The Structure of Our Galaxy

Since the 17th century it has been known that the Earth, the Sun, and the planets of the Solar System, together with billions of stars, form our Galaxy – the Milky Way. Their light, when observed far from the interference of civilization, merges into a bright band across the night sky. Yet if we could look at the Milky Way from the outside, we would see that it actually has the shape of a disk. Describing the structure of the Galaxy from within it is not an easy task. That is why it became one of the goals set by the OGLE project team.

Classical Cepheids are perfectly suited for mapping the Milky Way – they are young (less than 250 million years old), pulsating stars with regular brightness variations. By knowing the frequency of their pulsations, one can calculate their intrinsic brightness. Comparing this to the star’s observed brightness makes it possible to determine its precise distance from the observer.

Based on such distance measurements for more than 2,400 Cepheids, the OGLE team created the first three-dimensional map of the Galaxy. The map shows the distribution of these young stars in the Galactic disk. Importantly, the majority of them are newly discovered objects. This would not have been possible without the instruments at the Las Campanas Observatory in Chile.

This extraordinary model – a three-dimensional map of the Galaxy – shows us that the Sun is located about 50 light-years above the Galactic disk plane. The Galactic disk itself is not perfectly flat. At a distance of around 25,000 light-years from the Galactic center, the disk roughly maintains its shape, but farther out it becomes warped (the so-called “disk warp”). In the outer parts of the Milky Way, stars can be shifted by as much as 4,500 light-years relative to the disk plane defined in its central regions. This warping may be caused by interactions with other galaxies, the influence of intergalactic gas, or the effects of dark matter.

For the first time, the OGLE project has also precisely described “disk flaring” – the increasing thickness of the Galactic disk with distance from the center. Thanks to these studies, we now know that in the vicinity of the Sun, i.e. in its central region, the Milky Way’s thickness is about 500 light-years, while at its edges it exceeds 3,000.

The OGLE project has thus not only produced the first three-dimensional map of the Milky Way but also described its warped shape, providing a deeper understanding of the structure and evolution of our Galaxy.