Final answer:
Interstellar dust blocks starlight due to its ability to scatter blue light more than red, which causes interstellar reddening and extinction. Despite being sparse, dust accumulates over distances to obscure starlight, similar to smog in a city. Infrared observations help astronomers study dusty regions by reducing this obscuration.
Step-by-step explanation:
The density of interstellar dust is very low, yet it is able to block starlight due to a few key reasons. Despite its sparsity, when dust builds up over vast distances, it can obscure the light of more distant stars much like smog does in a polluted city. The dust particles are roughly the same size as the wavelength of light, and consist of rocky cores with mantles of ice, such as water, ammonia, and methane. These particles particularly scatter blue light more efficiently than red, which causes distant stars to appear redder—a phenomenon known as interstellar reddening. Additionally, this scattering effect leads to interstellar extinction, making stars look both redder and fainter.
Dust is detected not only by blocking light but also by emitting heat radiation in the infrared. This emission allows us to identify and study the dust particles. Moreover, the fact that dust grains are not much larger than the wavelength of light, maintains the reddening effect. If they were larger, they would block all colors of light equally. Thus, the unique size of interstellar dust particles is crucial to the observed reddening of starlight.
For scientists, knowing that interstellar dust scatters different wavelengths unequally is insightful in understanding the structure of our Galaxy. When viewing the Galaxy in the infrared spectrum, the obscuration by dust is less severe, which permits astronomers to observe distant cosmic features otherwise hidden by dust in visible light.