1. Electromagnetic Radiation
(How we get information about the cosmos)
Examples of electromagnetic radiation?
Light
Infrared
Ultraviolet
Microwaves
AM radio
FM radio
TV signals
Cell phone signals
X-rays

2. The Electromagnetic Spectrum
1 nm = m , 1 Angstrom = m
c =
l n

3. Refraction of light
All waves bend when they pass through materials of different densities. When you bend light, bending angle depends on wavelength, or color.

4. The "Inverse-Square" Law Applies to Radiation
Each square gets 1/4 of the light
Each square gets 1/9 of the light 1 D2
apparent brightness α
α means “is proportional to”. D is the distance between source and observer.

5. Why is the Sun yellow? The Black-Body Spectrum
We form a "spectrum" by spreading out radiation according to its wavelength (e.g. using a prism for light).
What does the spectrum of an astronomical object's radiation look like?
Many objects (e.g. stars) have roughly a "Black-body" spectrum:
Asymmetric shape
Broad range of wavelengths
or frequencies
Has a peak
Brightness
Frequency
also known as the Planck spectrum.

6. Approximate black-body spectra of stars of different temperature
cold dust
average star (Sun)
Brightness
infrared visible UV
“cool" stars
very hot stars
infrared visible UV
frequency increases, wavelength decreases

7. Laws Associated with the Black-body Spectrum
Wien's Law: 1 T
λmax energy α
(wavelength at which most energy is radiated is longer for cooler objects)
Stefan's Law:
Energy radiated per cm2 of area on surface every second α T 4
(T = temperature at surface)
1 cm2

8. The total energy radiated from entire surface every second is called the
luminosity. Thus
Luminosity = (energy radiated per cm2 per sec) x (area of surface in cm2)
For a sphere, area of surface is 4πR2, where R is the radius.
So Luminosity α R2 x T4

9. Clicker Question: Compared to blue light, red light travels: A: faster
B: slower
C: at the same speed

10. Clicker Question: A star much colder than the sun would appear: A: red
B: yellow
C: blue
D: smaller
E: larger

11. Betelgeuse
Rigel

12. The Doppler Effect
Applies to all waves – not just radiation. The frequency or wavelength of a wave depends on the relative motion of the source and the observer.

13. Applies to all kinds of waves, not just radiation.
The Doppler Effect
Applies to all kinds of waves, not just radiation.
at rest
velocity v1
velocity v1
velocity v2
you encounter more wavecrests per second => higher frequency!
velocity v1
velocity v3
fewer wavecrests per second => lower frequency!

14. Waves bend when they pass through material of different densities.
Things that waves do
1. Refraction
Waves bend when they pass through material of different densities.
air
water
swimming pool
prism
glass
air
air

15. 2. Diffraction
Waves bend when they go through a narrow gap or around a corner.

16. Clicker Question:
If a star is moving rapidly towards Earth then its spectrum will be:
A: the same as if it were at rest
B: shifted to the blue
C: shifted to the red
D: much brighter than if it were at rest
E: much fainter than if it were at rest

17. Spectroscopy and Atoms
How do we know:
- Physical states of stars, e.g. temperature, density.
- Chemical make-up and ages of stars, galaxies
- Masses and orbits of stars, galaxies, extrasolar planets
expansion of universe, acceleration of universe.
All rely on taking and understanding spectra: spreading out radiation
by wavelength.

18. Types of Spectra and Kirchhoff's (1859) Laws
1. "Continuous" spectrum - radiation over a broad range of wavelengths (light: bright at every color). Produced by a hot opaque solid, liquid, or dense gas.
2. "Emission line" spectrum - bright at specific wavelengths only. Produced by a transparent hot gas.
3. Continuous spectrum with "absorption lines": bright over a broad range of wavelengths with a few dark lines. Produced by a transparent cool gas absorbing light from a continuous spectrum source.

19. The pattern of lines is a fingerprint of the element (e. g
The pattern of lines is a fingerprint of the element (e.g. hydrogen, neon) in the gas.
For a given element, emission and absorption lines occur at the same wavelengths.
Sodium

20. The Particle Nature of Light
On microscopic scales (scale of atoms), light travels as individual packets of energy, called photons. (Einstein 1905). c
photon energy is proportional toradiation frequency:
E  (or E  )
example: ultraviolet photons are more harmful than visible photons. 1 

21. The Nature of Atoms The Bohr model of the Hydrogen atom (1913):
electron _ _ + +
proton
"ground state"
an "excited state"
Ground state is the lowest energy state. Atom must gain energy to move to an excited state. It must absorb a photon or collide with another atom.

22. But, only certain energies (or orbits) are allowed:_ _ _
a few energy levels of H atom +
The atom can only absorb photons with exactly the right energy to boost the electron to one of its higher levels.
(photon energy α frequency)

23. When an atom absorbs a photon, it moves to a higher energy state briefly
When it jumps back to lower energy state, it emits a photon - in a random direction

24. Other elements Helium Carbon
neutron proton
Atoms have equal positive and negative charge. Each element has its own allowed energy levels and thus its own spectrum. Number of protons defines element. Isotopes of element have different number of neutrons.

25. Ionization
Hydrogen _ +
Energetic UV Photon _
Helium + +
Energetic UV Photon _
Atom
"Ion"
Two atoms colliding can also lead to ionization. The hotter the gas, the more ionized it gets.

26. So why do stars have absorption line spectra?
Simple case: let’s say these atoms can only absorb green photons. Get dark absorption line at green part of spectrum. . . . . . . . . . . .
“atmosphere” (thousands
of K) has atoms and ions
with bound electrons
hot (millions of K), dense interior
has blackbody spectrum,
gas fully ionized

27. Stellar Spectra
Spectra of stars differ mainly due to atmospheric temperature (composition differences also important).
“hot” star
“cool” star

28. Why emission lines?
hot cloud of gas . . . . . .
- Collisions excite atoms: an electron moves to a higher energy level
- Then electron drops back to lower level
- Photons at specific frequencies emitted.

29. We've used spectra to find planets around other stars.

30. Star wobbling due to gravity of planet causes small Doppler shift of its absorption lines.
Amount of shift depends on velocity of wobble. Also know period of wobble. This is enough to constrain the mass and orbit of the planet.

31. Now more than 300 extrasolar planets known
Now more than 300 extrasolar planets known. Here are the first few discovered.

32. So why absorption lines?. . . . .
cloud of gas . . . . . .
The green photons (say) get absorbed by the atoms. They are emitted again in random directions. Photons of other wavelengths go through. Get dark absorption line at green part of spectrum.

33. Molecules Two or more atoms joined together.
They occur in atmospheres of cooler stars, cold clouds of gas, planets.
Examples
H2 = H + H
CO = C + O
CO2 = C + O + O
NH3 = N + H + H + H (ammonia)
CH4 = C + H + H + H + H (methane)
They have
- electron energy levels (like atoms)
- rotational energy levels
- vibrational energy levels

34. Molecule vibration and rotation

35. Types of Spectra
1. "Continuous" spectrum - radiation over a broad range of wavelengths
(light: bright at every color).
2. "Emission line" spectrum - bright at specific wavelengths only.
3. Continuous spectrum with "absorption lines": bright over a broad range of wavelengths with a few dark lines.

36. Kirchhoff's Laws (1859)
1. A hot, opaque solid, liquid or dense gas produces a continuous spectrum.
2. A transparent hot gas produces an emission line spectrum.
3. A transparent, cool gas absorbs wavelengths from a continuous spectrum, producing an absorption line spectrum.