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Lecture 12. ASTR 111 – Section 002. Measurements in Astronomy. In astronomy, we need to make remote and indirect measurements Think of an example of a remote and indirect measurement from everyday life. Using Light.

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lecture 12

Lecture 12

ASTR 111 – Section 002

measurements in astronomy
Measurements in Astronomy
  • In astronomy, we need to make remote and indirect measurements
    • Think of an example of a remote and indirect measurement from everyday life
using light
Using Light
  • Light has many properties that we can use to learn about what happens far away
  • Light interacts with matter in a special way
slide4
Only photons with special wavelengths will interact with atom

How will this affect what a person will see at point X?

When is the atom “hotter”?

X

slide5
Why is UV light usually blamed for skin cancer? What is special about it compared to other light sources?
slide7

Cloud

of

Gas

A prism bends photons more or less depending on their wavelength

slide8

Cloud

of

Gas

A prism bends photons more or less depending on their wavelength

slide15
What type of spectrum is produced when the light emitted from a hot, dense object passes through a prism?
  • What type of spectrum is produced when the light emitted directly from a cloud of gas passes through a prism?
  • Describe the source of light and the path the light must take to produce an absorption spectrum
  • There are dark lines in the absorption spectrum that represent missing light. What happened to this light that is missing in the absorption line spectrum?

From Lecture Tutorials for Introductory Astronomy, page 61.

slide18
Stars like our Sun have low-density, gaseous atmospheres surrounding their hot, dense cores. If you were looking at the spectra of light coming from the Sun (or any star), which of the three types of spectra would be observed?
  • If a star existed that was only a hot dense core and did not have a low-density atmosphere surrounding it, what type of spectrum would you expect this particular star to give off?
  • Two students are looking at a brightly lit full Moon, illuminated by reflected light from the Sun. Consider the following discussion between two students about what the spectrum of moonlight would look like:
    • I think moonlight is just reflected sunlight, so we will see the Sun’s absorption line spectrum.
    • I disagree, an absorption spectrum has to come from a hot, dense object. Since thie Moon is not a hot, dense object, it can’t give off an absorption line spectrum.

Do you agree or disagree with either or both of these students? Explain your reasoning.

slide19
Imagine that your are looking at two different spectra of the Sun. Spectrum #1 is obtained using a telescope that is in a high orbit far above Earth’s atmosphere. Spectrum #2 is obtained using a telescope located on the surface of Earth. Label each spectrum below as either Spectrum #1 or Spectrum #2.
slide20
Would this make sense?

This dark line was removed

energy and electromagnetic radiation
Energy and electromagnetic radiation

Planck’s law relates the energy of a photon to its frequency or wavelength

E = energy of a photon

h = Planck’s constant

c = speed of light

l = wavelength of light

The value of the constant h in this equation, called Planck’s constant, has been shown in laboratory experiments to be

h = 6.625 x 10–34 J s

slide22
Which electromagnetic wave has a higher energy: one with f=10 cycles per second or f=1 cycles per second?
blackbody definition
Blackbody Definition
  • Does not reflect incoming radiation, only absorbs
  • Emits radiation, depending on temperature
  • Temperature and emitted radiation intensity follow a special relationship

One way of creating a blackbody

Photon enters

If hole is very small, what is probability that it exits?

wien s law and the stefan boltzmann law are useful tools for analyzing glowing objects like stars
Wien’s law and the Stefan-Boltzmann law are useful tools for analyzing glowing objects like stars
  • A blackbody is a hypothetical object that is a perfect absorber of electromagnetic radiation at all wavelengths
  • Stars closely approximate the behavior of blackbodies, as do other hot, dense objects
slide30
Blackbodies do not always appear black!
    • The sun is close to being a “perfect” blackbody
    • Blackbodies appear black only if their temperature very low
special relationship
Special Relationship

For Intensity, think photons/second on a small area

Intensity

Wavelength

question
Question
  • Why is photon/second similar to energy/second? How are they related?
slide34
Flux

Flux is a measure of how much “stuff” crosses a small patch in a given amount of time. Can have flux of green photons, red photons, etc.

blackbody laws
Blackbody Laws
  • Stefan-Boltzmann Law – relates energy output of a blackbody to its temperature
  • Wein’s law – relates peak wavelength output by a blackbody to its temperature
special relationship1
Special Relationship

For Intensity, think photons/second on a small area

Energy Flux Intensity

Wavelength

stefan boltzmann law
Stefan-Boltzmann Law
  • A blackbody radiates electromagnetic waves with a total energy flux F directly proportional to the fourth power of the Kelvin temperature T of the object:
special relationship2
Special Relationship

Stefan-Boltzmann Law tells us that if we add up the energy from all wavelengths, then the total energy Flux

Energy Flux Intensity

Wavelength

special relationship3
Special Relationship

Wien’s law tells us that lmax depends on temperature

Max intensity at lmax

Energy Flux Intensity

Wavelength

lmax

special relationship4
Special Relationship

Sketch this curve for larger and smaller T

Energy Flux Intensity

Wavelength

slide43

Wavelength of peak decreases as temperature increases

At high wavelengths, intensity goes to zero

Overall amplitude increases with Temperature

As wavelength goes to zero, intensity goes to zero

slide48

Near this temperature, this special combination of intensities is what we call white. Also, the real

curve is a little flatter near the peak

  • Can this figure help us explain?
so what color is the sun in space
So, what color is the sun in space?
  • http://casa.colorado.edu/~ajsh/colour/Tspectrum.html

Left side is white

Right side is (should be) a

little “pinker”

slide51

If blue light has higher energy, and energy is proportional to temperature, why are my cold spots blue?

slide52

5

A

B

4

C

3

Energy Flux

2

1

0

slide54
If the object in Figure 1 were increased in temperature, what would happen to curves A, B, and C?
slide55
Curve C is more jagged. The locations where the curve C is small correspond to
    • Spectral lines of a blackbody
    • Spectral lines of atmospheric molecules
    • Instrumentation error
    • Diffraction lines
    • Spectral lines of the lens used to the light into colors
slide56
What is the intensity of curve B at 550 nm?
    • Impossible to tell; 550 nm is not shown in this figure
    • Nearest 0.2
    • Nearest 0.1
    • Nearest 0.05
    • Nearest 0.0
slide57
Venus has no atmosphere. If you measure the spectrum from its surface,
    • Curves B and C would not change
    • Curve C would look more like A
    • Curve C would look more like B
    • Curve B would look more like A
    • Curve B would look more like C
slide58
White light is composed of
    • Equal intensities of all colors of the rainbow
    • Unequal intensities of all colors of the rainbow
    • Equal number of photons of all colors of the rainbow
    • Unequal number of photons of all colors of the rainbow
    • Equal numbers of red, green, and blue photons
slide59
Does a blackbody have color?
    • Yes, and they all appear the color of the sun
    • No, you cannot see a blackbody
    • Yes, but its depends on its temperature
    • Maybe, it depends on if it is an ideal blackbody
slide60
Why is the best reason for putting a telescope in orbit?
    • Closer to stars
    • Better view of celestial sphere
    • The speed of light is higher in space
    • Less atmospheric interference
    • Cost