, so nearly circular.
Two Kinds of “Classical” Planets
"Terrestrial"
"Jovian"
Mercury, Venus,
Earth, Mars
Jupiter, Saturn,
Uranus, Neptune
Close to the Sun
Small
Mostly Rocky
High Density (3.3 -5.3 g/cm3)
reminder: liquid water is 1 g/cm3
Slow Rotation (1 - 243 days)
Few Moons
No Rings
Main Elements Fe, Si, C, O, N:
we learn that from the spectra
Far from the Sun
Large
Mostly Gaseous
Low Density (0.7 -1.6 g/cm3)
Fast Rotation (0.41 - 0.72 days)
Many Moons
Rings
Main Elements H, He
Dwarf Planets compared to Terrestrial
Planets
"Terrestrial"
Dwarf Planets
Mercury, Venus,
Earth, Mars
Pluto, Eris, many
others
Close to the Sun
Small
Mostly Rocky
High Density (3.3 -5.3 g/cm3)
Slow Rotation (1 - 243 days)
Few Moons
No Rings
Main Elements Fe, Si, C, O, N
Far from the Sun
Very small
Rock and Ice
Moderate Density (2 - 3 g/cm3)
Rotation?
Few Moons
No Rings
Main Elements Fe, Si, C, O, N
And an icy surface
Early Ideas
René Descartes (1596 -1650) nebular theory:
Solar system formed out of a "whirlpool" in a "universal
fluid". Planets formed out of eddies in the fluid.
Sun formed at center.
Planets in cooler regions.
Cloud called "Solar Nebula".
This is pre-Newton and modern science. But basic idea correct,
and the theory evolved as science advanced, as we'll see.
A cloud of interstellar gas
a few light-years,
or about 1000
times bigger than
Solar System
The associated dust blocks starlight. Composition mostly H, He.
Too cold for optical emission but some radio spectral lines from
molecules. Doppler shifts of lines indicate clouds rotate at a few km/s.
Clumps within such clouds collapse to form stars or clusters of stars.
They are spinning at about 1 km/s.
Now to make the planets . . .
Solar Nebula:
98% of mass is gas (H, He)
2% in dust grains (Fe, C, Si . . .)
Condensation theory: 3 steps:
1) Dust grains act as "condensation nuclei": gas atoms stick to
them => growth of first clumps of matter.
2) Accretion: Clumps collide and stick => larger clumps.
Eventually, small-moon sized objects: "planetesimals".
3) Gravity-enhanced accretion: objects now have significant
gravity. Mutual attraction accelerates accretion. Bigger objects
grow faster => a few planet-sized objects.
Result from computer simulation of planet growth
Shows growth of terrestrial planets. If Jupiter's gravity not included, fifth
terrestrial planet forms in Asteroid Belt. If Jupiter's gravity included, orbits
of planetesimals there are disrupted. Almost all ejected from Solar System.
Simulations also suggest that a few Mars-size objects
formed in Asteroid Belt. Their gravity modified orbits of
other planetesimals, before they too were ejected by
Jupiter's gravity.
Asteroid Ida
The Structure of the Solar System
L3
L5
L4
~ 5 AU
~ 45 AU
Interplanetary Matter: Asteroids
Asteroids and meteoroids have rocky composition;
asteroids are bigger.
(below)
Asteroid
Gaspra
(above) Asteroid
Ida with its
moon, Dactyl
(above)
Asteroid
Mathilde
Interplanetary Matter: Comets
Comets are icy, with some rocky parts.
The basic components of a comet
Oort Cloud
The size, shape, and orientation of cometary orbits
depend on their location. Oort cloud comets rarely
enter the inner solar system.
Meteor Showers
Meteor showers are
associated with comets –
they are the debris left
over when a comet
breaks up.
Earth's Internal Structure
How do we know? Earthquakes. See later
Crust: thin. Much Si and Al
(lots of granite). Two-thirds
covered by oceans.
Mantle is mostly solid, mostly
basalt (Fe, Mg, Si). Cracks in
mantle allow molten material
to rise => volcanoes.
Core temperature is 6000 K.
Metallic - mostly nickel and
iron. Outer core molten, inner
core solid.
Atmosphere very thin
Earthquakes
They are vibrations in the solid Earth, or seismic waves.
Two kinds go through Earth, P-waves ("primary") and S-waves ("secondary"):
Like all waves, seismic waves bend when they encounter changes in
density. If density change is gradual, wave path is curved.
S-waves are unable to travel in liquid.
Thus, measurement of seismic wave gives info on density of Earth's
interior and which layers are solid/molten.
Zone with no S waves:
must be a liquid core
that stops them
But faint P waves
seen in shadow zone,
refracting off dense
inner core
No P waves too:
they must bend sharply
at core boundary
Curved paths of
P and S waves:
density must slowly
increase with depth
Earthquakes and volcanoes are related, and also don't occur at random
places. They outline plates.
Plates moving at a few cm/year. "Continental drift" or "plate tectonics"
What causes the drift?
Convection! Mantle slightly fluid and can support convection.
Plates ride on top of convective cells. Lava flows through cell
boundaries. Earth loses internal heat this way.
Cycles take ~108 years.
Plates form lithosphere (crust and solid upper mantle).
Partially melted, circulating part of mantle is asthenosphere.
When plates meet...
1) Head-on collision
(Himalayas)
side view
2) "Subduction zone"
(one slides under the other)
(Andes)
3) "Rift zone"
(two plates moving apart)
(Mid-Atlantic Ridge)
4) They may just slide past each other
(San Andreas Fault)
top view
=> mountain ranges, trenches, earthquakes, volcanoes
The Greenhouse Effect
Main greenhouse
gases are H2O and
CO2 .
If no greenhouse
effect, surface
would be 40 oC
cooler!