Please be sure that you review your notes on a regular basis. This material is more difficult than
material that was on the first exam.
Lecture Summary (10/26 & 10/28)
The positions of emission lines and absorption lines are placed at the same wavelength in
the spectrum given the same gas. The wavelength is the same, but the line appears bright in
emission and dark in absorption. Atomic structure models are used to account for the
composition link.
The origin of spectral lines was explained using the Bohr atomic model and the
transitions of electrons between energy levels in the atom. Some lines due to hydrogen lie in the
visible portion of the spectrum. Emission lines of hydrogen and other atoms were studied in Lab
#7. It is the H alpha emission that gives the reddish tint to nebulae that contain hydrogen. See the
lecture handout on Origin of Spectral Lines.
Observations of our nearest star, the Sun, begin with the photosphere. Scans of the solar
spectrum have revealed the composition of the Sun and the temperature of the photosphere.
Spectra of other stars can also provide information on their composition and temperatures.
The solar surface, photosphere, is opaque gas. High resolution images of the photosphere show
the granulation pattern. Convection brings energy up from below.
Sunspots appear dark because they are cooler than the surrounding photosphere. The
number of sunspots seen in the photosphere increases and decreases regularly. Sunspots have
strong magnetic fields associated with them, which inhibit convection and disrupt the granulation
pattern. The Sun last reached solar maximum in 2001. The solar sunspot or solar activity cycle
takes 11 years to go from a maximum number of sunspots to minimum and back to maximum.
The Sun’s cycle is connected to the amount of energy it produces and there are variations
between solar maximum and minimum
The secret of the Sun's power lies deep within its interior. The electromagnetic force was
demonstrated and the strong nuclear force was introduced. Understanding what is happening
inside the Sun (illustrated on the lecture handout) involves these forces and occurs due to the
extreme conditions found in the solar core. Hydrogen fusion occurs in the core. As the core
temperature climbs to 15 million K and with density at 150 g/cc, protons moving at high
velocities collide and fuse together as the strong nuclear force dominates over the
electromagnetic repulsion. The proton-proton chain combines 4 protons to yield a helium-4
nucleus, subatomic particles and energy in a 3-step process. The Sun produces a tremendous
amount every second. Some of the Sun’s mass becomes energy as a result of the proton-proton
chain nuclear reactions (hydrogen fusion).
TERMS: Bohr's atomic model, energy levels, ground state, transitions, ionization, H-alpha, Hbeta, H-gamma, photosphere, sunspots, granulation, conduction, convection, radiation, sunspots,
solar cycle, solar maximum, solar core, strong nuclear force, electromagnetic force, hydrogen
fusion, deuterium, positron, neutrino, E= mc2
QUESTIONS:
1. Using Bohr's model of the atom, account for emission and absorption lines in
hydrogen.
2. Explain why absorption lines and emission lines for an element like hydrogen always
occur at the same wavelength positions.
3 How do astronomers explain the changing granulation pattern (from hour to hour) of
the photosphere?
4. Explain why the Sun cools in a region in which a sunspot forms.
5. Describe the conditions inside the Sun (from photosphere to solar core).
6. Why does the Sun shine?
7. Explain why fusion occurs only in the solar core.
8. Explain in words what happens during the proton-proton chain (hydrogen fusion in the
Sun).
9. How are temperature and composition determined from stellar spectra?