hard · Enhanced ACT science
Two astronomers debate the fate of a distant star cluster.
Astronomer 1: The cluster is gravitationally bound. Its total kinetic energy is less than the magnitude of its total gravitational potential energy, so the stars will remain orbiting one another indefinitely rather than drifting apart.
Astronomer 2: The cluster is unbound and dispersing. Recent velocity measurements show the stars' kinetic energy now exceeds the magnitude of the gravitational potential energy, so the cluster will fly apart over the next few hundred million years.
New data show the average stellar velocity in the cluster has decreased by 40% since the last survey, with stellar masses and positions essentially unchanged. This finding would most directly support which astronomer, and why?
- Astronomer 1, since lower velocity lowers kinetic energy, favoring KE below the potential energy magnitude
- Astronomer 2, because a velocity decrease confirms the stars are already flying apart from each other
- Astronomer 1, because gravitational potential energy also decreases when velocity drops, keeping the cluster unbound
- Astronomer 2, since lower velocity always means weaker gravitational binding between stars
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