Scientists have known for a long time that the shape of the Milky Way galaxy we call home is a typical barred spiral galaxy. The galaxy has a bar-shaped core region surrounded by a flat disc of gas, dust, and stars. The disc of our galaxy has four spiral arms and spans a diameter of about 127,000 light-years.
Our solar system is located within the disc and is about 27,000 light-years from the Galactic center. Our location in the disc is why when we view the disc’s stars, they look like a faint band in the sky. Scientists at the University of Warsaw say that the current knowledge of the Milky Way is based on various tracers like star counts or radio observations of gas molecules informed by the extrapolation of structures seen in other galaxies.
What the scientists want to do is study the shape of the Milky Way by directly measuring distances to a large sample of stars of a specific type to allow the construction a 3D map of the galaxy. The specific type of stars the team plans to measure the distance to are Cepheids, which are young pulsating supergiants.
The brightness of the Cepheids changes in a very regular pattern, with a well-defined period, that can range from several hours to several dozen days. The team says that the Cepheids follow a relation between the pulsation period and luminosity allowing the intrinsic luminosity of a Cepheid to be inferred from its period. The team says that the distance can then be determined by comparing the apparent and intrinsic brightness of the stars.
The process is not as straight forward as it sounds, the team has to contend with the presence of interstellar dust and gas that could dim the brightness of the star, but observations using infrared bands reduce the uncertainties. Scientists say that the distances to Cepheids can be determined with an accuracy better than 5%. Using the technique, the team developed a three-dimensional map of the Milky Way using a sample of more than 2,400 Cepheids. The map shows that the Milky Way isn’t flat, and it is warped as distances of greater than 25,000 light-years from the galactic center.