AST101H5 Lecture Notes - Cosmic Ray, Mass Number, Gustav Kirchhoff
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Astronomy Ch. 5 - Light: The Cosmic Messenger (Oct 7)
Astronomical Evidence
- Meteorites = solar system material that hits Earth
- Moon rocks brought back by astronauts
- Cosmic rays = very high energy subatomic particles from the Sun and deep space
Meteorites + moon rocks + cosmic rays = almost nothing
- Light = nearly all information, and only “connection” with distant objects
Spectrum of Light
- White light = visible light (seen by our eye) contains all colors - deep violet through
deep red
- White light has limited information
-Sorting light by colors → spectrum of light = first step in getting info from light first done
by Newton
Properties of Light
- Light is a wave
- has a length, after which it repeats
- has a height or amplitude above and below the central line
- has a speed
- has a frequency = number of waves passing a fixed location each second =
speed/wavelength
Light Waves
- Wave speed = speed of light
!= 300,000,000 m/s = 3 x 10^8 m/s
!= 300,000 km/s = fastest possible speed
- All wavelengths of light have the same speed in a vacuum
- Symbol for the speed of light is “c” (constant)
- Each color has its own wavelength
- red light, wavelength = 0.0000007 m
- violet light, wavelength = 0.0000004 m
- Wavelengths of light are so small that using the meter, “m”, is inconvenient
- Create new units
- Micrometer = 0.000001 m = 10^ -6 m = μm
!Ex. red light = 0.7 μm
!- Nanometer = 0.000000001 m = 10^ -9 m = nm
!Ex. red light = 700 nm
!- Ångstrom = 10^ -10 m = Å
!Ex. red light = 7000 Å
Frequency of Light
-Frequency = speed of light / wavelength of light
!!= highest possible speed / very tiny distance
!→ frequencies are extremely large, too fast to measure
!wave/s is called Hertz = Hz
Light Waves of What?
-Light waves travel through space, but space is almost a perfect vacuum
-Recall, light = radiation = energy
-Energy in electric and magnetic fields
-Wave = varying strength of electric and magnetic field = electromagnetic radiation
-Only the electric component interacts with our eye or a camera
-Brightness of light = (electric amplitude)^2
Many forms of “Light”
-Our eye sees violet - red
-Beyond red (longer wavelengths) we still receive energy = infrared radiation that we
cannot see, but can feel
-Below violet (shorter wavelengths) we still receive energy = ultraviolet radiation
-Using different detectors, we find many other forms of electromagnetic radiation
-Radiation often named after the detector
Oct. 12
The Full Spectrum of Radiation
-Radio - wavelength 1 m and longer
!frequency = 3 x 10^8 waves/s and less
-Microwave - wavelength about 1 cm
-Infrared - wavelength about 10 μm
-Visible - wavelength about 500 nm
-Ultraviolet - wavelength about 200 nm
-X-rays - wavelength about 1 nm
-Gamma (y) Rays - wavelengths less than about 0.01 nm
!frequency = 3 x 10^19 waves/s and greater
Golden Age of Astronomy
- Before 1950, all astronomy was done using visible light
- Since then, astronomy has used all forms of radiation
→ tsunami of new info is coming in from using all forms of radiation
→ the most rapid advances in the history of astronomy = Golden Age
Other Properties of Light
- Light = energy wave
- Light is also a particle = “photon”
- Light is both a wave and a photon at the same time
- Photon’s energy depends on the wave’s frequency
- radio wave = low energy photons
- gamma rays = high energy photons
-Brightness of light ∝ number of photons/s
What is Matter?
- Light is important because it brings us info about distant matter
- Fundamental form of matter = atom
- About 100 different kinds of atoms = chemical elements
- Hydrogen = most common element (NOT on earth)
- Carbon, nitrogen, oxygen, iron, gold, etc...
- On Earth, and in the Solar System, the temperature is cool enough that most atoms
combine to form molecules
- Nitrogen molecule - N:N = N2
- Oxygen molecule - O:O = O2
- Carbon Dioxide molecule - C:O:O = CO2
What is the Structure of Atoms?
- To connect light to matter, we need to understand the structure of atoms
- Three basic components:
!1. Proton: positive electric charge, p+
!2. Neutron: no electric charge, n0
!3. Electron: negative electric charge, e-
Note: within the atom the electric force dominates and gravity is negligible
How are Atoms Organized?
- Mass of proton = mass of neutron
- Protons and neutrons are confined to a tiny nucleus, about 10^-15 m across
!Note: nucleus has a + charge
- Electron’s mass is negligible compared to the proton or neutron mass
- Negative electrons fill out the volume of the atom, about 10^10 m across
Oct. 14
Atomic Rules
1. Number of protons = “atonic number” → the chemical element
!1 proton = hydrogen, 2 protons = helium ... 6 protons = carbon ... ...
2. Number of electrons = number of protons
!An atom has an exact balance of - and + electric charges
!→ the whole atom has no electric charge
3. Number of protons + number of neutrons = “atomic mass number”
!ex. Carbin with 6 p+ + 6 n0 has a mass number of 12 = carbon-12 = 12C
4. Number of neutrons is approximately equal to the number of protons