Does the Milky Way move like a spinning top?

The Milky Way is a spiral galaxy, which means that it consists among other components of a disk of stars, gas and dust in which the spiral arms are immersed. Initially it was assumed that the disk was completely flat, but it has been known for some decades that the outermost part of this region has some distortions, which is known as ''spiral arms''.warping': in one direction it is twisted upwards, and in the opposite direction, downwards.

The warped stars, gas and dust are therefore in a different plane from the central extended part of the disk, and the axis perpendicular to the new planes defines its axis of rotation.

In 2020, a research study announced the detection of the precession (spinning top-like motion) of the warping of the Milky Way's disk, i.e., the deformation of this outermost region of our galaxy is not static, but, like a spinning top spinning on itself, the orientation of its axis of rotation also changes with time.

In addition, the researchers found that it is faster than theories predict: one cycle every 600-700 million years, about three times the time it takes the Sun to rotate around the center of the Milky Way.

Precession is not a phenomenon unique to the Milky Way, but also happens to our planet. In addition to the annual translational motion of the Earth around the Sun and the rotational motion of slightly less than 24 hours, the Earth's axis precesses and does so with a periodicity of about 26,000 years, which means that the pole star has not always been close to the north celestial pole, since 14,000 years ago it was close to the star Vega.

Now, a paper conducted by Žofia Chrobáková and Martín López Corredoira of the Instituto de Astrofísica de Canarias and the University of La Laguna has taken into account the variation of the warping amplitude with the age of the stars. The study, published in The Astrophysical Journal, concludes that using the warping of old stars, whose velocities are measured, could result in precession disappearing or being slower than currently believed.

Gaia mission data

To arrive at this result, the researchers have made use of data from the Gaia mission of the European Space Agency (ESA), analyzing the positions and velocities of hundreds of millions of stars in the outer disk.

"It had not been taken into account in previous work that the warping of stars a few tens of millions of years old, such as the Cepheids, is much larger than the warping for stars visible with the Gaia mission, which are billions of years old," explains Chrobáková.

"This does not mean that the warp does not precess at all, it could, but much more slowly, and we probably won't be able to accurately measure that motion until we have better data," concludes López Corredoira.