When an H ll region is observed, it signals what stage in stellar formation?
A.
When an H II region is observed, it signals the stage in stellar formation known as the ionization of surrounding gas by a young and hot star.
To understand how an H II region signals this stage in stellar formation, we need to break down the steps involved:
1. Formation of a molecular cloud: Stellar formation begins with the collapse of a molecular cloud, which is a dense region of interstellar gas and dust.
2. Protostar formation: As the molecular cloud collapses under its own gravity, it forms a dense core known as a protostar. This protostar is surrounded by an opaque envelope of gas and dust.
3. Accretion and growth: The protostar grows by accreting more mass from the surrounding envelope of gas and dust. The infalling material becomes heated as it spirals onto the protostar, emitting radiation.
4. Increasing temperature and ionization: As the protostar continues to accrete mass, it becomes hotter and more luminous. At some point, it reaches a temperature high enough to ionize the surrounding gas. This ionization process creates an H II region.
So, when an H II region is observed, it indicates that a young and hot star has formed and is now able to ionize the surrounding gas. This stage in stellar formation is significant because it marks the transition from a collapsing molecular cloud to the birth of a young star.