Our ability to understand our universe is constantly frustrated by our inability to get a different perspective on the 3-D structures that litter the sky. If you are lucky enough to live in the Southern Hemisphere, a peek into a telescope at the Tarantula Nebula reveals clouds of gas and dust and starlight that are arranged just right to trick the mind into seeing a spider’s face. This is just a fancy game of shadows, however, and the actual structure of the clouds, if we could build them from clay, look nothing like the creepy crawlies that cause so many people to look away.

This lack of being able to readily see in 3-D means that when astronomers look at objects like the California Cloud and Orion A Cloud, they see similar-looking silhouettes. We also see very different amounts of star formation. This dichotomy of similar-looking clouds doing very different things has led folks to ask, “Why?”

Honestly, part of what makes scientists successful is their internal dialogue sounds a lot like a two-year-old who just keeps asking, “But WHY?” Unlike your standard two-year-old, astronomers recognize they have to find answers for themselves, and with enough technology, they do.

For this particular “but why don’t they form stars the same” question, researchers led by Sara Rezaei combined stellar locations from the Gaia mission, with maps of gas and dust from the WISE and 2MASS catalogs to determine how much dust and gas are between us and stars at different distances. 

If two stars are side by side in the sky and both have the same amount of dust between us and them, we know all that material is closer to us than the closer star. If instead, we see a small amount of dust between us and the nearer star, we know the bulk of the material is between that star and the more distant one.

Altogether, this team measured the amount of gas and dust along the line of sight to nearly 200,000 stars and used that information to come up with three-dimensional shape models of these nebulae. The Orion A Cloud was found to be denser, with both filament-like structures and a prominent ridge of material. These dense regions drive star formation. At the same time, what appeared as filaments in the California Cloud were actually alignments of a sheet of material that is being shaped by a bubble pushing up. Where parts of the sheet overlap, we see filaments.

At first glance, our view of the universe is very much Plato’s shadows dancing on a wall, but with hard work, as is shown in a pair of papers in The Astrophysical Journal Letters and Astronomy & Astrophysics, we can transform that shadow play into a 3-D reality – which is admittedly a projection of a more than elven-dimensional reality – but that is a story for another day.

More Information

MPIA press release

“Three-dimensional Shape Explains Star Formation Mystery of California and Orion A,” Sara Rezaei Kh. and Jouni Kainulainen, 2022 May 16, The Astrophysical Journal Letters

“Detailed 3D structure of Orion A in dust with Gaia DR2,” Sara Rezaei Kh., Coryn A. L. Bailer-Jones, Juan D. Soler and Eleonora Zari, 2020 November 18, Astronomy & Astrophysics