Most telescopes have what we call a B-V bandpass — a generic, high-energy blue and yellow light filter that gives us the star’s colour through the differences in the apparent brightness through the two filters. To get the total reddening of a star, we just measure the mass of the star using data we have now — this gets us an accurate value for the brightness without the extinction, allowing us to correct for the extinction!
A Case Study — The Cygnus Rift
Our Milky Way is a huge galaxy. Stretching almost 200,000 light years across, this big behemoth of stars and gas has much to offer. However, if we look at the constellation of Cygnus , there’s this big band of dust blocking our view towards its center. While this place is a prime target for infrared astronomy, pretend you haven’t brought your thermometer with you today. Let’s brush away the thorns of the region to unlock the secrets of its El Dorado!