Which chemical element is named after the Sun?

Helium, from helios - and it is the only element discovered somewhere other than Earth before being found here. In 1868 astronomers observing a solar eclipse found a spectral line at a wavelength matching no known element, and Norman Lockyer proposed a new one existing in the Sun. It was ridiculed for years and confirmed on Earth only in 1895.

It is the second most abundant element in the universe and genuinely scarce here, because it is light enough to escape the atmosphere entirely and does not come back. What we have is dug up: it accumulates underground from the radioactive decay of uranium and thorium over hundreds of millions of years and is extracted as a by-product of natural gas from a handful of fields.

Which makes party balloons an awkward use of a finite resource. Liquid helium a few degrees above absolute zero is the only practical coolant for the superconducting magnets in MRI scanners, with no substitute, and shortages have repeatedly disrupted hospitals and semiconductor plants. It also has a genuinely bizarre property below about two kelvin: it becomes a superfluid with no viscosity, capable of flowing up the walls of a container and out.