The model of the global four-arm spiral pattern in the Galaxy is defended, for example, in the works of Vallée (2017b, 2018). Some authors prefer so far (before the appearance of more high-precision data) to consider individual segments of spiral arms with individual pitch angles (Nikiforov & Veselova 2018). In these models, the pitch angle lies in the range from −10 • to −15 • . Light absorption strongly distorts the spatial distribution of objects and makes it difficult not only to determine their photometric distances, but also to calibrate the PLR even by high-precision trigonometric distances. To determine the absolute magnitude, it is necessary to know the color excesses, and for the Cepheids of the Milky Way this is a much more serious problem than for stars of constant brightness (Rastorguev et al. 2013;Lazovik & Rastorguev 2020) or for LMC Cepheids. Supposing a unified absorption law, we can solve these problems and reduce the effect of differential absorption by using two-color Wesenheit indices (WV I as an example) instead of absolute magnitudes (Madore 1976). To use Wesenheit indexes of Cepheids instead of PLR, we should know the "period-normal color" relation, which reflects the shape and width of the instability strip and strongly depends on the metallicity of young populations. In addition, the use of Wesenheit indices or tricolor indices like QUBV to reduce the effects of differential absorption, is justified only for a small optical depth of dust in the broadband (heterochromic) photometry, which does not take place for Milky Way disk in optics.

### Language ###
English

### Assessment ###
Can ask about the range of pitch angles mentioned in spiral arm models.

### Query ###
What is the typical pitch angle range for spiral arms in the Milky Way galaxy?
What is the typical pitch angle range for spiral arms in the Milky Way galaxy?