Emission
Astronomy: The Science of Seeing
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Goals
•
•
•
•
•
What is light?
What are the types of light?
Where does the light we see come from?
Understanding the light of heat.
On a sunny day:
– Why does it seem hotter wearing a black T-shirt versus
a white one?
– Why are they different?
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A Spectrum
Emission lines
Continuum
Absorption lines
• A spectrum = the amount of light given off by an
object at a range of wavelengths.
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Temperature and Light
• Warm objects emit light.
– Thermal radiation
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Kelvin Temperature
•
•
•
•
•
•
Kelvin: an absolute scale.
Kelvin is Celsius + 273 degrees.
Water freezes: 0 C  273 K
Water Boils: 100 C  373 K
Room Temp: 80 F  27 C  300 K
Surface Sun: 5800 K
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Atoms in Motion
• Everything is composed of atoms which are
constantly in motion.
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Temperature
• The hotter the object, the faster the average
motion of the atoms.
HOTTER
COOLER
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Atoms and Light
• As atoms move they collide (interact, accelerate).
• Collisions give off energy.
• But light IS energy.
E = hc/l
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Light and Temperature
• The hotter the object the faster the average atom and
the more energetic the average collision.
• The faster the atoms the more collisions there are.
HOT
COLD
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Energy and Intensity
•
The more energetic the average collision the bluer the
average light that is given off.
– Since E = hc/l
• The more collisions that occur the more light that is
given off per surface area.
1. Hotter is bluer.
2. Hotter is brighter.
(peak at shorter
wavelength)
(more intense at all
wavelengths)
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Thermal Radiation Laws
1. Hotter is bluer.
–
(peak at shorter
wavelength)
2. Hotter is brighter.
–
(More intense at
all wavelengths)
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Concept Test
Which of the two stars (A or B) is
at a higher temperature?
Star A
visible
range
a. Star A
b. Star B
c. The two stars have the same
temperature.
d. It is not possible to infer this
relationship.
Star B
VIBGYOR
Wavelength
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Concept Test
Which of the following best
describes how Star A would
appear compared to Star B?
a. Star A would appear more red than Star
B.
b. Both stars would appear more red than
blue.
c. Both stars would appear more blue
than red.
d. Star A would appear more blue than
Star B.
e. None of the above.
Star A
visible
range
Star B
VIBGYOR
Wavelength
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Thermal versus Reflection
• Thermal radiation is light given off because of an
object’s temperature.
• Don’t confuse with reflected light:
– Buses are yellow not because they are hot enough to
emit visible radiation but rather they reflect the yellow
light given off by the Sun.
• What kinds of thermal radiation do we see in our
everyday life?
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The IR Universe
Orion - visible
Orion – by IRAS
• Everyday
things that
are hot
radiate in the
IR:
• Dust – There
are
interstellar
clouds of
dust.
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The IR Universe
Io from IRTF.
Orion – by IRAS
• Molten Rock –
There are lava
flows on a moon
of Jupiter.
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The IR Universe
The Moon in eclipse.
Orion – by IRAS
R. Gendler
• In eclipse, there is
no reflected light.
• Only thermal
radiation.
• Differences in
composition lead to
differences in
temperature.
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Continuum Concept Test
•
The sun shines on a cold airless asteroid made of black
coal. What light from the asteroid do we detect?
a.
b.
c.
d.
e.
No light at all.
A little reflected visible light.
A little reflected visible, plus more emitted visible light.
A little reflected visible, plus mostly emitted infrared light.
A little reflected visible, plus emitted visible and emitted infrared
light.
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A Spectrum
Emission lines
Continuum
Absorption lines
• A spectrum = the amount of light given off by an
object at a range of wavelengths.
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Spectral Line formation?
• Electron has
different energy
levels: Floors in a
building.
• Lowest is called the
Ground State.
• Higher states are
Excited States.
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Changing Levels
• If you add the RIGHT amount of energy to an
atom, the electron will jump up energy floors.
• If the electron drops down energy floors, the atom
gives up the same amount energy.
• From before, LIGHT IS ENERGY: E = hc/l
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Kirchhoff’s Laws
• Light of all wavelengths shines on an atom.
• Only light of an energy equal to the difference between
“floors” will be absorbed and cause electrons to jump up in
floors.
• The rest of the light passes on by to our detector.
• We see an absorption spectrum: light at all wavelengths
minus those specific wavelengths.
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Absorption
• Dark hydrogen absorption lines appear against a
continuous visual spectrum, the light in the spectrum
absorbed by intervening hydrogen atoms
• Compare with the emission spectrum of hydrogen.
From "Astronomy! A Brief Edition," J. B. Kaler, Addison-Wesley, 1997.
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Emission Lines
• Every element has a DIFFERENT finger print.
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Different stars, different spectra
Stellar Spectra
Hot
• Different stars have
different types of
spectra.
• Different types of
spectra mean
different stars are
made of different
elements.
Cool
Annals of the Harvard College Observatory, vol. 23, 1901.
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To Sum Up…
• EVERY element has a SPECIAL set of lines.
– Atom’s fingerprint.
• Observe the lines and you identify the component
elements.
• Identify:
– Absorption spectrum
– Emission emission
Learn about the environment of the element
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Concept Test
•
The sunlight we see is thermal radiation caused by the
extreme heat of the sun’s surface. However, the very top
thin layer of the sun’s surface is relatively cooler than the
part below it. What type of spectrum would you expect to
see from the sun?
a.
b.
c.
d.
e.
A continuous spectrum.
A continuous spectrum plus a second, slightly redder continuous
spectrum.
A continuous spectrum plus a second slightly bluer continuous
spectrum.
A continuous spectrum plus an absorption spectrum.
None of the above.
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The Sun
Courtesy of NOAO/AURA
You
HOT
Cooler
Low Density
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Helium
• The element Helium (He) was first discovered on
the Sun by its spectral lines.
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Doppler
Shift
• The greater
the velocity
the greater
the shift.
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So Now…
• From the presence and position of Spectral Lines
we can know:
– Composition (H, He, H2O, etc.)
– Movement through space (towards or away)
– How fast?
l
l
c V
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Concept Test
•
I spin an object emitting a constant tone over my
head. What do you hear?
a. A constant tone.
b. A tone that goes back and forth between high and low
frequency.
c. A constant tone of lower intensity.
d. Two constant tones, one of higher frequency and one of
lower frequency.
e. One tone going smoothly from low to high intensity.
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Concept Test
•
I spin an object emitting a constant tone over my
head. What do I hear?
a. A constant tone.
b. A tone that goes back and forth between high and low
frequency.
c. A constant tone of lower intensity.
d. Two constant tones, one of higher frequency and one of
lower frequency.
e. One tone going from smoothly from low to high
intensity.
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Homework #8
• For Wednesday 24-Sept: Read B5.3-5.5 and
B13.1-13.2.
• Do: B5 Problems 38, 57
• Do: B13 Problems 19, 27, 32, 53
• Don’t forget Topic of Confusion
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