I have re-calculated grades and if you have any questions about how you are doing in ASTR 101,
please stop by during office hours next week. The last day to withdraw from class with a “W” is
April 14.
Reminders: Homework #4 is due April 10 in lecture, see syllabus.
Observatory Open House April 5, 8 pm. See syllabus for assignment. The shuttle bus leaves
from Farm Dr. behind CSS building and across from Cambridge Hall. Be there by 7:15. Be sure
to sign in for both lecture and observing session—here’s hoping it is clear.
Exam 2 is coming—April 24. I am planning a review session from 4-5:15 on April 23. You
need to keep up with the new material. Start reviewing now!
Lecture Summary (4/1 & 4/3)
Astronomers study light from objects to explore the universe. Electromagnetic radiation
(light) was described in terms of wavelength and photon energy. In 1873 Maxwell developed a
theory involving oscillating electric and magnetic fields to describe light. A 3 dimensional model
was used to illustrate this concept. There is much more to this phenomenon than the range which
our eyes are sensitive to—the visible. Colors result from tiny wavelength differences. Blue light
has shorter wavelength and greater photon energy than red light. A prism was used to split light
into its component colors—a rainbow. Mrs. Deming demonstrated invisible forms of
electromagnetic radiation using a fluorescent screen (ultraviolet radiation) and an infrared
detector.
Our atmosphere prevents some types of electromagnetic
radiation from reaching Earth's surface. Today telescopes are used to collect electromagnetic
radiation both from Earth and above the atmosphere. Some have been placed in orbit above the
atmosphere. Light comprises the data astronomers must analyze in the study of stars.
Astronomers find it useful to spread light into a spectrum.
Different conditions lead to the 3 kinds of spectra: continuous, emission and absorption.
The positions of emission lines and absorption lines are identically placed in the spectrum for the
same gas. The wavelength is the same, 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 were explained using the Bohr atomic model and the
transitions of electrons between energy levels in the atom. The Balmer lines of hydrogen lie in
the visible portion of the spectrum. Emission lines of hydrogen and other atoms are studied in
Lab #7. It is the H alpha emission that give the reddish tint to nebulae that contain hydrogen.See
the yellow sheet on Origin of Spectral Lines. Spectra reveal much information about stars.
Wien's law can be used to probe stellar surface temperatures.
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. (A blue sheet was given out on the Sun.) 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 will involve these
forces and what occurs under 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 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. Solar flares can develop
and release tremendous amounts of energy. The incidences of and extent of auroras increase
during solar maximum. The activity on the Sun has a cycle of 11 years. Stars that are fusing
hydrogen in their central cores are called main sequence stars. As the energy flows outward, the
heated gas exerts an outward pressure, which approximately balances the inward force due to
gravity and the star is in equilibrium.
TERMS: electromagnetic radiation, wavelength, photon energy, Angstrom,
continuous spectrum, emission spectrum, absorption spectrum, Bohr's atomic model, energy
levels, ground state, transitions, photosphere, sunspots, solar cycle, solar maximum, auroras,
solar core, strong nuclear force, electromagnetic force, hydrogen fusion, deuterium, positron,
neutrino, E= mc2
QUESTIONS:
1. Be able to discuss electromagnetic radiation in terms of wavelength and photon
energy. Describe which kinds of electromagnetic radiation can be observed from Earth's
surface.
2. How are the three types of spectra produced?
3. Using Bohr's model of the atom, account for emission and absorption lines in
hydrogen.
4. Explain why absorption lines and emission lines for an element like hydrogen always
occur at the same wavelength positions.
5. Why does the Sun shine?
6. Explain why fusion occurs only in the solar core.
7. Describe the conditions inside the Sun (from photosphere to solar core).
8. How does energy get out of the Sun?
9. Explain why the Sun cools in a region in which a sunspot forms.