Imagine standing in a dense woodland. How would you know the size and the shape of that forest from your limited viewpoint? Not seeing the woods for the trees is a good analogy for what astronomers experience when trying to assess the size and shape of our galaxy — we occupy a small star system embedded inside the Milky Way's disk. It's not like we can fly above the galactic plane to peek at our galaxy's overall shape (although how cool would that be?).
Researchers, however, were determined to figure out the true shape of the Milky Way, while being embedded inside of it. Here's how they did it: A team from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) and Macquarie University in Australia studied 1,339 bright pulsating stars called Cepheid variables to create a 3-D map of the disk of our galaxy.
What they found came as a surprise: We live in a warped galaxy.
"We usually think of spiral galaxies as being quite flat, like Andromeda which you can easily see through a telescope," said astronomer and research collaborator Richard de Grijs, of Macquarie University, in a statement. But our galaxy isn't like Andromeda. It has an S-shaped bend that gets more twisted the farther you move away from the galactic center. At this bend, the galaxy's gravitational pull becomes weaker, making it look like an old vinyl record that has become warped.
The study, which has been published in the journal Nature Astronomy, used data from NASA's Wide-field Infrared Survey Explorer (WISE) to precisely determine the locations of the Cepheids throughout our galactic disk and turned them into a powerful tool to cut through the dust, gas and other stars that are obscuring our view.
"It is notoriously difficult to determine distances from the sun to parts of the Milky Way's outer gas disk without having a clear idea of what that disk actually looks like," said Chen Xiaodian, a researcher at NAOC and lead author of the study, in a separate press release. "However, we recently published a new catalogue of well-behaved variable stars known as classical Cepheids, for which distances as accurate as 3 to 5 percent can be determined."
Cepheids are young stars that are four to 20 times the mass of our sun, and they live fast and die young, consuming all their fuel in the span of only a few million years, burning up to 100,000 times brighter than our star. But what they lack in lifespan, they make up for in regular pulses in brightness that can be used by astronomers to accurately measure their distances and, in this case, they acted as tracers to map out the warped Milky Way disk.
Although the Milky Way doesn't conform to the standard, flat disk exhibited by other spiral galaxies (like Andromeda), it's not alone. From earlier observations, the researchers identified a dozen other galaxies with a similar S-bend shape, which have given them a clue as to why our galaxy is warped.
"Combining our results with those other observations, we concluded that the Milky Way's warped spiral pattern is most likely caused by 'torques' – or rotational forcing – by the massive inner disk," added Liu Chao, co-author of the study. Basically, the orbital motions in the massive central region of the Milky Way gravitationally bully the less massive outer regions, causing them to buckle and bend out of shape.
Ultimately, this new finding could help us better understand the dynamics of orbital motions inside the Milky Way, thereby providing a glimpse of how our galaxy evolved.
Originally Published: Feb 6, 2019