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Wednesday, December 11, 2019

 

Betelgeuse Fades

Evening sky at 22:10 ACDST on Saturday December 14 (90 minutes after sunset), Orion is readily visible. Betelgeuse is the bright red star below the "saucepan" of Orion.

Betelgeuse is a red giant star which forms a distinctive part of the Constellation of Orion the Hunter.

While familiar to almost all casual observers of the night sky, not may people know it is a variable star, with small fluctuations in brightness not visible to the casual observer.


A 9 year light curve of Betelgeuse from the light curve generator at the AAVSO.  It's a bit messy because of all the observations , but if you pick out the yellow line you can see the recent dip in brightness that has everyone excited.

 The variability of Betelgeuse is complex, with a
dominant period of 420 days, superimposed on a long period of 5-6 years and a shorter term variability of around 180 days. The 420 day variability was likely observed by Indigenous Australians and incorporated into their sky stories.

However now it has been reported that Betelgeuse has dimmed substantially, (see curve above) to a 25 year low (even though it is still bright to us). It is well worth keeping an eye on it, to see if it dims further.

What the dimming means is not clear. It is thought that  Betelgeuse's  variability is due to the star expanding and contracting in size, with dimming due to contraction. The unusual dip in brightness may be due to its' multiple cycles lining up, or possibly dust being brought up to the surface.

Betelgeuse may be getting a bit more attention as we expect it to go Supernova, but we don't expect that to happen for tousands, if not millions, of years from now.

At the moment getting a good fix on Betelgeuse's brightness will be a bit difficult, as the nealy full moon will make determining its brightness harder, but by the week end there should be enough time before the moon rise to get a good feel for it. Achernar (magnitude 0.45), Alpha Crucis  (magnitude 0.61) and Beta Centauri (magnitude 0.55) and the red giant star Aldebaran (magnitude 0.85) are good comparison  stars. All in all  Betelgeuse will be an interesting object over the coming month.

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