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.

In the early stages of star formation, a protostar forms when a region within a molecular cloud collapses under gravity. Protostars are in the process of accumulating mass and are not yet hot enough to trigger nuclear fusion. During this time, the surrounding gas and dust start to form an accretion disk around the new star. This disk is crucial because it channels material onto the protostar, helping it grow. The accretion disk's mass and material, combined with the energy output of the protostar, lead to various observable phenomena that astronomers study to understand star formation better.

Protostars are often hidden within dense clouds of gas and dust, making them challenging to observe directly in visible light. However, they emit strongly in infrared wavelengths, which can penetrate these clouds. This makes infrared observations vital for studying protostars and their development.

Observational astronomy involves collecting and analyzing data about celestial objects and phenomena using telescopes and other instruments. It's the primary method by which we gather empirical evidence about objects in space, such as protostars and their surrounding accretion disks.

Astronomers use various types of telescopes, including those that observe visible light, infrared light, and radio waves, to capture a complete picture of distant phenomena. For studying protostars and accretion disks, infrared telescopes are particularly important because they can detect the heat emitted by young stars and the surrounding dust.

Advanced techniques such as spectroscopy allow astronomers to determine the properties of celestial objects, like their composition, temperature, density, and motion. Through these observations, scientists can validate theoretical models and improve our understanding of the universe.

Accretion disks are essential components in the formation of celestial bodies like stars and planets. The evidence for their existence around protostars comes from several observational techniques.

* **Dark Bands**: One significant piece of evidence is the presence of dark bands across protostars. These are caused by the dense dust in the accretion disk blocking the light from the protostar, making it appear as a dark lane against the brighter background. This phenomenon is crucial in identifying accretion disks.

* **Infrared Emissions**: Since dust in the accretion disk absorbs visible light and re-emits it in the infrared spectrum, astronomers use infrared telescopes to detect these emissions. The specific patterns of these emissions help confirm the presence and characteristics of accretion disks.

* **Spectral Lines**: Spectroscopy can reveal the movement and composition of the material in the accretion disk. By observing redshifted and blueshifted spectral lines, astronomers can infer the rotational dynamics of the disk, further supporting its presence.

Dusty accretion disks are particularly important in the star formation process. These disks consist of gas and dust that orbit the protostar and gradually accrete onto it, helping the protostar grow in mass.

The dust within these disks plays several crucial roles. It acts as a cooling agent by radiating away the heat generated during the accretion process, which allows more material to fall onto the protostar without dispersing. This cooling is essential for the disk's long-term stability.

Dust grains within the disk can also stick together and coagulate, forming larger particles that may eventually become the seeds for planet formation. This process is the first step in building the complex structures that will result in planetary systems.

Studying dusty accretion disks around protostars thus provides invaluable insights into how stars and planets form and evolve, helping us understand our own Solar System's origins.