Tuesday, October 11, 2011

Seeing the Light - Astonishing Mira

I was perusing the Sky and Telescope website the other week, and ran across an observing report prompting sky watchers to go out and observe the variable star Mira, now at maximum brightness in the eastern sky in early evening. 

Mention of the star transported me back a decade or more back in time to a night I was out observing with my buddy Roger at the TAAA's observing site at Empire Ranch.  I know it was 12 years ago because Jupiter was very near it's present position, which takes 12 years to circle our sky!  I had just finished an 8" diameter objective prism, and had it mounted on my 11" Newtonian for some visual spectroscopy - breaking the light of stars into a rainbow.  Because the prism deviates the light a considerable amount, I was sweeping the sky, looking for the bright spectrum of Jupiter when I accidentally swept up a remarkable spectrum!  Instead of most stars which showed the rainbow colors with only a few dark narrow absorption lines, this one, also pretty bright, was crossed by much wider absorption bands and also had emission lines in the spectrum!  It was very distinctive and astonishing.

With a little study to locate what we had found on the star charts, it was Mira, also then near maximum light.  Mira is a long period red variable, and since it was discovered hundreds of years ago and is the first of it's kind, is called a Mira-type variable.  Over a period of 330 days it varies from 2nd to 10th magnitude - from easy naked eye visibility to invisibility even with binoculars (a factor of about 1600!).  Interestingly, in the past 12 years since swept up in my spectrograph, it has gone through 13 cycles, so reappears now at maximum brightness...

This last January, I posted about objective prism spectroscopy, and showed some spectra of the Hyades star cluster.  Using a similar setup last weekend (used a longer focal length lens for more color dispersion), I again imaged the spectrum of the bright blue star Vega to compare it to the Mira spectrum.  The result is shown here with a spectrum of the star Fomalhaut for comparison.  Fomalhaut is about twice the mass of the sun and as a result burns about 3,000K degrees hotter (about 9,000K vs about 6,000K for the sun).  With the hotter temperature, the spectrum of Fomalhaut is dominated by hydrogen absorption lines.  Mira, at a much cooler 3,000K, has a much more complicated spectrum, the cooler temperatures allow heavier elements and even molecules in it's atmosphere, dominated by the molecular bands of Titanium Oxide.  Also noteworthy are Hydrogen Beta and Gamma lines are in emission rather than absorption.  It makes for a very striking spectrum visually.

Mira should be easily visible for a few more weeks in the eastern sky to the south (right) of Jupiter.  The picture here was taken on the first of this month with brilliant Jupiter and Mira over the lights of Sierra Vista and Fort Huachuca.  If you are interested in checking out a variable star, hunt it down and watch it disappear over the next couple months.

1 comment:

David A. Harvey said...

Wonderful post Dean. I remember observing this star back in 1969 after I got my first telescope (a 2.4" Sear refractor) mistaking it for Saturn (which was nearby). Thanks for the memories.