 Dust. Interstellar dust in our galaxy is critically important for calculating intrinsic star luminosity to get its distance. Dust is also what enables star formation by acting as a catalyst for producing molecular gas. And dust is the only galaxy content that we can use to accurately calculate the galaxy's rotation curve that star velocities at different distances from the galactic center. The Milky Way's rotation curve is one of the reasons scientists propose the existence of dark matter. The dust is made of thin, highly flattened flakes of graphite and silica. That's carbon and rock-like minerals coated with water ice. Each dust flake is roughly the size of the wavelength of blue light, more smaller. The dust is probably formed in the cool outer layers of red giant stars and dispersed in the red giant winds and planetary nebula. The dust absorbs and scatters the light that passes through it, with blue being scattered more than red. The further the light has to travel, the more of this dust it encounters and the dimmer and redder it gets. Stromers call this extinction. Due to this extinction effect, stars and the galactic disk can lose half their luminosity every 3,000 light-years. It wasn't until we measured the amount of dust between us and the stars that we could accurately use standard candles to determine how far away they were. It was the astronomer Robert Trumpler who first quantified this phenomenon in the 1930s. In fact, it has been said that the increased understanding of dust marked the beginning of modern astronomy. Here's an image of two overlapping galaxies named VV191, discovered years ago by the Galaxy Zoo Project that has attracted over 400,000 volunteers who have performed over 11 million galaxy classification tasks since it started in 2007. Now Hubble and Webb have teamed up for a study of the galaxy pair to better understand the spiral galaxy's dust. The galaxy pair is particularly useful for dust studies because the background elliptical has its brightest region right behind the edge of the outermost dust lanes in the spiral. In such a configuration the properties of the dust in the foreground spiral galaxy can be measured to very low dust densities. A wide range of electromagnetic waves were used to construct the image with ultraviolet and visible light from Hubble and near-infrared light from Webb. This light from the elliptical galaxy suffers extinction as it passes through the dust in the spiral galaxy. Astronomers map the extinction and determine its dependence on the wavelength of the light. Here are the major steps in the dust analysis. First the original image is used to determine the observed light intensity. Next a smooth and symmetric model for the foreground spiral is used to figure the foreground galaxy's light intensity. Next a model for the background elliptical's light intensity is developed from light not passing through the spiral. And last these models are combined to get the derived light transmission map. This process is repeated for six available light wavelengths from Webb and Hubble. The wavelengths were chosen to trace the way dust extinction falls off towards longer wavelengths, which is affected both by the size of the interstellar dust grains and how strongly they are clumped together. What they found is that the light extinction in this distant galaxy is behaving very much like it works in our part of the Milky Way. This was a bit unexpected because VV191B is twice the size of the Milky Way and we are examining its outermost spiral features. It looks like dust is the same everywhere. There's one other non-dust related item in this image. Note the faint red arc in the inset at 10 o'clock. This is a very distant galaxy over 10 billion light years away whose appearance is warped by gravitational lensing. Its duplicate appears as a dot at 4 o'clock. These images of the lens galaxy are so faint and so red that they went unrecognized in Hubble data but are unmistakable in Webb's near infrared image.