There's Water on the Sun?

The Sun’s a great ball of fire. Well, okay, it’s more like a giant ball of hydrogen and helium plasma, plus a sprinkling of other ingredients all swirling around at temperatures that would feel like taking a blowtorch to the face. But one of those ingredients might sound a little weird.

Turns out, after decades of debate, astronomers found evidence of water on the Sun. Water is all over the solar system. In fact, some water molecules are older than the solar system itself. They were just hanging out in a big cloud of dust and gas which eventually collapsed into a bunch of smaller bodies. So of course the Sun, which holds over 99% of the solar system’s mass, had to have scooped up some of that water when it formed.

The question was, could those molecules stay water? Or were the scorching temperatures so high that every bit of H2O was doomed to crack apart into hydrogen and oxygen? The Sun has an average surface temperature of about 5,500 degrees Celsius, which yeah, is too hot. But I did just say average.

What about those dark patches? Those are sunspots. And they look darker than the surrounding bits because they’re cooler, around 3,500 degrees Celsius. Yes, that’s still super hot to you or me, but it’s cool enough for water molecules to survive, in vaporized form. But just because water vapor can survive there, doesn’t mean it’s actually there. Hence the long and arduous hunt for water on the Sun.

Now, scientists can figure out a star’s composition using spectroscopy, measuring the exact blend of light wavelengths that the star gives off. Every element and molecule has its own unique spectroscopic signature, but water’s is a bit complicated. And at certain wavelengths, it can look eerily similar to other chemical compounds that are very much not water. So around the turn of the 20th century, some scientists were arguing over what compounds they were actually spotting in sunspots. Like was it water, or was it…titanium?

Then, in the 1970s, astronomers detected water vapor’s signature in the light of red giant stars. These stars are red because their surfaces are way cooler than our Sun’s. In fact, their average surface temperature is in the same range as the Sun’s sunspots. And finally, in the 90s, scientists found what they were looking for. H2O can indeed hang out in those cooler, darker patches.

Now, this might all sound like a bit of fun, counterintuitive trivia. That water can exist atop a fiery ball of plasma. But this fact can seriously complicate our exploration of worlds beyond this solar system. By watching a planet pass in front of its star, and measuring how starlight gets filtered through the planet’s atmosphere, astronomers can figure out what that atmosphere is made of.

And as you may have guessed, one of the compounds they want to find is water. But any detection of water vapor has to come with a caveat. The signal could be coming from the planet’s atmosphere… or it could be coming from the star’s. So if you hear a bit of news announcing that we found water on another world, maybe pause and wonder whether that water is actually on a star.

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