Astro 201: Sept. 14, 2010

• Read: Hester, Chapter 4Chaos and Fractal information on class web page

• On-Line quiz #3: available after class, due next Tuesday before class

• HW #3 on line• Today:– Light

LIGHTand

Telescopes

LIGHT• Astronomers study the Universe by analyzing the

light from cosmic objects• LIGHT: Maxwell’s equations predict that moving

charged particles will produce periodic waves of electric and magnetic field

Periodic function,Travels through space

Ocean wave = fluctuation in height of water.

Sound wave = fluctuation in air pressure.

Electromagnetic wave = fluctuation in electric and magnetic fields.

Wavelength, Frequency, c

The speed of a wave equals wavelength times frequency.

fc

c for “celeritas”, the Latin word for “speed”

The speed of light in a vacuum is ALWAYS c = 300,000 km/sec (186,000 miles/sec).

Figure 4.6: Wavelength, Speed, Frequency, and Amplitude

Light shows some properties of particles, such as the photoelectric effect.

Particles of light, called PHOTONS, kick electrons out of atoms.

Light is made of PARTICLES.

fhE E = energy of photon f = frequency of light waveh = Planck’s constant (a number)

The ENERGY of a photon is related to the FREQUENCY of the wave.

Light forms a spectrum from short to long wavelength.

Roy G. Biv

ROY G BIV

Optical, or Visible Light

Bright at all wavelengths

Bright at specific wavelengths

Bright at all except some specificwavelengths

An example of a Continuum Spectrum: Blackbody

• A “blackbody” is an object that absorbs all light that falls on it

• It radiates light which has a distinctive spectrum, specified by its temperature

• The peak of the spectrum for blackbodies with T=98.6 F is in the infrared part of the spectrum

• T=6000 degree black bodies have peaks in the visible part of the spectrum (e.g. the SUN)

• Incandescent light bulb filament: 2000-3000 deg.

Stars have spectra that are more or less like those of blackbodies

Produced when light interacts withatoms

A nucleus, consisting of protons and (usually) neutrons, is surrounded by a cloud of electrons.

Absorption and Emission Lines in Spectra:

electron

proton

Behavior on subatomic scales is governed by QUANTUM MECHANICS.

Rule: electrons can only exist in orbits of particular energy. (Small orbit = low energy, big orbit = high energy).

HYDROGEN: One Proton, one electron.

Electron falls from high energy to low energy orbit: energy is carried away by a photon.

λ=656.3 nm

121.6 nm

102.6 nm

Photon has a fixed ENERGY, corresponding to a specific WAVELENGTH.

Consider a hot, low density glob of hydrogen gas.

λ = 656.3 nm (3→2)

λ = 486.1 nm (4→2)

λ = 434.0 nm (5→2)

Light is emitted with wavelengths or frequencies corresponding to energy jumps between electron orbits.

1) Hot, low density gas produces an emission line spectrum.

Spectrum of hydrogen at visible wavelengths.

3→25→2 4→2

Carina Nebula: a cloud of hot, low density gas about 7000 light-years away.

Its reddish color comes from the 656.3 nm emission line of hydrogen.

A cool, low density glob of hydrogen gas in front of a light source.

Light is absorbed at specific wavelengths corresponding to energy jumps between electron orbits.

2) Cool, low density gas produces an absorption line spectrum.

Spectrum of hydrogen at visible wavelengths.

3←25←2 4←2

Every type of atom has a unique spectrum.

The spectrum of the Carina Nebula:

Hydrogen line at 656.3 nm

Radial velocity = how fast an object is moving toward you or away from you.

The RADIAL VELOCITY of an object is found from the DOPPLER SHIFT of its spectrum.

Christian Doppler (1803-1853)

DOPPLER SHIFT: If the wave source moves toward you or away from you, the wavelength changes.

A modern recreation of Doppler’s famous experiment: Doppler effect of sound

http://www.youtube.com/watch?v=ZlCcX697Twg

The reason for Doppler shifts:

Wave crests are “bunched up” ahead of wave source, “stretched out” behind wave source.

Figure 4.16: The Doppler shift of light

If a light source is moving TOWARDS you, the wavelength appears shorter (called “blueshift”).

If the light source is moving AWAY from you, wavelength is longer (called “redshift”).

Doppler shifts are easily detected in emission or absorption line spectra.

Figure 4.17: Doppler shifts of astronomical objects

Size of Doppler shift is proportional to radial velocity:

c

V

0

Δλ = observed wavelength shift = λ-λ0

λ0 = wavelength if source isn’t movingV = radial velocity of moving sourcec = speed of light

= 300,000 km/sec

One of the nifty applications of the Doppler effect has been the detection of Planets orbiting other stars: “Extrasolar Planets”

The star and planet orbit around their “center of mass” periodic red and blue shifts of the lines in the star’s spectrum

To date: about 358 planets have been discovered, mostly larger than Jupiter

Another nifty application of the Doppler effect : Doppler radar to predict weather

Not so nifty: Police radar guns to catch you speeding