Light and Matter Astronomy 315 Professor Lee Carkner Lecture 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

  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=3X10 8 m/s))  obs – rest )/ rest = v/c  n.b., in calculator 3X10 8 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