Cepheids are a type a variable star for which the periodicity in the brightness changes is related to the intrinsic brightness of the star, this relation is known as Leavitt’s Law. This makes Cepheids excellent standard candles, that is stars for which distances can be determined directly from their apparent brightness and the known intrinsic brightness. Cepheids have thus been used extensively to map the structure of our Milky Way, in particular to understand the structure of its spiral arms and the shape of the Milky Way disk (which is know to be warped).
The map below shows the locations of the known Cepheids in the Milky Way and provides a good illustration of how the selection function for Gaia limits our view of the Milky Way disk structure on the far end of the Galactic centre (notice that there are much fewer Cepheids on the right hand side of the map).
The interactive version of the map below lets you toggle the different components of the plot.
