Light and Matter Astronomy 315 Professor Lee Carkner Lecture 6

Using Light  Our only means of learning about stars is to analyze the light they emit  We want to know something about the properties of the material that makes up the star  Temperature  Composition  Motions  many more …

How Do Light and Matter Interact?  Light can be absorbed, emitted and reflected by atoms and electrons  The properties of the photons change as this happens  How?  We need to know something about atoms

The Nature of Matter and its Antecedents  Matter is made up of protons, neutrons and electrons  Protons and neutrons form the nucleus  The number of protons determine the type of element (the atomic number)  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  It takes large amounts of energy to break up the nucleus, so nuclei do not change much  The most common atoms are:  Hydrogen (one proton, one electron)  Helium (2 protons, 2 neutrons, 2 electrons)  An atom can become ionized by losing one or more electrons

Electron States  Electrons orbit the nucleus  Each orbit has a very specific energy  For any particular atom there are only a few permitted energies or orbits  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  If the atom is hit with a photon it may absorb it and use the energy to move the electron up a level, however:  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

 After an atom’s electrons have been excited they may drop down in energy emitting a photon  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  Absorption lines are dark  Emission lines are bright

Absorption Lines

Identifying Atoms  If you excite some atoms, electrons will move up in energy  Atoms can be excited by radiation or collision  After excitation the electrons will drop back to their original energy levels and produce photons that we observe as an emission spectrum  Each atom has its own distinct emission spectrum and can be thus identified

Elemental Emission Spectra

Types of Spectra  For a dense gas (or a solid or liquid) the atoms collide so much that they blur the lines into a continuous blackbody spectrum  Light at all wavelengths  A low density gas excited by collisions or radiation will produce an emission spectrum  Light only at specific wavelengths  A low density gas in front of a source of continuous radiation will produce an absorption spectrum  A continuum with dark lines at specific wavelengths

Absorption + Continuum

Pure Emission Spectrum

Kirchhoff’s Laws  If we look at a hot dense object (like a star) we see a continuous spectrum  If we look at a hot dense object with cooler low density gas in front of it (like the thin atmosphere above a star) we see a continuum with absorption lines  If we look at hot low density gas directly, with no star behind it (such as an emission nebula) we see just emission lines

Kirchhoff’s Laws

The Doppler Effect  When you observe a moving object, the wavelengths of light you observe change  Moving away -- wavelength increases -- red shift  Moving towards -- wavelength decreases -- blue shift  Example: the change in a car’s sound as it moves past you  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

Spectral Line Shifts  For objects moving away from us the spectral lines move to larger wavelengths  This is called a red shift  For objects moving towards us the spectral lines move to shorter wavelengths  This is a blue shift

Stellar Doppler Shift

Line Broadening  The spectral lines emitted by atoms are not at one precise wavelength, they are broadened in different ways  Doppler broadening results from the atoms being in motion so some photons are a little red shifted and some a little blue  The hotter the gas the more broadening  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 color of the star  From the Doppler broadening  Composition:  From the spectral lines  Motions:  From the Doppler shift of spectral lines  Density:  From the collisional broadening

Next Time  Read Chapter 4.4, Chapter  Change from syllabus