ANALYSIS
Data reduction
Many stars had high radial velocities, and this altered the position of the spectral lines. First, the shift caused by them must be accounted for by using the non-relativistic Doppler formula:
λ’ = λ (1+ v/co)
λ’ is the shift that is caused by the Doppler Effect. λ is the initial (rest) wavelength. V is the radial velocity (which was highly negative and therefore caused the shift) and co is the speed of light in units of m/s. The shifted line positions needed correction.
Secondly, the heliocentric radial velocities given in each of the catalogs made two corrections: a) for the Earth’s orbit around the Sun and b) for Earth’s rotation. After the radial velocities are amended, adjustment is still need to compensate for these changes. In order to calculate the amount of change, some identified lines were taken from the spectrum and compared to a rest spectrum. The real wavelengths, compared with the theoretical wavelengths give an idea of how many angstroms the spectra has shifted. With a resolution of 1 Angstrom, such shifts if left uncorrected could lead to the misidentification of elements. One spectrum was corrected for their shift by comparing the observed wavelength of specific lines and comparing it to its lab wavelength. It had the second highest negative radial velocity in this survey.
In order to portray the
entirety of the photon counts, each of the text files were put into Excel
and files for the same star were compared to each other for cosmic rays.
Cosmic rays are light that saturates the pixel when it enters the
spectroscope. These are not a part of the star’s spectra and must be taken
out. In order to do so, the cosmic ray is subtracted. However to do this,
both graphs must have the same intensity (photon count). In order to account
for this, the stars are put in a proportion to have them both represent their
respective amounts on a level field - then the cosmic ray subtraction is done.
After the subtraction, the spectra can then be stacked (added together) and
then graphed using PSI Plot. These higher photon counts then increased
the signal to noise ratio which more accurately portrayed the spectra of the
star.
Here is a demonstration of stacking.