Saved on this device

Follow your curiosity

What discovery has been shared with you?

FACTREE gives you something interesting to discover every day — and something worth talking about together.

Start with one fact. Explore it, go deeper, then follow whichever branch catches your imagination.

Choose subjects for a surprise

Exploring any topic

Begin your discovery

Your next discovery is one click away.

Choose one or more subjects above, or leave Any Topic selected and let curiosity decide.

Astronomy

Extreme Ultraviolet Spectroscopy of the Solar Corona

Quick fact

By analyzing extreme ultraviolet (EUV) light from the corona, scientists have measured temperatures spanning 1 to 10 million K—far hotter than the Sun's visible surface at ~5,500°C. These observations rely on spectral lines from highly ionized elements like iron that emit only in the EUV range.

Why this is interesting

The Sun's outer atmosphere, the corona, is millions of degrees hotter than its surface—yet it's invisible to the naked eye. How do we know its temperature, or even that it exists?