During last observing session I pointed telescope with attached spectroscope to well known Pleiades open cluster. It is also called “Seven Sisters” cluster, because this is the usual number of the cluster stars you may spot with naked eye. I wanted to check their spectra and how much similar they are 🙂
Here is annotated image of the M45 Pleiades open cluster in Taurus. I made this photo over four years ago. It was shot with TS Photoline 130mm apochromatic refractor and Atik383L mono camera.
I captured spectra of six Pleiades members: Alcyone, Merope, Electra, Maia, Taygeta and Atlas. Total exposure time for each target was a few minutes – these are bright stars, and that was enough to get decent signal to noise ratio. LowSpec spectroscope was equipped with 600 lpmm grating that gives resolution about R~1500. And the full visual range spectra you may see below.
These are really very similar. But all these bright stars of M45 are B type indeed, and they differ in details only. When you enlarge the spectra and know what to look for, then it is possible to classify star to proper type. Resolution R~1500 is enough for it.
But despite similarities, the most exciting feature is probably the hydrogen alpha line at 6563 angstroms. At this area the differences are striking.
These emission/absorption shapes indicates, that these bright B type stars in M45 cluster are shell stars. They are surrounded with molecular cloud. Rotating speed of the cloud and the star causes the line broadening. You may notice, that the width of the line is different for different stars. However it may be not quite easy to compare, because line height also differs.
Also the shape of the H alpha line differs among these stars. Emission line is superimposed over absorption, and for Electra there is another small absorption line at the top. The shape of the shell around the star and the angle the rotation axis is inclined to the line of sight affects the way this spectrum line looks like.
Here is hydrogen beta line (at 4861 angstroms) zoomed. As you may see these do not differ so much. You may only notice, that they width differ, but shape remains very similar.
And here is the LowSpec device attached to the telescope ready to capture spectra 🙂