Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:
ULTRA-COOL DWARF STAR
At some point, I want to profile each of the planets in the TRAPPIST-1 system one by one for my Exoplanet Explorer series. But it’s too early for that. Right now, we don’t know much about these planets except that they’re there.
But I can say something about TRAPPIST-1 itself. It’s a type of star called an ultra-cool dwarf star.
Apparently TRAPPIST-1 has just barely enough mass to cause hydrogen fusion in its core. That means that for a star, it doesn’t produce a whole lot of energy, and thus its temperature is relatively low. Based on my rough math and statistics I got from Wikipedia, it looks like TRAPPIST-1 is less than half the temperature of our Sun.
This is one of the things that makes TRAPPIST-1 so interesting to me, and why it’s really starting to capture my imagination. It’s not just about all those Earth-like planets. The star itself helps set a lower limit for just how small and cold stars can be.
Planet Pailly 
It sounds like these things are just above brown dwarfs. I know something like 75% of the stars in our galaxy are red dwarfs. I wonder what proportion are the ultra-cool variety.
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That’s my impression as well. Just a little less mass, and this would be a brown dwarf.
Wikipedia says 15% of the astronomical objects in our stellar neighborhood are ultra-cool dwarfs. I’m not sure what they’re including in the category of “astronomical objects” or how large our “stellar neighborhood” is, but I guess that’s at least the start of an answer.
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