Humans are kind of like goldfish.
I mean that in a celestial sense. Just like a goldfish can’t see its bowl from the outside, our position in the universe means we can’t see our home galaxy, the Milky Way, as the rest of the universe sees it.
Thankfully, scientists have built the most accurate map of the Milky Way yet, using a collection of huge, bright stars. They have found that the Milky Way is not exactly as artist’s impressions might have you believe — it’s actually twisted and warped, bending at the edges.
It’s the first time that a map was built using young stars — and the first measurement of how twisted our home galaxy is.
The paper, published in Nature Astronomy on Feb. 4, details work by Australian and Chinese astronomers to examine the classical “Cepheids” — a collection of huge, young stars in the Milky Way that can be up to 100,000 times brighter than the sun. Looking at the Cepheids over time you can see the stars pulse, sometimes dipping or increasing in brightness. By studying the length of these pulses, the distance between us and the Cepheids can be found, helping to construct a map of the Milky Way.
The team plotted the locations of 1,339 Cepheids on a 3D map, building out the most accurate representation of how the Milky Way is shaped. We know the Milky Way is a spiral galaxy — a thin disk of a hundred billion stars that circle around a huge supermassive black hole at the galaxy’s centre. The gravity of all these objects hold the galaxy together.
But as you get further out, the gravity weakens and the gas that makes up the disk doesn’t fall neatly into the thin plane that stars closer to the centre do.
The research team showed, with the help of the Cepheids, how the Milky Way isn’t a flat cosmic disk shaped like a lipless frisbee or a pancake, but instead it’s markedly warped into an S-like shape. So, going with the pancake analogy, the Milky Way looks more like a pancake, if you’ve just lifted it out of the pan before letting it awkwardly dangle on the spatula.
And the twisted shape isn’t necessarily unexpected — other spiral galaxies in the universe exhibit similar features.
“Many large spiral galaxies actually show warped (S-shaped) disks in the distribution of the gas in their disks,” says Richard de Grijs, a co-author on the study and astronomer at Macquarie University.
“The “twisting” of the disk is, we think, caused by gravitational forcing of the rotating inner disk, dragging along the outer disk,” de Grijs explained.
How exactly this shape came to be requires a little more work, however.
“We would need to do (numerical) simulations of a realistic galaxy embedded in a dark matter halo to see if we can reproduce the observations and figure out how this came about,” says de Grijs. And with the amount of stars in the Milky Way increasing thanks to observations by spacecraft such as the European Space Agency’s Gaia, there’s always room to improve the model even more.
“It would also be good to confirm the shape using larger samples of objects.”
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