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