Variable Stars Help Astronomers Build 3D Map Of The Milky Way

The outer disk of the Milky Way is warped. J. Skowron / OGLE / Astronomical Observatory, University of Warsaw

If you picture the Milky Way as a whole, you probably think of it as a flat structure with a small bulging region in the middle and beautiful spiral arms stretching out. The reality is a fair bit more complicated. There is a bar, not a bulge, and the Milky Way’s outer disk is not flat but warped.

It is not easy to work this out, given that we are inside the Milky Way. Many groups of astronomers have been working on 3D maps of the Milky Way and researchers from the Astronomical Observatory of the University of Warsaw have constructed an advanced map using a particular type of star, known as a Cepheid variable. The work is published in the journal Science.

Cepheid variables are pulsating stars whose day-to-month-long changes have an extremely stable period and are closely linked to their true luminosity. By combining measurements of their observed brightness you can get a pretty good estimate of their distance. This relation between pulsation, brightness, and distance, devised by American astronomer Henrietta Swan Leavitt, turned these objects into the first standard candles, a sort of cosmic milestone.

Armed with this knowledge, the team estimated the distance of 2,431 Cepheid variables in the Milky Way and, using their position in the sky, were able to create an important 3D map of our galaxy. This gave them a good idea of what our galaxy actually looks like.

The distribution of the Cepheid variables and 3D map of the Milky Way. J. Skowron / OGLE / Astronomical Observatory, University of Warsaw

“Our map shows the Milky Way disk is not flat. It is warped and twisted,” co-author Przemek Mroz comments in the video below. “This is the first time we can use individual objects to show this in three dimensions.”

Earlier this year, another study employed Cepheid variables to look at the warping of the Milky Way, but it did not incorporate as many objects as the newer work. The origin of the outer disk's skewness is currently unclear. Gravitational interactions with the small galaxies surrounding the Milky Way or a past merger might have played a role but many believe that internal forces from the massive inner disk could have caused the warp we see.

More detailed maps of the Milky Way are coming thanks to the European Space Agency's Gaia mission. These could provide even more details on the structures in the Milky Way and provide clues to where they come from.

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