Astronomers believe that stars develop from the condensation of interstellar gas clouds (IGCs) that are initially hundreds to even thousands of times more massive than the Sun. Two astronomers discuss the star formation process in these clouds.Astronomer X:Star formation begins when large fragments of a gas cloud collapse. Each of these fragments typically contains hundreds of times more gas than exists in an individual star. This collapse occurs when the gravitational attraction between the gas particles overcomes the sum of all outward forces. When collapsing gas particles in each fragment get close together, heat builds up. As a protostar begins with the inward movement of particles from the exterior of the protostar. Its collapse stops when outward forces, especially forces that result from the heating of the gas being compressed, balance the gravitational force. This brings a small region of the original cloud into equilibrium and a new star into a stable form.Astronomer Y:Stars develop when small fragments of gas clouds collapse. Calculations show that when fragmentation occurs, the fragments that form are much smaller in size than mature stars. Thus, stars develop from relatively small masses and volumes of the large IGCs called cores. A cold core collapses because of its own gravity. However, 95% of all localized cores are called protostars. The collapse of a cold core is stopped by outward forces that develop when the gas heats up. The protostar then gravitationally attracts for outer layers of the cold core together until the gas has completely transformed into a star. At early stages in development, the outer layers of the cold core are so thick that they prevent the core's light from escaping into space. Only after the outer layers have been incorporated into the star does the star become visible.Is the observation by Astronomer X that almost all young stars are found in clusters consistent with the theory of Astronomer Y?

Correct answer: D. No: cold cores pull in all material close to them

• A. Yes: the sub-fragments of fragments of the cloud that form stars attract each other gravitationally
• B. Yes: the gas cloud where individual stars form is a relatively compact object
• C. No: the stars. Once formed drift away from each other
• D. No: cold cores pull in all material close to them

Last updated 3 September 2026