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Diffuse interstellar bands were first discovered in 1919 and since then scientists have found nearly 500 of them. How many do we understand? Only one.

Host: Caitlin Hofmeister

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[ ♪ Intro ].

For thousands of years, people have looked into the night sky and found stuff they didn’t understand. That’s kind of how astronomy happened.

But not very many of those mysteries have lasted for a hundred years. And even if they have, most of them aren’t as simple as “what am I even looking at?” Then, there are the diffuse interstellar bands. These are patterns of light in space that don’t match any known atom or molecule.

There are hundreds of them, and we’ve been trying to figure out what they are for nearly a century. After decades of work, scientists finally identified one of these bands in 2015, but even then, it’s just downright bizarre: a cage-like carbon molecule containing sixty atoms. So what else is floating around out there?

The diffuse interstellar bands, or DIBs, were first uncovered in 1919 by astronomer Mary Lea Heger. She was studying faraway stars by measuring their spectra, or the distribution of their light by wavelength. Different molecules absorb and emit different types of light, so patterns in these readings typically tell astronomers what objects are made of.

But this time, Heger noticed a pattern that had never been seen before, and it wasn’t clear which molecules were the culprits. Their light absorption showed up as fuzzy gaps, or diffuse bands in spectra taken with early photographic plates, so these signals quickly picked up a nickname. But it wasn’t like astronomers had never found strange readings before.

In 1868, the observation of an unknown spectral band in the Sun’s atmosphere had actually led to the discovery of helium. It’s science: New stuff comes up all the time. The weird part with DIBs is what happened next: nothing.

Scientists were unable to find any matching atom or molecule here on Earth. In fact, they still haven’t, at least, for those specific signals. Instead, over the next century, astronomers found nearly 500 more DIBs.

And until 2015, they hadn’t explained a single one. That year, a team of physicists were messing around with an exotic molecule called buckminsterfullerene. Rather than repeating that word over and over, people usually just call them buckyballs, but they are not the same stuff as the popular magnetic toys.

Buckyballs had been discovered during lab experiments in the 1980s designed to simulate the gas flowing out of carbon-rich stars. They’re made of 60 linked carbon atoms, look remarkably similar to a soccer ball, and were cool enough to earn their discoverers the 1996 Nobel Prize in Chemistry. The 2015 experiment cooled down a gas of buckyballs to nearly absolute zero and an ultra low pressure, essentially, the conditions of deep space.

And in that situation, the researchers found that buckyballs emit a spectrum of light that matched a set of diffuse interstellar bands discovered in 1994. Which was awesome! It meant we finally had at least one answer about these signals.

Researchers think these molecules may be formed in the atmosphere of a carbon-rich star. Then, they could be carried into space as part of the star’s stellar wind, where they might survive for millions of years. Whenever that gas floats between us and another star, it absorbs a bit of that star’s light, and leaves behind its imprint in the form of a DIB.

So what about all the other diffuse bands? Well, many researchers are investigating molecules that contain carbon as potential culprits. Carbon is capable of forming four strong chemical bonds, which allows it to anchor the largest and most diverse collection of molecules in nature.

And it’s been a promising lead so far. Besides buckyballs, we now think some DIBs are probably created by other fullerenes, the category of large, cage-like carbon molecules that buckyballs belong to. Complex molecules like these can come in a bunch of forms that are only slightly different from each other, but each of those tiny variations may be enough to create a new diffuse band.

Other DIBs might come from a different class of large carbon molecules: the polycyclic aromatic hydrocarbons. They’re large, net-like structures of carbon and hydrogen that astronomers know are common throughout the universe. But just focusing on carbon doesn’t mean it will be easy for scientists to match more molecules with DIBs.

Unfortunately, it’ll probably be slow going. To get a spectrum, chemists and physicists have to recreate the conditions of deep space in the lab. That process is both really hard and a little different for every substance.

And I’m not a professional scientist, but that combination frankly doesn’t sound that fun. To top it all off, there’s a virtually unlimited number of potential molecules, which isn’t the best situation when you’re essentially doing guess-and-check. For now, it seems likely that astronomers will keep finding new DIBs faster than lab scientists can explain them.

There is, however, a silver lining: Each new, unexplainable band is a sign that an unknown, maybe even undiscovered molecule is floating out there in the cosmos. And that’s pretty cool. A little unnerving for researchers maybe, but cool.

Thanks for watching this episode of SciShow Space! If you’d like to learn about another mystery in the world of astronomy and planetary science, you can watch our episode about the Hypatia stone. [ ♪ Outro ].