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Monday, December 08, 2008


Watching Mira Brighten - Record Your Observations.

The star Mira shines at roughly magnitude 4.2 in this image of the sky taken on 28 November 2008 (click to enlarge, otherwise the stars are a bit to dim to see).

While most stars seem to shine with a constant brightness, there are some that undergo regular, dramatic change in brightness. Few of these are visible to the unaided eye, but two classics are Mira and Algol.

Algol is a bit hard for most of us in the southern hemisphere, but Mira is very easy, and Mira (omicron ceti), a star in the constellation of Cetus the whale, is a long period pulsating red giant and changes brightness from below naked eye visibility to a peak of round magnitude 2 (roughly as bright as gamma Crucis in the Southern Cross) in around 330 days. Mira will peak magnitude later this December, and will brighten visibly over this month.

The image above shows Mira on 28 November, when it was around magnitude 4.2, as of last night it was around magnitude 3.9. At the beginning of November I couldn't see it at all, so it was probably magnitude 5.0.

Here's a activity for the next few weeks, record the changes in Mira's brightness. This will be difficult for the next few days, as the bright Moon will be nearly on you o f Mira, making the stars hard to see. However, after a few days you will be able to see the stars with reasonable ease.

But I don't know anything about magnitudes, you say. Estimating magnitudes is relatively easy, the image to the left is a chart of Cetus with Miras position marked, and the surrounding stars labelled with their magnitude. To estimate the magnitude of Mira, compare it to the surrounding stars. Is it brighter than a star labelled 3.5, and is it dimmer than a star labelled 3.0? then its magnitude is somewhere around 3.2-3.3.

Click on the chart to enlarge it and print it out. Use a torch with red cellophane wrapped around the business end to view it while star gazing (to keep your night vision), and wait for at least 5 minutes outside for your eyes to dark adapt. Cetus is just above the easily recognized Taurus above the northern horizon in the early evening. This would be an ideal activity for kids.

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ice piece, Ian. I've included it in the AAVSO Writers Bureau for December. Write more variable star stuff!
G'Day Mike.

I fyou liked the Mira piece, then you might like the piece I have just done on Algol.

Spaeking of the AAVSO writers bureau, I have edit access, but should I let AAVSO folks nominate a post of mine, or can I put up my own posts?
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