Astronomers know that there are dusty accretion disks around protostars because a. there is often a dark band across the protostar. b. there is often a bright band across the protostar. c. theory says accretion disks should be there. d. there are planets in the Solar System.

Protostars are the early building blocks of stars. When astronomers observe the skies, they look for young stars at the beginning of their lifecycle. These observations play a crucial role. By studying protostars, scientists gain insights into star formation.
One key aspect they observe is the presence of dusty accretion disks. These disks are made of gas and dust and are fundamental in feeding material to the growing star.
Telescopes equipped with advanced instruments help us see these disks more clearly. When astronomers notice a protostar, they often detect a 'dark band'. This band signifies the presence of dust blocking the light from the protostar, an indication of an accretion disk.

The dark band across a protostar is one of the most telling signs of an accretion disk. This band occurs because the disk, full of dust and gas, obscures some of the light emitted by the protostar.
When we view the protostar from the side, the disk appears as a dark line across the bright protostar. This effect happens due to the dust within the disk absorbing and scattering the light.
This dark band is crucial because it directly reveals the disk. While theories predict the existence of these disks, seeing the dark band provides observational backing.
This evidence helps confirm that the theoretical models of star formation align with real-world observations.

In the world of astronomy, theories act as guiding principles. They outline how we believe certain processes, like star formation, should occur. However, theories need testing and validation.
Observations serve as the ultimate test for any theoretical model. When astronomers propose that dusty accretion disks accompany protostars, they use observations to test this idea.
Discovering a dark band supports the theory. It shows that what was predicted on paper exists in reality. This process of validation strengthens the theory and helps scientists trust it.
Observational evidence like this is especially valuable. It bridges the gap between mathematical models and the actual universe, ensuring our understanding remains accurate and reliable.