A thread!
arxiv.org/abs/2002.10463
1) cooler objects emit less light (and emit it at longer wavelengths as temperature drops)
2) a cooler surface means more titanium oxide (TiO) molecules in the atmosphere; these molecules absorb lots of visible light.
![](https://pbs.twimg.com/media/ERuKt6wX0AELTgz.png)
We could use the strengths of the TiO bands to estimate its temperature and fit the spectrum with some red supergiant atmosphere models to see if anything looked odd.
Um:
![](https://pbs.twimg.com/media/ERuLQeUXsAIGBH4.png)
Here's a zoomed-in look of the 2004 and 2020 spectra.
The TiO bands are a LITTLE deeper in the 2020 spectrum, but not much:
![](https://pbs.twimg.com/media/ERuLeLfX0AUHDFP.png)
In the end, Betelgeuse looked the most like a 3600 K red supergiant; the TiO bands were a little deeper than in a 3650 K star, but not as deep as a 3550 K star:
![](https://pbs.twimg.com/media/ERuLp0JW4AIw2e6.jpg)
Combining this with our star comparisons, we settled on a Feb 15 2020 T=3600 K for Betelgeuse.
![](https://pbs.twimg.com/media/ERuL7K2XUAMUuoY.jpg)
Nope. After doing the math, it turns out a 50K decrease would make Betelgeuse about 0.4 mag dimmer. It had gotten 1.1 MAG DIMMER since October!
Dust.
RSG-produced dust grains tend to be bigger, which means they act "gray" - they absorb red AND blue light and wind up making objects look dimmer.
We think this could be happening because there's some new large-grain dust blocking our view down there.
![](https://pbs.twimg.com/media/ERuM7EIWsAEZGjW.jpg)
![](https://pbs.twimg.com/media/ERuNFjRXkAAg1XL.jpg)
arxiv.org/abs/astro-ph/0…
However, it IS still an awesome and fascinating star, and we should all keep studying it!