Mizar, Alcor and some background stars and their spectra, scaled 0.75x.
When astronomers started to take spectra of stars in the middleof the 19th century, it seemed as though different stars were made of differentelements. As usual in science it was good practice to develop a classificationscheme, which, while it developed, gave insight into the inner workingsof the objects under investigation leading to a deeper understanding.
Most of this work was done at Harvard Observatory. The so called Harvardsequence of stellar spectra was developed between the 1890s and the1920s and it is since then used by astronomers all over the world. Starsare classified according to the presence of absorption lines which areproportional to the abundance of chemical elements in the outer stellaratmospheres.
Spectral classifying stars is something of an art form that comes withpractice. Each spectral class is defined by the spectrum of a standardstar against which the other stars are compared.
Several catalogs of digital spectral examples are available on theWeb like the ElectronicUniverse Project or, ANew Library of Optical Spectra (Silva + 1992).
If I leck patience I use Guide to obtain the spectral type ofa star on a mouse click.
Here is a large collectionof spectra from Fujii Bisei Observatory.
And here two java applets follow which display a periodic system and for every chemical element anassociatedspectrum,.
Here I present my first spectroscopic results.Spectra of several main sequence stars, one Wolf-Rayet star (WR136) and of the ring nebula M57 were obtained.
The following image shows Mizar and Alcor (upper left side), a famous couple from the handle of the big dipper, along with two associated streaks which are their first oder spectra.This raw image was obtained on 05/14/2000 with my 8"/F4.5 Newtonian, camera was my ST8E in 2x2 binning mode, exptime 10sec. Rainbow-Optics grating at nose piece of camera
Dispersion: 15.6 Angstroms per pixel
Full Width at Half Maximum
Spectral coverage per frame: about 5550 Angstroms
Wavelength Range: 3500 to 9000 Angstroms
Sensitivity: Signal to noise ratio of 10:1 for a 7.7th Mag star, 45
second exposure using a non-ABG ST-8E and a 8 inch (20 cm) aperture.
Mizar, Alcor and some background stars and their spectra, scaled 0.75x.
The above image is rotated 18.8 deg counter clock wise and Alcor and its spectrum is cropped.
A little bit of image processing shows the prominent Balmer series of Hydrogen more clearly, which dominate the spectra of all A-Type stars. This type of spectroscope has strong field curvature, therefore one does not focus on stars but on spectral lines.
Allthough it is not difficult to identify the most prominent lines, the spectrum should be calibrated for wavelength at least. If you want more, you can correct for atmospheric extinction and the instruments spectral response. Results are shown in the next image. Observe the left (blue) shift of the brightest parts of the continuum after correcting for atmospheric transmission as well as spectral response of the CCD ...
The following plot corresponds to the spectrum above. The green lines are the positions of Balmer lines of hydrogen (computed according to the Rydberg formula), which converge against the series limit at 3646A.
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