 Before we leave the galaxy's dusty disk, we'll take a closer look at the dust itself. It's critically important for calculating intrinsic star luminosity, and it's the only galaxy content that we can see to accurately calculate the galaxy's rotation curve. That's star velocities as a function of how far from the center of the galaxy they are. The Milky Way's rotation curve is one of the reasons scientists have proposed the existence of dark matter. The dust is made of thin, highly flattened flakes of graphite and silicon. 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. The further the light has to travel, the more of this dust it encounters, and the dimmer it gets. Astronomers call this extinction. Due to this extinction effect, stars in the galactic disk can lose up to half their luminosity every 3,000 light-years. Probably the brightest stars can be seen more than 10,000 light-years away. These clouds are best viewed using radioastronomy. This is because gas clouds radiate radio waves. And radio waves pass through dust particles untouched, because their wavelength is much larger than the size of these particles. What's more, the hydrogen in these regions emit a spectral line in the radio frequency band, and this spectral line exhibits Doppler shifts, enabling us to measure the cloud's radio velocity relative to us. In this line of sight reading, we see a number of peaks. Each one represents a cloud. Peaks have different frequencies because the clouds have different radio velocities. The maximum peak is from a cloud that's radio velocity is close to its total orbital velocity.