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: where T is the temperature, F is the flux, and  (sigma) is called the Stefan- Boltzmann 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.  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.”

 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 =  R 2 * L/4  d 2 * (1 – a) Energy Radiated = 4  R 2 *  T 4

 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.

 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. CONNECTIONS 5.1

 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 5.3

 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: MATH TOOLS 5.4

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 Refractors use lenses to bend the light to a focus Reflectors use mirrors to reflect the light to a focus Catadioptric telescopes use both lenses and mirrors

Telescopes Telescopes have three functions: 1. Gather light LGP ∝ Area = πR 2 2. Resolve objects Θ = 2.06 X 10 5 ( λ/D) 3. Magnify EXTENDED objects

The most important property of any telescope is to gather large amounts of light and concentrate it to a focus.

Refraction is the bending of light when it goes from one medium to another “n” is the index of refraction. Refraction is governed by Snell’s Law:

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.