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Chapter 19: Problem 54

Discuss how H-R diagrams of star clusters could be used to set limits on the age of the universe. Could they be used to set lower limits on the age? Could they be used to set upper limits? Explain your reasoning.

Short Answer

Expert verified
H-R diagrams can be used to set both lower limits and upper limits on the age of the universe. The lower limit is determined from the main-sequence turn-off point of the oldest star clusters whereas the upper limit could be inferred assuming that no star can be older than the lifespan of the longest lived star.

Step by step solution

01

Understanding H-R diagram

H-R diagram is a graph in which the absolute magnitudes (luminosities) of stars are plotted against their spectral types (colors). Main sequence stars, white dwarfs, giants and supergiants all occupy specific regions of the H-R diagram. Normally, an H-R diagram can be used to estimate the age of a star cluster by looking at the turn-off point from the main sequence. The turn-off point indicates the stars that have exhausted their main source of fuel and are leaving the main sequence.
02

Setting Lower Limits

The H-R Diagram can indeed be used to set a lower limit to the age of the universe. The oldest globular star clusters have main sequence turn-off points at stars with 0.8 solar masses. This suggests the universe is at least as old as these stars. Their estimated age, calculated from stellar evolution models, gives us a minimum age for the universe.
03

Setting Upper Limits

Setting an upper limit to the age of the Universe using H-R diagrams could be more challenging because it is contingent on the assumption that the Universe was filled with stars at its onset, an assumption that might not be fully accurate. However, this argument assumes that the first stars formed shortly after the birth of the universe. With our current understanding of stellar evolution, we could argue that there is an upper limit to the lifespan of a star. Hence, the absence of any star older than this limit can set an upper limit to the age of the universe.

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