- Pick an IRAS source from the list on the Excel spread
sheet (the row labeled IRAS ID). For more on IRAS see Introduction to
- You are an astronomer now so you need to look at the
source. Go to the amazing SIMBAD viewer to see
the sky at the source. This is maintained by the Centre de Donnes Astronomiques de
Strasbourg. In the entry box under "1 enter an identifier, type
IRAS followed by a space and the IRAS source ID. Press the SUBMIT
button five or six lines down. Shortly a page will appear with a lot or
information about the IRAS source. Now it gets slightly tricky
depending on your computer and its firewall. First try the Aladin Java
applet button on the right. If you are lucky a picture will appear
after a minute or so showing the sky over a square several arc minutes
on a side. If you are unlucky you can use the Aladin Previewer. The
porblem is the sky view is too large and there is not much flexibility.
A third path if you are a computer whiz is to install the Aladin
stand-alone viewer. Taking the lucky case wait until the picture
appears. Right click on the picture and click grid to put a grid on.
It helps to go down to the lower right and put the zoom at 2. Click the
load button on the upper left to bring up the 2MASS and IRAS
information. Get 2MASS by clicking the VizieR buttor on the right and
then clicking 2MASS on the list that appears and then the submit button
on the lower left.. There are lots of 2MASS sources so this takes
awhile. Scroll down the list and repeat for IRAS. If you are lucky you
will have your IRAS source near the center of the picture. Click on the
IRAS source and information will appear at the bottom of the page.
(This may take some practice.) A pure Dyson Sphere will not have an
optical source in the survey optical picture.
- See if there is a 2MASS source nearby. Shift click to
bring up its parameters at the bottom of the page. The xxx.xxxxxx 2MASS
identifier is on the left. The values for the three IRAS filters in
magnitudes appear in the third, fourth, and fifth boxes. These can be
converted to values in Janskys by using the formulas in the 2MASS
All Sky Explanatory Supplement VI.4a. Alternatively, the flux
values for the three filters in magnitudes can be taken from the Excel
spread sheet. They must still be converted to Janskys. Basically F =
F0*10^(-M/2.5) where M is the magnitude and F0 = 1594 (for 1.235 micron
filter), 1024 (1.662) and 666.7 (2.159). Plot F (y axis) vs landa
(microns) as x axis. Typically a good pure Dyson Sphere would be
expected to have a small value.
- Plot the ratio F[i]/k[i] where F, in Janskys is the
flux and k
is the "color correction" for each of the four IRAS filters. In the
IRAS database the fluxes are called FLUX. In the spread sheet these
are at IRAS filter information. The wavelength (lamda) values are
listed on the left and are the x values for the points. The color
corrections are given just below the fluxes. Typically the color
corrections are approximately 1. (Information on the color correction
is given in Section VI.C.3 of the IRAS Explanatory Supplement.) The y
values are the flux/color correction for each source.
- Plot the corrected raw spectra from the IRAS Low
Resolution Spectrometer (LRS) times
the wavelength in microns times 3.34E11 (10E20/c where c is the speed
of light in m/second). The spectra are given in the chart but can be
obtained for other IRAS sources using Kevin
Volk's Calgary page.
Put in the IRAS ID and remember to use "corrected raw text". There are
two spectra, one for shorter wavelengths, one for longer. They appear
on the spread sheet at IRAS LRS - Calgary "corrected raw text". The
associated wavelengths in microns are on the left.
- Fit the IRAS 12, 25, and 60 micron filters with
Use the frequency version of the Planck distribution. This sounds
strange since the x axis is in wavelength. However, this approach more
easily matches the other distributions. To do this it is
fit the peak value (related to apparent magnitude of the source) and
temperature. One way to estimate the temperature is to use so-called color-color
temperatures. These two temperatures are
given in the spread sheet under temperature and fit. The spead sheet
gives a fit at the trial temperature, the coarse best fit that was done
for the Dyson sphere search of the entire data base. A typical fit is
shown on the "working" page of the spread sheet.
- Is the source a pure Dyson Sphere? For this search a
Dyson Sphere is assumed to be thick enough to adsorb all the radiation
from the star and reradiate it as a pure black body spectrum. If there
are any absorbtion or emission lines the source is not a pure
Sphere. If the spectra from the IRAS filters, the LRS, and the 2MASS
points do not match a Planck distribution it is not a pure Dyson
Sphere. If it did closely match the Planck distribution (as a few
post-AGB sources seem to) then one may need to really look in detail as
other frequencies like radio bands to rule out other possibilities.