Astronomy 1010-H
Planetary Astronomy
Fall_2015
Day-19
Course Announcements
•
How is the sunset/sunrise observing going?
•
Dark Sky nights – Wed. 10/7 starting at 7:30pm – at the
Observatory.
•
Exam-2 will be Friday, Oct. 9; Ch. 3, 4, & 5
•
SW-chapter 5 posted: due Fri. Oct. 9
•
No lab next week – Fall Break
Emitted Light
 Luminosity: amount of light leaving a
source.
 The amount and type of light leaving a
source changes as an object heats up or
cools down.
 The hotter an object is, the more luminous
it is.
 The hotter an object is, the bluer it is.
 Dense objects emit
a blackbody (or
Planck) spectrum.
 Continuous.
 Gives light at all
wavelengths.
 Example:
incandescent light
bulb.
 For two objects of
the same size, the
hotter one will:
• Emit more total light
at all wavelengths.
• Emit more total
energy every
second.
• Emit light at shorter
wavelengths, on
average.
Stefan’s Law
 Flux is the total amount of energy emitted
per square meter every second (the
luminosity per area).
 Then:
F  T
4
where T is the temperature, F is the flux,
and  (sigma) is called the StefanBoltzmann constant.
 Hotter objects emit much more energy (per
square meter per second) than cool objects.
Wien’s Law
 The peak wavelength of a blackbody is
inversely proportional to its temperature.
2
,
900
,
000
nm
K


peak
T
 Peak wavelength peak : the wavelength of
light of a blackbody that is emitted the
most.
 Here the wavelength is in nanometers and
the temperature is in kelvin.
 “Hotter means bluer.”
L
B
2
4r
 Brightness is the
amount of light
arriving at a
particular place.
 Decreases as the
distance from a light
source increases,
obeying an inverse
square law.
 The light spreads
out over a greater
area.
 Temperature is a measure of the average
speed of the motions of atoms.
 Kelvin scale: Water freezes/boils at 273 K /
373 K.
 Absolute zero is when thermal motion stops.
Equilibrium Temperature
 Balance between absorbed and radiated energy.
 Albedo (reflectance) of a planet.

a = 1: 100% reflection

a = 0 : 100% absorption
 Energy absorbed = R2 * L/4d2 * (1 – a)
 Energy Radiated = 4R2 * T4
 Radiation laws help figure out the equilibrium
temperatures of the planets.
 Distant planets are cold mainly because of
the inverse square law of light.
 Balance of heating and cooling.
CONNECTIONS 5.1
 A stable equilibrium is a balance that is unlikely to
change, while an unstable equilibrium can easily
be nudged away from its balance point.
 Static equilibrium involves a situation where
forces are balanced and opposing each other, so
nothing changes.
 Dynamic equilibrium involves a situation that is
always changing, but remains in balance.
MATH TOOLS 5.3
 With the Stefan-Boltzmann law, you can find
Earth’s flux using its average temperature of
288 K.
 Using Wien’s law, you can find the Sun’s
surface temperature using the fact that its
peak wavelength is around 500 nm.
MATH TOOLS 5.4
 The equilibrium temperature of a planet
depends on the energy it receives and its
albedo (a), its reflectivity.
MATH TOOLS 5.4
 The equilibrium temperature of a planet
depends on the energy it receives and its
albedo (a), its reflectivity.
 Simplifying:
PROCESS OF SCIENCE
 Confirmation of an idea from different
fields of science can be a strong indication
of the truth of that idea.
Exam-2 To Here!
Telescopes & Instruments
 The telescope is the
astronomer’s most
important tool.
 Purpose: to gather
light of all kinds.
 Two kinds of optical
telescopes:
reflecting and
refracting.
 Invented in 1608 by
Hans Lippershey.
Telescopes come in three
general types
Reflectors use mirrors to reflect
the light to a focus
Refractors use lenses to bend
the light to a focus
Catadioptric telescopes use both lenses and
mirrors
Telescopes

Telescopes have three functions:
1.
Gather light

2.
Resolve objects

3.
LGP ∝ Area = πR2
Θ = 2.06 X 105 (λ/D)
Magnify EXTENDED objects
The most important property of any
telescope is to gather large amounts
of light and concentrate it to a focus.
Light Gathering Power  Area  radius 2
Refraction is the bending of
light when it goes from one
medium to another
“n” is the
index of
refraction.
c
n
v
Refraction is
governed by
Snell’s Law:
n1 sin 1  n2 sin 2
If we curve the surface and
make a lens, we can get the
light to concentrate to a point
 Refracting telescopes
use lenses.
 Objective lens: refracts
the light.
 Aperture: size of the
objective lens (larger
aperture gathers more
light).
 The objective lens is
placed in the aperture.