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Wednesday, February 12, 2020


Will Betelguese Brighten in the Coming Fortnight?

Recent light curve of Betelgeuse from the light curve generator at the AAVSO. I have used a mean of 3 days observations (with error bars) to give a better idea of the average brightness over time (click to embiggen).

As has been reported widely, the red-giant Betelgeuse has been dimming and is now dimmer than has been measured visually in the last 50 years.

This has caused considerable excitement in the astronomical community, but although Betelgeuse is "close" to the end of its lifespan, it is unlikely to go supernova any-time soon (maybe 100,000 years from now).

There has been a lot of speculation over why Betelgeuse is dimming. The leading theory is that it is a chance alignment of it's natural variability cycle. Betelgeuse is a variable star with a complex cycle of dimming and brightening, there is a dominant period of 420 days, superimposed on a long period of 5-6 years and a shorter-term variability of around 180 days. An early Astronomers telegram at the beginning of the "fainting"suggests that the "the current faintness of Betelgeuse appears to arise from the coincidence of the star being near the minimum light of the ~5.9-yr light-cycle as well as near, the deeper than usual, minimum of the ~425-d period".

A more recent Astronomers telegram has reported improved modelling of these cycles and predicts if the cycles are the cause, Betelgeuse should start brightening  on February 21 (± 7 days). That means it could start brightening as early at Feb 14 or as late as the 29th. Some more explanation and graphs are available at this Space Weather article (scroll down).

Whatever happens in the next fortnight and beyond, observation by multiple observers are required during this critical time period.  See spotters maps and brightness guide stars below.

Regardless of cause, Betelgeuse is the dimmest and coolest it has been since photometry began 25 years ago, and may even have shrunk to 92% of its previous diameter.

Evening sky looking north at 21:42 ACDST on Saturday, February 15 (90 minutes after sunset). Similar views will be seen elsewhere in Australia at the equivalent local time. Click to embiggen.

Orion is readily visible. Betelgeuse is the bright red star below the "saucepan" of Orion. Red Aldebaran is almost the same height above the horizon as red Betelgeuse, making brightness comparisons easy.

In order to avoid the Purkinje effect, where red stars seem to become brighter the longer you stare at them, you need to keep shifting your gaze around. Try bracketing the star with observations of stars brighter and dimmer. to get a good comparison. A more comprehensive guide to observing variable stars is here. Different observers will have slightly different estimates. My last estimate was Betelguese was a trace under 1.7 and Les Dalrymple had it a trace over. Try not to let your expectations bias what you are seeing. Comparison sttar magnitudes are given below.

Spotters chart of stars suitable for estimating the brightness of Betelgeuse. Nearby Aldebaran (magnitude 0.85) also red, is a good comparison star. Bellatrix, the other shoulder star of Orion opposite Betelgeuse is magnitude 1.6. The middle star of Orion's belt, Alnilam is magnitude 1.7 and Adhara in Canis Major is Magnitude 1.5. Wezen, near Adhara, is 1.8 and Saiph in Orion is 2.1.

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