Light and atoms --- Light is intimately connected with the arrangement of
electrons in atoms: light is the by-product of the internal rearragement of
atoms at the electronic level.
Atoms --- consist of a positively charged nucleus, containing positively
charges protons and neutral neutrons, surrounded by the same number of
negatively charged electrons as there are protons, making the atoms
electrically neutral. Most of the mass of the the atom is contained in its
nucleus. The number of protons (or electrons) is the atomic number of the
element and corresponds to the position of the element in the periodic table,
known from chemistry. Nuclei also contain neutrons, electrically neutral
heavy particles. An atom of a given element can have different numbers of
neutrons in its nucleus, but will still be the same element; we call these
isotopes of the element. Finally, the number of electrons preent in an atom
of a given element can vary. When electrons are missing, we call this an ion
of the element. As long as the proper number of protons are present, it's
still the correct element --- e.g. 6 protons make a carbon nucleus, if all 6
electrons are present, we have a neutral carbon atom, if one or more electrons
are missing we have a carbon ion, if 6 (7) neutrons are in the nucleus, we
have an atom or ion of carbon-12 (carbon-13), and so on.
Energy levels --- The key point is that electrons cannot have arbitrary
energies in the atom, but are constrained to be in orbits with discrete
energies characteristic of the atom or ion. When an atom or ion interacts
with light, it does so by changing the orbit of an electron and since only
certain energy transitions are possible for any atom or ion, so only photons
with the corresponding energies (wavelengths, frequencies of light) can be
involved. We call a diagram of the possible electron energies arranged
vertically with the lowest at the bottom an energy level diagram. The lowest
possible energy is called the ground state of the atom or ion and the others
are called excited states. When an atom or ion absorbs light, an electron
moves from a lower to a higher state, and the opposite is true for emission of
light. The allowed energy levels get closer together (in energy) at the
higher levels and eventually come to a limiting energy, the ionization
potential of the atom or ion, which is the amount of energy needed to
ccmpletely remove an electron from it. The observed series of spectral lines
are now explained as transitions to a given energy level. For Hydrogen, the
Lyman series is all transitions to the ground state, the Balmer series is all
transitions to the first excited state, and so on.
Line Radiation --- Under the right conditions, a substance will
emit light at discrete frequencies (wavelengths, photon energies) that are
completely unique to the element in question, and also to its state of
ionization (how many electron(s) are left in the atom). We talk about
emission lines from the element and we know through nearly two hundred years
of work a lot about what lines are given off by what elements. The simplest
example is pure Hydrogen gas which shows series of absorption or emission
lines, some in visible light (the Balmer series), some in the UV (the Lyman
series), IR (the Brackett series) and so on. The longest wavelength line in
each series is labelled as alpha, then beta, gamma and so on. (So the Balmer
series is H_alpha, (6563 Angstroms), H_beta, and so on. The Lyman series is
L_alpha (1216 Angstroms), L_beta, and so on.) The lines get closer together
at shorter wavelengths and finally come to a limiting wavelength. Each
element or ion has its own unique series of lines.
Kinetic energy --- is energy of motion of a body, = 1/2 x (mass) x
(velocity)^2.
Heat --- is another name for the random kinetic energy of atoms or molecules
and the amount of heat is measured by the temperature of the body. Only at
absolute zero is there no random motion; at all normal temperatures, this
random motion occurs.
Temperature scales --- In astronomy, we mostly use the Kelvin temperature
scale, which is the same as the Celsius (centigrade) scale, but with the zero
point defined to be at absolute zero. In this scale, water boils at 273K and
"room temperature" is about 300K.
States of matter --- As the temperature is increased, an initially solid
substance will become liquid, and finally gaseous. If enough heat is added,
the atoms will ionize, producing an ionized gas, or plasma. The Sun is a big
ball of plasma.
Emission line spectrum --- A collection of atoms that is in the form of a hot
rarefied gas will emit a line spectrum, as we saw in the demo of gas tubes.
This is what the light is from nebulae --- radiation from hot ionized gas. We
will see that matter under other conditions has a different spectrum.