An oven with three racks, each baking a different dish: that's the image that comes to mind when you look at the galaxy I Zw 18. The top rack is blazing at 21,200 kelvins, the middle one at 17,290, and the bottom at 16,170. Such unevenness would surprise any chef, but that's exactly what astronomers found when they directly measured the electron temperature in three different glowing gas zones for the first time.
This galaxy is a true relic: it has almost no elements heavier than helium, which astronomers call "metals." It's like a kitchen where the only ingredients are water and flour. By studying such objects, scientists peer into the Universe's past, when stars hadn't yet had time to forge heavy elements. Using the DESI spectrograph, mounted on a telescope in Arizona, they managed to catch rare emission lines of oxygen and sulfur — thermometers of sorts that reveal the gas temperature.
The precise temperatures allowed recalculating the oxygen abundance: 12 + log(O/H) = 7.066 ± 0.046. That's about 50 times less than in our Sun. They also refined the ratios of nitrogen and sulfur to oxygen. Such data serve as an anchor for calibrating methods used on distant galaxies where direct temperature measurements are impossible. So, a tiny galaxy-oven helps fine-tune thermometers for the entire Universe.
A temperature of 21,200 kelvins is four times hotter than the Sun's surface. At that temperature, oxygen atoms glow green, like the aurora borealis.