The Spectra of Stars

The light from a star can tell professional astronomers much about a star's temperature and chemical composition.  The colour alone can provide information as to temperature, the bluer the hotter and the redder the cooler, but it requires spectroscopy to provide information as to the chemical composition.

To obtain a spectrum from light you require a prism or a diffraction grating.  A diffraction grating is comprised from a number of fine parallel lines etched on glass or metal.  A CD disc can perform as a diffraction grating and if you look at one obliquely you will see the typical colours of the spectrum of white light.

You can purchase diffraction gratings for use with amateur sized telescopes but they cost about £100.  I wondered whether I could make one and whilst surfing the internet found a useful programme on the COAA website.

Sirius through my eyepiece
with spectrum visible in offset spikes

Anyway, armed with only a piece of automotive carpet, a couple of sheets of printer acetate, a sheet of thin card, a tube of impact adhesive, a scalpel, my laptop and an inexpensive Epson printer, I set about making a grating to go over the objective lens of my 127mm Meade apo refractor.

Well it took me about an hour and I had what looked like a diffraction grating. Would it work?

One of the main problems of obtaining a spectrum from starlight is that the amount of light you can collect is very small.

On the 9th of March, whilst I was awaiting the shadow transit of Jupiter by its moon Ganymede, I decided to try and capture the spectrum of Sirius and that of Betelgeuse.  I chose these stars as they are both bright stars and at opposing ends of stellar classification. The following images were taken with a handheld compact camera, held to the telescope eyepiece and using the video setting . I converted the mov. clips to avi format and then stacked the frames to create stills using AutoStakkert.

Betelgeuse through my eyepiece
 with spectrum visible in offset spikes

Spectra of the two stars side by side for comparative purposes
using my homemade diffraction grating

The above Spectra cleaned up, enlarged and saturation increased in APS

These two spectrums can be seen to be quite different, indicating the very disparate temperatures and compositions of the two stars.

All in all I was quite surprised by how well my homemade diffraction grating works. Calibrating my spectra however, remains another issue to which I need to give more thought. Not bad though for an old fine arts graduate!

http://www.coaa.co.uk/software_astronomy.html