Transcript Test 2 review session

Review for Test #2 October 27
Topics:
• Telescopes
• The Solar System and its Formation
• The Earth and our Moon
• The Terrestrial Planets
• The Jovian Planets
• Moons, Rings, Pluto, Comets, Asteroids, Dust, etc.
Methods
• Conceptual Review and Practice Problems Chapters 3 - 8
• Review lectures (on-line) and know answers to clicker & HW questions
• Try practice quizzes on-line (in MA)
•Bring:
• Two Number 2 pencils
• Simple calculator (no electronic notes)
Reminder: There are NO make-up tests for this class
Test #2 Review
How to take a multiple choice test
1) Before the Test:
• Study hard
• Get plenty of rest the night before
2) During the Test:
• Draw simple sketches to help visualize problems
• Solve numerical problems in the margin
• Come up with your answer first, then look for it in the choices
• If you can’t find the answer, try process of elimination
• If you don’t know the answer, Go on to the next problem and
come back to this one later
• TAKE YOUR TIME, don’t hurry
• If you don’t understand something, ask me.
Test #2 Useful Equations
Kepler’s laws, including:
P2  a3
Gravitation:
F=
G m1 m2
R2
Equivalence of Matter and Energy:
E = mc2
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.
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.
Prime focus
(GMRT)
Offset Cassegrain
(VLA)
Beam Waveguide
(NRO)
Reflector TypesCassegrain focus
(AT)
Naysmith
(OVRO)
Dual Offset
(ATA)
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.
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.
X-ray Optics
X rays and gamma rays will not reflect off mirrors as other
wavelengths do; need new techniques.
X rays will reflect at a very shallow angle, and can therefore be
focused.
Gamma Rays cannot be focused at all; images are coarse
The Structure of the Solar System
L3
L5
L4
~ 5 AU
~ 45 AU
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
initial gas and dust
nebula
dust grains grow by
accreting gas,
colliding and sticking
continued growth of
clumps of matter,
producing
planetesimals
planetesimals collide
and stick, enhanced
by their gravity
result is a few large
planets
Hubble observation of
disk around young star
with ring structure.
Unseen planet sweeping
out gap?
Terrestrial - Jovian Distinction
Terrestrial planets:
Inner parts of Solar Nebula hotter (due to forming Sun): mostly gas.
Accretion of gas atoms onto dust grains relatively inefficient.
Jovian planets:
Outer parts cooler: ices form (but still much gas), also ice "mantles" on
dust grains => much more solid material for accretion => larger
planetesimals => more gravity => even more material.
Jovian solid cores ~ 10-15 MEarth . Strong gravity => swept up and
retained large gas envelopes.
Composition of Terrestrial planets reflects that of initial dust – it is
not representative of Solar System, or Milky Way, or Universe.
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
The Greenhouse Effect
Main greenhouse
gases are H2O and
CO2 .
If no greenhouse
effect, surface
would be 40 oC
cooler!
The Moon
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Tides
A feature of oceans (but solid material has small tides too).
Two high and two low tides per day.
Tides are due to Moon's gravitational pull being stronger on
side of Earth closest to it (Sun causes smaller tides).
Earth-Moon gravity keeps them
orbiting each other. But side of Earth
closest to Moon has slightly stronger
pull to Moon => bulges towards it.
Other side has weaker pull => bulges
away compared to rest of Earth.
The Earth spins once a day while the
bulge always points towards and away
from the Moon => high and low tides.
Tides
Mercury
Mass = 3.3 x 1026 g
= 0.055 MEarth
Radius = 2439 km
= 0.38 REarth
Density = 5.4 g/cm3
Semimajor axis = 0.39 AU
Venus
Mass = 0.82 MEarth
Radius = 0.95 REarth
Density = 5.2 g/cm3
Average distance from Sun = 0.72 AU
Orbital period = 225 days
Rotation period = 243 days (longer
than orbital period, and retrograde!)
Mars
Mass = 0.11 MEarth
Radius = 0.53 REarth
Density = 3.9 g/cm3
Average distance from Sun = 1.52 AU
eccentricity = 0.093
Range in distance from Sun = 1.38 1.66 AU
Rotation Period = 24.6 hours
Orbital Period = 687 days
Pathfinder site was an outflow channel
Red arrows: rounded boulders indicating water erosion?
White arrows: "conglomerate" rock, like in Earth's riverbeds?
Blue arrows: sharp-edged boulders, volcanic rock?
The Jovian Planets (Gas Giants)
Jupiter
Saturn
Uranus
Neptune
(roughly to scale)
Storms on Jovian Planets
Jupiter's Great Red Spot: A hurricane
twice the size of Earth. Has persisted for
at least 340 years.
New storm “Oval BA”
Neptune's Great Dark Spot: Discovered by Voyager 2 in 1989. But
had disappeared by 1994 Hubble observations. About Earth-sized.
Why do storms on Jovian planets last so long?
On Earth, land masses disrupt otherwise smooth flow patterns. Not a
problem on Jovian planets.
Storms on Jovian Planets
Jupiter's Great Red Spot: A hurricane
twice the size of Earth. Has persisted for
at least 340 years.
New storm “Oval BA”
The Galilean Moons of Jupiter
(sizes to scale)
Io
Closest to Jupiter
Europa
Ganymede
Callisto
Furthest from Jupiter
Radii range from 1570 km (Europa, slightly smaller than our Moon), to 2630
km (Ganymede - largest moon in Solar System).
Orbital periods range from 1.77 days (Io) to 16.7 days (Callisto).
The closer to Jupiter, the higher the moon density: from 3.5 g/cm3 (Io) to 1.8
g/cm3 (Callisto). Higher density indicates higher rock/ice fraction.
Saturn's Rings (all Jovians have ring systems)
- Inner radius 60,000 km, outer radius
300,000 km. Thickness ~100 m!
- Composition: icy particles, <1 mm to
>10m in diameter. Most a few cm.
- A few rings and divisions distinguishable
from Earth.
Oort Cloud is a postulated huge, roughly spherical reservoir of comets
surrounding the Solar System. ~108 objects? Ejected planetesimals.
A passing star may dislodge Oort cloud objects, plunging them into
Solar System, where they become comets.
If a Kuiper Belt object's orbit takes it close to, e.g., Neptune, its
orbit may be changed and it may plunge towards the inner Solar
System and become a comet.
Study hard and do well!