Chapter 5Light: The Cosmic Messenger

Different Energies of Light or “Electromagnetic Radiation”

Electromagnetic Radiation

The peak-to-peak size is the wavelength, . The number of peaks that pass by in a given time is the frequency, , measured in Hz (1/s).The speed of light in a vacuum is constant, c.

c = 3 x 105 km/s c = is inversely proportional to

Electromagnetic Radiation

Light also acts like a particle.

These “energy packets” are called photons The energy of a photon is given by:

E = h = hc/where h is Planck’s constant

higher wavelength = lower energy

Wien’s Law: the peak wavelength

of EMR emitted by a blackbody is inversely proportional to its temperature.

= the wavelength

max = the peak wavelength in ÅT = temperature of source in K

max x T = 2.9 x 106 nm K max / T

5000 10,000 20,000

Å)

Blackbody Radiation - hot things glow!

Thought Question

Which is hotter?

• A blue star

• A red star

• A planet that emits only infrared light

Wien’s Law

• A human body has a temperature of about 310 K. At what peak wavelength does it radiate light?

• What part of the spectrum does this correspond to?

• The Sun has a temperature of about 5800 K. At what peak wavelength does it radiate, and what color is this?

Wien’s Law: max = 2.9 x 106 / T

max = 2.9 x106 / 310 = 9.4 x 103 nm = 9.4 m

This peak wavelength lies in the far infrared. The military uses night-vision devices sensitive near 10 m to see people in absolute darkness.

The Sun: max = 2.9 x106 / 5800 = 500 nm

This peak wavelength is yellow-green, but the human eye sees sunlight as white because its response has evolved to make maximum use of the Sun’s entire spectrum.

Stefan-Boltzmann Law

E = T4

L = 4 R2 T4

Hotter things emit moreenergy and are more luminous (brighter)

A big thing at a given Tis more luminous than a smaller thing at the same T

or

: longer wavelength shorter wavelength

: smaller frequency larger frequency

E: smaller energy larger energy

T: lower temperature higher temperature

color: redder bluer

[R: for same radius (size)]L: lower luminosity higher luminosity

Thought Question

The higher the photon energy,

• the longer its wavelength.

• the shorter its wavelength.

• Energy is independent of wavelength.

ATOMS

Nucleus:protons (+)neutrons

Orbiting electrons(-)

If an atom loses anelectron, it is said tobe ionized, an ion.

Electrons also getexcited.

Atoms are distinguished by howmany protons are in their nucleus.All hydrogen atoms have one proton.

Helium - two protons

Atomic Terminology

• Atomic Number = # of protons in nucleus • Atomic Mass Number = # of protons + neutrons

Atomic Terminology

• Isotope: same # of protons but different # of neutrons (4He, 3He)

• Molecules: consist of two or more atoms (H2O, CO2)

The Bohr Hydrogen Atom

Chemical Fingerprints

• Downward transitions produce a unique pattern of emission lines.

Chemical Fingerprints

• Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths.

Chemical Fingerprints

• Each type of atom has a unique spectral fingerprint.

Composition of a Mystery Gas

Most of the spectra we observe are not continuous. They contain emission and absorption lines. This is because the Universe is filled with atoms and ions and they absorb and emit photons.

Kirchhoff’s Laws

Production of Absorption Lines

How do light and matter interact?

• Emission

• Absorption

• Transmission:— Transparent objects transmit light.— Opaque objects block (absorb) light.

• Reflection or scattering

Introduction to Spectroscopy

Thought Question

Why is a rose red?

• The rose absorbs red light.

• The rose transmits red light.

• The rose emits red light.

• The rose reflects red light.

Galileo and his Telescope

• 1609• first to look up and publish• Moon - rough, mountains,

craters, etc.• moons of Jupiter• Sunspots• phases of Venus• “ears” of Saturn• stars as points - far away?

Refracting Telescope

Use lenses to focus the light.

M = fo / fe

fofe

Reflecting Telescopes

What Shape is the Mirror?

How do telescopes help us learn about the universe?

• Telescopes collect more light than our eyes light-collecting area

• Telescopes can see more detail than our eyes angular resolution

• Telescopes/instruments can detect light that is invisible to our eyes (e.g., infrared, ultraviolet)

Bigger is better

1. Larger light-collecting area

2. Better angular resolution

Light-Gathering Power

• Depends only on the Area of the primary mirror:

Area = r2 = (D/2)2 = D2/4where D is the diameter of the primary mirror

• Your eye: Area = m)2/4 = 5 x 10-5 m2

• Palomar: Area = (5m)2/4 = 20 m2

Palomar is 400,000 times more powerful than your eye. Plus, if you use an electronic detector and long exposure times, you can see even fainter objects.

Remember: Light gathering area is proportional to D2

if D = 1 m, light gathering power ~ 1if D = 2 m, light gathering power ~ 4if D = 5 m, ~ 25if D = 10m, ~ 100

So, a 10m telescope has 25 times the light gathering power of a 2m telescope.

Angular Resolution

• Theoretically, the resolution is proportional to:

/D where D is the diameter of the telescope• Bigger space telescopes have better (smaller)

resolution.• For ground-based telescopes, the angular resolution is

limited by atmospheric “seeing” effects to 1-2 arcsec for ordinary telescopes.

Why do we put telescopes into space?

It is NOT because they are closer to the stars!

Recall our 1-to-10 billion scale: • Sun size of grapefruit• Earth size of a tip of a ball

point pen,15 m from Sun• Nearest stars 4,000 km

away• Hubble orbit

microscopically above tip of a ball-point-pen-size Earth

Observing problems due to Earth’s atmosphere

1. Light Pollution

Star viewed with ground-based telescope

2. Turbulence causes twinkling blurs images.

View from Hubble Space Telescope

3. Atmosphere absorbs most of EM spectrum, including all UV and X ray and most infrared.

Telescopes in space solve all 3 problems.

• Location/technology can help overcome light pollution and turbulence.

• Nothing short of going to space can solve the problem of atmospheric absorption of light.

Chandra X-ray Observatory

Optical Telescopes

Infrared Satellites

RadioTelescopesare

Interferometers

VLA 27 scopes26m in diameter36 km baseline

Chandra X-ray Observatorylike skipping stones…

The Doppler Effect

blueshift redshift

Doppler shift tells us ONLY about the part of an object’s motion toward or away from us.

How a Star's Motion Causes the Doppler Effect

Measuring the Shift

• We generally measure the Doppler effect from shifts in the wavelengths of spectral lines.

Stationary

Moving Away

Away Faster

Moving Toward

Toward Faster

Measuring Velocity

Determining the Velocity of a Gas Cloud

Thought Question

• It is moving away from me.

• It is moving toward me.

• It has unusually long spectral lines.

I measure a line in the lab at 500.7 nm. The same line in a star has wavelength 502.8 nm. What can I say about this star?

The Doppler Effect

What can we learn?

Measure

Spectrum – max

Brightness, L

emission/absorption lines

Blue/redshift

Find?Temperature, T

Radius, R

Chemical composition

rotation or radial velocity