Chapter 5 Light: 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 10 5 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 Å T = temperature of source in K  max x T = 2.9 x 10 6 nm K   max  / T ,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 10 6 / T max = 2.9 x10 6 / 310 = 9.4 x 10 3 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 x10 6 / 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 =  T 4 L = 4  R 2  T 4 Hotter things emit more energy and are more luminous (brighter) A big thing at a given T is more luminous than a smaller thing at the same T

or : longer wavelengthshorter wavelength : smaller frequency larger frequency E: smaller energylarger energy T: lower temperaturehigher temperature color: redderbluer [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 an electron, it is said to be ionized, an ion. Electrons also get excited. Atoms are distinguished by how many 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 ( 4 He, 3 He) Molecules: consist of two or more atoms (H 2 O, CO 2 )

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 = f o / f e fofo fefe

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 =  r 2 =  (D/2) 2 =  D 2 /4 where D is the diameter of the primary mirror Your eye: Area =  m) 2 /4 = 5 x m 2 Palomar: Area =  (5m) 2 /4 = 20 m 2 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 D 2 if D = 1 m, light gathering power ~ 1 if D = 2 m, light gathering power ~ 4 if D = 5 m, ~ 25 if 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

RadioTelescopes are Interferometers VLA 27 scopes 26m in diameter 36 km baseline

Chandra X-ray Observatory like 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 nm. The same line in a star has wavelength 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