We use Zulip (https://zulip.org/) for our corporate chat, and we've never looked back. It's been good, and it's fully open source. We self-host, but paid hosting is easy to get too if you want.
Wow. This project was the cause of a very long and intense discussion about mis-use of the term "open source". See https://github.com/n8n-io/n8n/issues/40#issuecomment-5397146... for details (lands mid-thread -- you might want to scroll back to see the start, and if you read the whole thing to the end then you deserve some sort of award!).
TL;DR: The author originally tried to call n8n "open source" but while using a non-open-source license. After much discussion, he kept the license but stopped using the label "open source", to the relief of many people.
That half-decade-old thread is still what I point to when I want to explain to someone why preserving the specificity of the term "open source" matters.
Most of the comments so far are about the temperature and the closeness to the sun, and, hey, I get it: those are both amazing to think about. But to me even more amazing is... 0.16% of the speed of light?? Yikes.