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

The Electromagnetic Spectrum 1 nm = 10 -9 m , 1 Angstrom = 10 -10 m c = l n

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.

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.

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.

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

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

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

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

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

Betelgeuse Rigel

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.

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!

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

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

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

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.

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.

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

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 

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.

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)

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

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.

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.

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

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

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.

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

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.

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

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.

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

Molecule vibration and rotation

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.

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.