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Light and Matter
Astronomy 315Professor Lee
CarknerLecture 6
Using Light
We want to know something about the properties of the material that makes up the star
Such as: Motion
How Do Light and Matter Interact?
The properties of the photons change as this happens
How? We need to know something about
atoms
The Nature of Matter and its Antecedents
Protons and neutrons form the nucleus
Electrons are in orbits (or shells or levels or states) surrounding the nucleus In a neutral atom the number of protons and
electrons are equal
Atoms Atoms interact with each other (and light)
through the electron shells
The most common atoms are: Helium (2 protons, 2 neutrons, 2 electrons)
An atom can become ionized by losing one or more electrons
Electron States
Each orbit has a very specific
energy
e.g. An electron in a hydrogen atom cannot be anywhere, only in the permitted state
Energy Levels
Electron Transitions Moving an electron from one state to another
involves energy
An atom will only absorb a photon if it is at the exact energy for a level transition Thus, any one type of atom is able to absorb
photons at a only a few specific energies
Absorption and Emission
Absorption and Emission
Again, any atom will only emit at certain specific energies
If we examine a spectrum of emitting or absorbing atoms, we see absorption and emission lines Emission lines are bright
Emission and Absorption Lines
Identifying Atoms
Atoms can be excited by radiation or collision
Each atom has its own distinct emission spectrum and can be thus identified
Kirchhoff’s Laws For a dense gas (or a solid or liquid) the atoms
collide so much that they blur the lines into a continuous blackbody spectrum
e.g. a light bulb
A low density gas excited by collisions or radiation will produce an emission spectrum
e.g., an emission nebula
A low density gas in front of a source of continuous radiation will produce an absorption spectrum
e.g., a star (due to its cool outer atmosphere)
Absorption + Continuum
Pure Emission Spectrum
Kirchhoff’s Laws
The Doppler Effect When you observe a moving object, the
wavelengths of light you observe change Moving towards -- wavelength decreases -- blue shift
The faster the motion the larger the change By measuring the shift of lines in a spectrum, you
can determine how fast the object is moving
Doppler Effect
Stellar Doppler Shift
Spectral Line Shifts Look at a spectral line at rest in the lab
Look a moving star and measure the
shifted wavelength
The ratio of the wavelengths is the ratio of the velocity of the star (v) to the speed of light (c=3X108 m/s))
obs – rest)/rest = v/c n.b., in calculator 3X108 is 3E8 or
3EE8
Line Broadening
Doppler broadening results from the atoms being in motion so some photons are a little red shifted and some a little blue
Collisional broadening results from atom-atom collisions in the gas A larger temperature and larger density produces more
broadening
Doppler Broadening
How Do We Use Light to Find Stellar Properties?
Temperature: From the Doppler broadening
Composition: From the spectral lines compared to
standards
Motions:
Next Time
Read Chapter 4.5, Chapter 17.2-17.3