Monday January 25th 1999
Announcements:

Lecture notes:

Class 7

Title of class is chemistry from afar
This seems odd this is not a chemistry course but astronomers do use chemical clues to learn what things are made of.
For example we can learn what elements are in a distant star just by studying its spectrum

First we have to explain the atomic model

• atom = building block of matter composed of protons neutrons and electrons
• atom not indivisible = made of subunits
• consider H atom - made of 1 proton and 1 electron

protons = positively charged particles in the nucleus of an atom.
The number of protons determines the type of chemical element
electrons = very light particles in "orbit" around nucleus - negatively charged by same amount that proton is positively charged

not a simple orbit - really quantum mechanically "fuzzy".  can't pin down precise position

atoms are small = 5E-9cm

More generally atoms have nuclei made of protons and neutrons with >1 electron going around nucleus

neutrons = neutral particles also located in the nuclei of many atoms - they have a slightly larger mass than protons

Example - helium atom
2 protons
2 neutrons
2 electrons

Carbon -> 6 protons 6 neutrons 6 electrons

In complex atoms the number of protons usually equals the number of neutrons but they need not be exactly the same

Electric force binds electrons to protons in atoms since opposite charges attract - when proton number = electron number the atom is neutral

Now electrons can only have certain energies in their "orbits" around nuclei.  Due to a set of scientific laws called quantum mechanics that you will have to take on faith. The particular
energies that an electron can have are called energy levels.

Normally electrons are in the state of lowest energy called the ground state.

ground state - state of lowest energy for electrons when they are close to the nucleus.  All electrons are in the lowest possible energy levels.

However if an atom absorbs energy it can go into an excited state.
excited state - electrons have higher energy than when in the ground state and are further from nucleus.

An important fact is that the energy levels of different types of atoms are different.  Energy levels of hydrogen are different from those of carbon.  Remember this since this is the key to
how astronomers figure out the compositions of objects.

Consider an H atom in ground state.  If energy is added the electron will "jump" to a higher level.  Alternatively if an electron falls to a lower level energy is released.

Energy released by emission of a photon.  The photon has the energy equal to the difference between the 2 levels.

Three Types of Spectra
continuous spectra - emits light at all wavelengths.  Get all colors of rainbow spread out
absorption line spectra - like a continuous spectrum but has a few particular wavelengths where no light and spectrum is dark.  - made when cool gas in front of a light source absorbs
radiation at particular wavelengths.  Also made in slightly different way in surface layers of stars.
emission line spectra - spectrum only bright at certain wavelengths.  Made when electrons go from higher to lower energy levels in atoms and only emit light at these wavelengths.

Now the particular absorption lines you see from stars tell you what elements are in the star.  Remember that the particular energy levels unique to each element.

Compare star absorption spectra to absorption spectra we make on earth.

Showed overhead showing spectra of different types of stars

• note lines different for each type of star
• not due mainly to abundance but rather T
• get T abundance - that's how we do chemistry from afar