Reading between the lines

Spectrum of the star Vega in the constellation Lyra taken from my backyard with my homemade spectrometer affixed to my 127mm. Meade Apo refracting telescope. The wavelength is given along the X-axis in Angstrom units where an Angstrom is one ten billionth of a metre.              The spectrum has not been corrected for the camera's response or the affects of the Earth's atmosphere.  Credit: Visual Spec free software by Valerie Desnoux used for processing the spectrum.

 There are three basic types of spectra:

1. Continuous Spectra - "A hot opaque solid, liquid or gas will emit a continuous spectrum when under high pressure" Kirchoff's Laws Rule 1 - produced by by hot bodies like stars and our Sun and the result of very hot gases comprising energetic atoms colliding.
2. Emission Spectra -"A hot gas under low pressure (ie. much less than atmospheric) will emit a series of bright lines on a dark background" Kirchoff's Laws Rule 2 - produced when atoms have less energy than those producing continuous spectra.
3. Absorption Spectra - "When light from a source that has a continuous spectrum is shone through a gas at a lower temperature and pressure, the continuous spectrum will be observed to have a series of dark lines superimposed on it." Kirchoff's Laws Rule 3 - produced when photons from a continuous spectrum pass through a gas made of one or more elements, lines for each element will absorb photons of the wavelengths specific to those elements. In other words this produces a continuous spectrum with dark lines where photons of specific frequencies are absorbed. Our Sun and other stars produce continuous spectra but as the photons, created at the stellar core, pass through the stellar atmosphere some are absorbed at specific wavelengths related to the elemental make up of the star.

 Analysis of the Absorption Spectra from stars enable astronomers and astrophysicists to determine the chemical fingerprint of stars, their temperature and radial velocity. A very powerful measurement tool!

Alpha Lyrae or Vega is the third brightest star visible from the northern hemisphere. It is only 25 light years from our Solar System  a veritable neighbour. It is a very hot star, approximately 9600 degrees Kelvin, or about twice as hot as our Sun. It has a mass approximately twice that of our Sun and is spinning so fast that it shape is oblate rather than spherical. It is thought to have a protoplanetary disk of dust and rocks that spins around it.

The visual spectrum of Vega is dominated by absorption lines of hydrogen; specifically by the hydrogen Balmer series with the electron at the n=2 principal quantum number. The lines of other elements are relatively weak, with the strongest being ionized magnesium, iron, and chromium. My low resolution spectrum shows the stronger Hydrogen Balmer lines clearly demonstrating the presence of Hydrogen in the stellar atmosphere of Vega. I believe some of the Helium lines can be seen and also two lines of Oxygen in the Earth's atmosphere (the photons I collect with my kit has to pass through our atmosphere to reach my telescope. (All of this could be wishfull thinking on my part)

This is my first real attempt at obtaining information from stellar spectra. Well what did I find out?

1. Vega has an absorbtion spectrum which exhibits the Balmer Series of Hydrogen absorption lines.
2. I tried applying Wiens Law using the wavelength at which it exhibits the greatest energy flux (taken from my spectrum) to determine Vega's temperature and obtained a value 100% different from the published temperature. As Wiens Law works only for 'blackbody emitters' I came to the conclusion that Vega does not radiate energy as a 'blackbody'.

Credits: I used information taken from the following sources - Jeffrey L. Hopkins 'Using Commercial Amateur Astronomical Spectrographs'- The Patrick Moore Practical Astronomy Series by Springer. Visual Spec free software by Valerie Desnoux. Three Hills Observatory.

My homemade spectrometer under construction - made from 100 lines per mm. transmission diffraction grating and an old Logitek Webcam with the infrared filter removed.

 I think I might construct a new design  mark 2 spectrometer utilising a camera with a bigger chip! The shed beckons.