Download Module 3: Exploring Other Stars Assignment 5: Estimating

Module 3: Exploring Other Stars
Assignment 5: Estimating Temperatures of Stars from their Spectral
Energy Distribution
In this module we will further explore techniques astronomers use to learn about
stellar temperatures from the spectrum of the star.
Look at the spectra below, (also found at the NOAO website with many other
nice images): www.noao.edu/image_gallery/images/d2/starsl.jpg.
These represent what color photographs of the stellar spectra would look like, if
we had really sensitive photographic material. But we don’t, and in the digital
age, astronomers have found it’s much more accurate to represent the spectrum
as a plot of intensity versus wavelength. This is what the applet in the last
module was doing for a theoretical blackbody. In the document
Module3_spectra.pdf you see the digital spectra of six real stars. The actual
spectra of stars are not perfect black bodies. The spectra exhibit sharp dips
called absorption lines that were described in the stellar_spectroscopy.pdf. In
module 4 you will learn how astronomers use just the pattern of absorption lines
to determine the temperature of a star but now we will gauge its temperature by
examining the overall slope and shape of its spectrum without paying attention to
the absorption lines. Astronomers would say you are examining the slope of the
“continuum”, which approximates the black body shape.
Examine each of the six stars with spectra in Module3_spectra.pdf file. (The
identification of the stars are given in the line just above the plot: HDxxx, or
SAOxxx, or Feigexxx). You might want to print them out and arrange them in
whatever logical pattern you see. Now calculate a temperature for each star from
the peak of the spectrum: in the cases where you can identify the peak
wavelength, you can use Wien’s law, as given in the spectroscopy pdf. Or you
may also want to use the java applet
(http://astro.unl.edu/naap/blackbody/blackbody.html ) to estimate where the peak
lies. Estimate your error in measuring this peak wavelength: what error does that
translate into for the temperature?
Then, using this temperature, you can also estimate the relative amount of
luminosity emitted by each star using the Stefan-Boltzmann Law (luminosity is
proportional to the 4th power of the temperature). To do this, let’s define the
luminosity of the coolest star to be 1.0. How much more luminous are the hotter
stars?
On the Blog post a list of the stars (identifying them by their HD, SAO or Feige
numbers) your estimates of temperature (in degrees K), and the number of
significant figures you believe are appropriate. Calculate the relative luminosity,
compared to the coolest star in your sample, for each of the other 5 stars. To do
this, select the coolest star in your sample and define its luminosity as 1, and
scale the remaining stars to this one.