Betelgeuse Dimmed

First, I apologize for the initial bizarre appearance of my last post. For some reason, some computer decided to slice and dice. I have no idea why, or for that matter, how. Hopefully, this post will have better luck.

Some will recall that around October 2019 the red supergiant Betelgeuse dimmed, specifically from magnitude +0.5 down to +1.64. As a variable star, its brightness oscillates, but it had never dimmed like this before, at least within our records. This generated a certain degree of nervousness or excitement because a significant dimming is probably what happens initially before a supernova. There has been no nearby supernova since that of the crab nebula in 1054 AD.

To put a cool spot into perspective, if Betelgeuse replaced the sun, its size is such it would swallow Mars, and its photosphere might almost reach Saturn. Its mass is estimated at least ten times, or possibly up to twenty times, the mass of the sun. Such a variation sparks my interest because when I pointed out that my proposed dependence of characteristic planetary orbital semimajor axes on the cube of the mass of the star ran into trouble because the stellar masses were not known that well I got criticised by an astronomer: they knew the masses to within a few percent. The difference between ten times the sun’s mass and twenty times is more than a few percent. This is a characteristic of science. They can measure stellar masses fairly accurately in double star systems, then they “carry over” the results,

But back to Betelgeuse. Our best guess as to distance is between 500 – 600 light years. Interestingly, we have observed its photosphere, the outer “shell” of the star that is transparent to photons, at least to a degree, and this is non-spherical, presumably due to stellar pulsations that send matter out from the star. The star may seem “stable” but actually its surface (whatever that means) is extremely turbulent. It is also surrounded by something we could call an atmosphere, an envelope of matter about 250 times the size of the star. We don’t really know its size because these asymmetric pulsations can add several astronomical units (the Earth-sun distance) in selected directions.

Anyway, back to the dimming. Two rival theories were produced: one involved the development of a large cooler cell that came to the surface and was dimmer than the rest of Betelgeuse’s surface. The other was the partial obscuring of the star by a dust cloud. Neither proposition really explained the dimming, nor did they explain why Betelgeuse was back to normal by the end of February, 2020. Rather unsurprisingly, the next proposition was that the dimming was caused by both of those effects.

Perhaps the biggest problem because telescopes could only look at the star sone of them however a Japanese weather satellite ended up providing just the data they needed. This was somewhat inadvertent. The weather satellite was in geostationary orbit 35,786 km above the Western Pacific. It was always looking at half of Earth, and always the same half, but the background was also always constant, and in the background was Betelgeuse. The satellite revealed that the star overall cooled by 140 degrees C. This was sufficient to reduce the heating of a nearby gas cloud, and when it cooled, dust condensed and formed obscuring dust. So both theories were right, and even more strangely, both contributed roughly equally to what was called “the Great Dimming”.

It also suggested more was happening to the atmospheric structure of the star before this happened. By looking at the infrared lines, it became apparent that water molecules in the upper atmosphere that would normally create absorption lines in the star’s spectrum suddenly changed to form emission lines. Something had made them become unexpectedly hotter. The current thinking is that a shock-wave from the interior propelled a lot of gas outwards from the star, leading to a cooler surface, while heating the outer atmosphere. That is regarded as the best current explanation. It is possible that there was a similar dimming event in the 1940s, but otherwise we have not noticed much, but possibly it could have occurred but our detection methods may not have been accurate enough. People may not want to get carried away with, “I think it might be dimmer.” Anyway, for the present, no supernova. But one will occur, probably within the next 100,000 years. Keep looking upwards!