Discussion Question Responses - Week 3

Note:  Assess your work on the basis of 3 points total.

1. What properties of the Sun can astronomers deduce from detailed study of the solar spectrum? In each case, explain the property, what measurement leads to the property, and how they make the measurement.
a. Dark lines in the Sun’s absorption spectrum tell us the elemental composition of the Sun’s outer layer - the chromosphere.
b. The wavelength at which the continuous (black-body) spectrum peaks tells us the surface temperature, assuming the Sun is a black body and using Wien’s law.
c. Comparison of the Sun’s spectral lines and relative intensities gives us an independent determination of surface temperature.
d. Detailed study of spectral absorption line shapes also gives information about the Sun’s magnetic field and material density at the solar surface.
e. Focusing on very small parts of the Sun and analyzing the Doppler effect tells us about the speed at which hot gas is rising and cooler gas is descending. We also learn about local magnetic field variation by careful study of spectral line shapes.  At sunspots, the magnetic fields are powerful enough to split a spectral line into several components (via the Zeeman effect).

2. Suppose a solar flare creates a radio disturbance on Earth a few minutes after going off and again several days later. Explain these disturbances.
The initial disturbance on Earth happens about 8 minutes after the flare and is the result of arrival at Earth of the first burst of light from the flare (Earth is 8 light-minutes distant from the Sun). This light includes high-energy components (UV and X-rays) that can cause increased ionization of atoms in our upper atmosphere, thereby affecting radio signals. The second disturbance hours to days later and signals the arrival of slower-moving charged particles (electrons and protons mostly). The arrival of these particles generates radiowave (and also magnetic) disturbances on Earth.  The charged particle arrival also leads to brighter aurora borealis effects and can cause northern lights bright enough to show up in Bellingham area.  If you wish to check daily to determine whether to expect a display on a given day, use the following website:
http://sec.noaa.gov/rt_plots/kp_3d.html
If the k_p value is at 6, you might see a relatively dim display, and if it is at 7 or more expect a bright display, typically the same night following values at 6 or 7.   To see a picture of the Sun in X-ray light to visualize why we on Earth might get a burst of particles, go to
http://www.lmsal.com/YPOP/ProjectionRoom/latest.html
This website shows simultaneous false-color images of the Sun in X-ray and other wavelength representations.  Unfortunately, the Yohkoh mission that made the X-ray images is now out of business - the spacecraft lost positioning control during the December 2001 solar eclipse, and mission scientists have given up regaining control.

3. What measurements would you make (assuming you have the money, time, & equipment) to determine a star’s surface temperature? Explain your answer.
You have a couple of choices here - in practice astronomers use them both.
a. Suppose you measure the shape of the continuous (blackbody) part of the star’s spectrum. You would plot the spectrum’s light intensity versus wavelength. From the wavelength at which the spectrum peaks, you can calculate the surface (photosphere) temperature using Wien’s law (wavelength is inversely proportional to the temperature).
b. Suppose you make a good measurement of all the absorption lines - location and intensity. You can determine the star’s surface temperature by finding the spectrum on a chart that most closely matches your star’s spectrum.